UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post

Darren-UK · 19th Oct 2011, 10:06 pm

Valves

It simply isn't practical here to describe each and every valve (vacuum tube), what it does and how it functions. However, the beginner needs to recognise the handful of types found in the record players which this guide embraces. Take, say, your run-of-the-mill Dansette or Fidelity machine; you may find a solid state rectifier and just one valve (an Output valve driving the loudspeaker), or you may find a rectifier valve and an output valve, or two output valves and an amplifier valve preceding the output valve. You will need to know which is which and, importantly, where they locate on the chassis/PCB. On the latter point, it's not uncommon for the 'phantom twiddler' who doesn't know what he's doing to twiddle about and insert valves in the wrong sockets - then wonder why the machine fails to function. The problem is quite common with radio receivers but isn't unknown with record players. Service data or, often, a diagram inside the cabinet, will show you where the valves are located.

To make things easy we'll stick herein to the more familiar valve codes as used by Mullard and a few other manufacturers. Codes can be cross-referenced via valve equivalent books or, perhaps more conveniently, via websites such as http://www.r-type.org

In the main, record players designed for AC mains (only) power will use valves with codes beginning with the letter 'E' whereas record players designed for use on AC/DC supplies will use valves with codes beginning 'U'. A few examples follow:

EL41 and EL84. These are Output valves for AC supply.
UL41 and UL84. As above but for AC/DC supply.
EZ40. Rectifier valve for AC supply.
UY85. Rectifier valve for AC/DC supply.

Note that if the second letter is either a 'Y' or a 'Z' then the valve is a rectifier.

Other valves found in record players from the period in question might include:

ECL82
ECL86
PCL82
PCL83
UCL82
UCL83

The above are Audio frequency/Audio amplifier valves. The two PCL's are actually, or rather were originally, television valves. Ignoring rectifier valves obviously, you may find your record player contains two identical valves. If this is the case you'll have what is known as a "Push Pull" output stage. However, to further describe this here would be drifting somewhat so we'll elaborate in a later post concerning P-P output stages and stereophonic machines (the two are not necessarily the same thing). Meanwhile, hereinafter, you can assume we're talking about the more common single-ended (ie single output valve) amplifiers.

However, before we press on to amplifiers proper further down this post, one further explanation would be in order. Beginners may by now have realised that Output valves are the ones which ultimately drive the loudspeaker. Some confusion may therefore arise when a valve such as an EL-something, or a UL-something is not present, but ECL - or UCL - type(s) are present on its, or their, own. This is because valves, put simply, are in fact two or more functions in one glass envelope - hence diode, triode, pentode and so forth. So if we take, say, an ECL82, this valve on its own acts as a preamplifier and an output valve; E = for AC supply, C = the preamp section and L = the output section.

The same applies to, say, a UCL82 with the exception of the "U" which, as you'll recall, signifies use in AC/DC machines. You cannot, therefore, mix E and U series valves on the same chassis irrespective of the remaining letters and numbers of a valve's code. Out of interest, there exists one or two odd expections to this in the radio world but you won't come across it in record players.

Valve and valve base faults

There exists, among 'Joe Public' an erroneous assumption that almost any fault with any valved equipment has to be down to a faulty valve or valves. This is probably an assumption stemming from the fact that valves are, since 1930-ish anyway, the only components which can be readily unplugged from and reinserted into a chassis.

Nevertheless some problems can - and do - occur with valves from time to time, albeit far less so in record players when compared to radios. This is because; record players usually contain less valves, because record players tend to see less use than radios and because record players usually have fewer other components which can instigate valve trouble. The latter, of course, is rather an obvious statement because the number of "other components" is directly related to the number of valves and the function of those valves.

Common and easily traced and remedied valve faults include:

Identification Perhaps not strictly a fault but a commom problem with valves is the inability to identify them due to the markings on the envelope being invisible. You won't get very far if you can't identify your valves. A common 'cure' is to place the anonymous valve in the fridge for a while, then breathing on it in the hope that the ghost of the markings will show up. If they do, then writing the details on the envelope with a CD marker pen is the usual next step. A quicker and better idea, however, is to shine an LED torch onto the valve. In 99% of cases the markings thus become visible - or at least the imprint of where the markings once were becomes visible.
Valve totally dead and not 'warming up'. Assuming none of the faults described below apply, the valve most likely, although not unquestionably, has a failed heater. Substitution will prove or disprove this, but first it's wise to grab your meter and check for voltages at the valve base. To do this you will need to obtain - and understand - valve data. Of course, you can use your meter to check heater continuity by first ascertaining which two pins connect to the heater. Be careful your meter doesn't pump more current into the heater than it's designed to handle though. If you have any uncertainties regarding this and/or any uncertainties about LT and HT supplies you're advised to seek help in the forums.
Lost vacuum. A valve (or vacuum tube) consists of a glass envelope, expelled of air, within which the electronics are located. A valve which has lost its vacuum will have a milky on the inside if its envelope. Replacement of the valve is the only solution.
Dirty valve pins and/or base sockets. Clean by spraying the base with switchcleaner spray and plugging in/unplugging the valve several times. This MUST be done with the mains supply isolated and do not reconnect the mains until the switchcleaner has dried.
Bent or broken valve pins. Assuming the valve to be 'all glass', you can straighten a bent pin be careful bending with your fingernail. This does, however, carry the risk of the glass cracking and the valve thereby losing its vacuum. Purpose designed gadgets for straightening valve pins were made but they're now scarce. Should you fail to remedy the problem, replace the valve. Same for broken pins, replace the valve - unless the broken pin happens to be an unused one.
Poor or non-existent connection(s) between valve pin(s) and base sockets(s). Assuming the valve pins to be in good order, this problem is usually the result of the socket(s) for some reason opening up and thus not contacting the pins. You can attempt to close up the affected sockets but they tend to break. If this happens, replace the base.
Tracking across valve base. This is the same as the tracking which can occur across the distributor cap of a petrol engine. Sometimes this can be cured simply by thoroughly cleaning the valve base, but sometimes the problem can persist and the easiest solution is to replace the base.
Internal short circuit. One or two valves can develop such between the heater and cathode. The most well known valve for this problem is the UL41. Replacement is the only solution although, perhaps fortunately, you're unlikely to find many record players which contain a UL41.
Low emission. Put simply, this means a valve is not pumping out what it's supposed to. Like Selenium rectifers, rectifier valves are not immune to the problem either. Replacement is the only solution. Perhaps I should add here that "emission" is a term more often applied to valves other than rectifiers. "Low output" would perhaps be more appropriate for a rectifier valve, but I stuck with "emission" to avoid any confusion with Output valves.
In connection with the low emission problem described previously, one point is worth a reminder. This reminder being that a defect with a valve can be, and often is, caused by faulty associated components such as capacitors. This is one reason why old capacitors (and resistors) cannot be ignored. Output valves are especially vulnerable to capacitor problems.

Power supplies, progressing onto amplifiers

The distinction between the power supply and amplifier if, say, the mains transformer is excluded, can become blurred. For example, some may see a smoothing capacitor and a rectifier as being part of the power supply whereas others may see these components as part of the amplifier. To sidestep this distinction and to assist newcomers in recognising components and their possible faults, this post is component, as opposed to unit, orientated.

At this point we are dealing with mains electricity, so if you are unfamiliar with this area, or cannot recognise components and/or understand their function, you are strongly advised to leave well alone and seek advice. Although much of it revolves around radio receivers, reading this and other pages of our main website will help you to understand components and their function. Pay particular, but not exclusive, attention to the capacitor section; illustrations within which will help you identify the different types and also recognise the potential troublemakers.

So what, you may be asking, has radio-orientated stuff got to do with record players? A fair amount is the answer; what you'll find inside your run-of-the-mill record player is effectively the power supply as found in a mains powered radio, married to an audio output stage as also found in said radio(s). Indeed, many radio receivers include a facility by which an 'add on' record deck can be connected to - and thus played through - a radio receiver. In such instances the radio receiver is then, for all intents and purposes, serving exactly the same function as a record player amplifier. Therefore, many power unit and amplifier components found in radios will be the same as, or similar to, those found in record players. For the moment, however, we'll stick to mains power supplies.

Put simply, this will consist of a mains transformer (if an AC mains only machine) or a dropper resistor (if an AC/DC mains machine), a rectifier (which may be a valve or a Solid State device) and smoothing/reservoir capacitors. In some circumstances the supply for the valve heater(s) may be via a tap off the motor windings. If the motor has three wires connected to it, then this will be the situation.

Regarding mains transformers, usually an 'isolating transformer' is used. This type, in simple terms, has two coil windings; one primary and one secondary. The windings are inductively coupled and thus 'isolated' from the mains power source. Beware though, on rare occasions an 'autotransformer' may be used and I've come across at least two record players so fitted (one being an Emerson). Autotransformers are little more than a large wire-wound resistor and do not, therefore, offer the aforementioned advantage of isolation.

The mains dropper resistor takes the place of a transformer in AC/DC machines, the latter meaning a machine designed to operate from either Alternating Current or Direct Current mains. There are various reasons for the provision to operate off DC mains but these are beyond the remit of this guide. What you do need to be aware of, however, is that AC/DC equipment (as with equipment using an autotransformer) is potentially more hazardous than AC-only equipment. This is especially so if your record player has a metal chassis (often a PCB is used though).

So it will by now be understood that should you need to do any work on your record player's electrics, you need to be armed with knowledge of what in fact you're dealing with - another reason to be familiar with the various components. Otherwise the relevant service data as advised back in post #1 will tell you, as will a little data plate affixed somewhere on the record player (these are not always present though).

A vaguely similar power supply arrangement to that of AC machines will be found in mains powered transistorised machines, with the obvious difference that there'll be just a low voltage supply to the amplifier but not necessarily to the motor - so beware.

At this point it's convenient to elaborate upon why power supplies and amplifiers are combined in this one post. See the image below.

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^ Here's the reason, power supply and amplifier components are mounted upon a common chassis - with the exception of the mains transformer. It is probably, therefore, an idea to seperate the components by explanation. This is a typical arrangement for AC machines although, as mentioned earlier, a PCB (Printed Circuit Board) is often used instead of the far better aluminium, or aluminium alloy, chassis seen here. On AC/DC machines using a dropper resistor, the dropper will likely be mounted on the chassis rather than seperately elsewhere as per the mains transformer. The reason for this will be explained shortly.

The image above shows what is, by domestic record player standards, rather a deluxe set-up. There are three valves; a halfwave rectifier (to convert the AC mains to a form of DC), an audio input amplifier and an output valve. That latter is the device which drives, via the output transformer, the loudspeaker.

Output transformers may be located on the chassis, as in this case, or on the loudspeaker itself, or on its own somewhere else within the cabinet. If you're a newcomer to this malarky, do not get the mains and output transformers mixed up! Also, do not run the amplifier with the loudspeaker disconnected unless a load such as a test meter is connected to the 'speaker wires.

At the bottom left of the image we can see the mains transformer, well away from the chassis. This component is mounted here for two reasons; 1. It facilitates the fitting of different transformers according the which market the machine is intended for, thus allowing the chassis to be of a fairly standard design. 2. Keeping the mains transformer away from amplifier components helps reduce the risk of induced mains hum.

This transformer provides LT voltage for the valve heaters (LT = Low Tension) as well as the feed to the rectifier, via a surge limiting resistor, which emerges as the HT (High Tension).

Moving to the chassis proper, at its left we can see a pilot lamp which tells us when the power is switched on at the on/off switch. This latter is combined with one of the two potentiometers seen next to the pilot lamp. Next we can see a large cylindrical alloy can. This is the reservoir and smoothing capacitors; the purpose of which is to smooth out the hum caused by alternating current mains. The rectifier alone cannot do this. At the bottom of the chassis can be seen the three valves, one of which is the rectifier (Rectifiers are called 'valves' for convenience. In this case the chassis contains, from the technical viewpoint, two valves and a rectifier. This would commonly be stated on technical data as being "2v + R". Do not confuse "2v" with "2V", the latter meaning "Two Volts").

Behind the chassis and thus out of view in the image lurks a bare minimum of other components, namely two or three capacitors and a couple of resistors. All very simple when compared to, say, a radio receiver. The components described in the above three paragraphs are to do with the power supply and we'll continue with some specific issues in this area.

Smoothing 'cans', as they tend to be called, should be treated with respect. Examine them at their solder-tag end; is that end 'pregnant' ie bulging? is there any mould or goo-like substance around the solder-tags (terminals)? If any of these visual faults are present the 'can' must be replaced as it's in a dangerous condition and could literally explode, resulting in at least a vile mess or at worst physical injury to yourself. These 'cans' are another reason you should not apply full mains power to a machine with an unknown history.

The failure to smooth out mains hum is another indication of a failed smoothing can, although in some cases the can may just need 'reforming'. Do a search for "Reforming", "Reforming electrolytics" etc. Although it's mentioned elsewhere, it is convenient now to repeat that with many mains powered record players a degree of 'warm' background hum is normal. Get hum thanks to a duff smoothing can and you'll know it - a very loud hum not unlike the sound of muffled, but very rapid, machine gun fire. Another aspect relevant here is 'Ripple Current'; rectifiers leave behind what you could call a 'residue' from the ingoing AC current. To handle this, capacitors have a 'Ripple Current Rating', quoted in mA (milliamps). Determining ripple current is really not something the beginner, so if you find yourself needing to replace some electrolytics it's easiest to select the highest rating for the voltage and capacitance you require. That said, most modern replacements seem to be perfectly adequate regardless so it probably won't be an issue you need to lose sleep over.

Finally on the smoothing can subject, another safety issue. When a machine has been powered up, these cans can retain their charge for a while after power has been disconnected. Therefore NEVER touch the terminals of these cans, or any other parts connected to them, for some minutes following power disconnection. A discharge resistor may or may not be present, either way take no chances as belts from these cans are nasty and potentially lethal.

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^ A little while ago we talked about mains transformers. With machines designed for AC/DC power supplies (that's AC or DC) you will find not a transformer but a 'Dropper resistor' (simply "dropper" hereinafter). The image on the left shows a typical dropper BUT one removed from a radio receiver. This type is mounted on, and perpendicular to, the chassis/PCB and has connection tags to suit various input (ie mains) voltages.

You may find this type of dropper inside an AC/DC record player or you may find a simpler 'fixed input voltage' axial type such as that shown in the image on the right. However, the latter type can be found elsewhere on the chassis for ballasting purposes so don't get confused (Earlier it was mentioned about some more capacitors and resistors being out of sight behind the chassis depicted; one of them is just such a resistor).

Droppers/ballast resistors are high power devices and thus run hot, very hot in fact. "High power" refers to their Wattage ie Volts x Amperes = Watts = Power. It is worth adding here that Output and Rectifier valves also run hot and this is normal.

Should a dropper fail, and they do fail, you have various options available to you. Should you find yourself in this position, plenty of information exists in our forums and main site. Alternatively and especially if you're unsure of what you're doing, you can create a forum thread to seek advice.

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^ In the rectifer department we've so far babbled on mostly about 'valve' rectifiers. Many old record players, and other equipments, will instead make use of a selenium rectifier. A 'Solid State' rectifier in other words. This image shows a typical selenium rectifier which, as it happens, was once installed in a record player (some joke of a low quality 'attache case' effort I believe it was). Other rectifiers (metal oxide) take the form of ganged discs with fins for air cooling, the flat Selenium type as illustrated are affixed to the (metal) chassis which cools by conduction ie the chassis acts as a heatsink. You will therefore not find flat selenium rectifiers PCB mounted.

Selenium rectifiers, being solid state, have some advantages over the 'valve' types. Unfortunately they can go wrong - and they do. The symptom of impending failure is low output, meaning the voltage output is lower than what it should be, but if a rectifier has reached this state then it's scrap anyway - impending total failure or not. Total failure cannot be mistaken as very unpleasant odour is emitted - and the fumes are toxic. Do a forum search for "selenium rectifier" and "rotten eggs" and you'll get the idea.

Failing or failed selenium rectifiers can easily be dealt with by replacing them with modern, easily obtainable diodes. As aforementioned, searching for "selenium rectifier" will give the details. If your amplifier has poor, or no, output, distorted audio and/or poor bass control (if fitted) one of the things you should check is the rectifier output. Set your meter to a range of at least 250V and check across the rectifier's output. Remember you're dealing with high voltages here, with AC mains in and DC output at, if all is well, a figure (voltage wise) approaching that of the input. If the output is less than 200V, the rectifier is failing.

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^ We'll move onto amplifier components now. This image appears earlier in this post but is repeated here to avoid the nuisance of repeated excessive scrolling of a long post.

As touched upon earlier, if you take the chassis of a radio receiver and rid it of it's RF and IF stages (look those up if unsure) you're left with just the audio and output stages. In very simple terms, this is what's inside a record player - or at least in the case of the example used in the images here; some really cheap, rubbishy offerings contain little more than an output valve and a few capacitors and resistors.

It is impossible to describe here how an amplifier works simply because amplifiers vary. For example, there may or may not be a tone control or there may be seperate bass and treble controls. There might be a single valve or there maybe two or three. The output may be mono or stereo (the latter is somewhat rare where portable domestic record players are concerned, but machines with the facility for an add-on speaker/amp unit for stereo reproduction are quite common).

So to simplify things, given too that we have only one amplifier picture to refer to, we'll give just a basic description of how the amplifier depicted functions. Remember that of the three valves seen, only two are part of the actual operation of the amplifier.

To begin, we can see a white cable descending to the amplifier. This is the wires from the cartridge, carrying the signal from the record to the amplifier. These wires connect to the volume control which, in this case, also incorporates the on/off switch. Across this is connected the tone control and its attendant 'top cut' capacitor. From here we have the 'input signal'. This signal then passes to the first valve (V1) where it's amplified. This stage is, in effect, a 'preamp'. From here the now-amplified signal passes to the second valve (V2, the output valve) where it is further amplified. The next stage is the 'output transformer'; this can be seen mounted at top right of the chassis. The signal is fed to the primary winding of this transformer, thence to the loudspeaker via the secondary winding.

As mentioned earlier, a small few resistors and capacitors also lurk for coupling and other purposes. Capacitors are well documented here and elsewhere but resistors, other than mains droppers (which are wire-wound) are worth a mention. Carbon resistors, especially high value examples, tend to go out of tolerance with age. Specifically they increase in value. This can cause all sorts of apparently strange problems and, as the common-or-garden record player contains only a minimum of components, it's quick and worthwhile to do a check of all resistors.

If you can't read resistor colour codes (some have their values actually marked on them) then you'll need the relevant service data, which will tell you values and locations. You can check resistor values in-circuit providing no other components are connected in parallel with them. Once again, service data will give you this information or, if data isn't available, a careful visual examination of the circuit will tell you. This isn't daunting on relatively simple record player circuits (as opposed to the far more complex radio circuits) but be aware that anything connected in parallel with a resistor may not necessarily be located slap bang next to it on the chassis/PCB.

If we divert for a moment and go to post #14, take a look at the second image therein which shows part of a transistorised amplifier. At the foreleft of the image can be seen a small low wattage resistor. This is one of those mentioned above which have the value printed upon them, in this case 10K (10,000 Ohms). The main reason for this momentary diversion is to highlight a problem which can occur with this type of resistor; namely the crimped terminals. These can go intermittent or fail completely, the only real cure being to replace the resistor. This type of resistor is common in transistoried equipement - far less so in valved. In fact, off the top of my head, I can't say if they appeared in valved equipment at all.

Another not uncommon fault with resistors is a peculiar and otherwise unexplainable, rustling sound at the loudspeaker (Dry solder joints, wiring faults etc usually materialise in the form of louder, harsher, crackles and pops). If you encounter this problem with a resistor, replacement is the cure.

The Push-Pull amplifier

Generally speaking, the type of record player this guide is concerned with will have a single output valve. Better quality machines may have a pair of output valves of the same type (in transistorised amplifiers a pair of output transistors is virtually the norm, albeit not necessarily of the same type; one PNP and one NPN for example).

Sticking with the Push-Pull valved amplifier, the setup is rather more complex (and one reason we're not detailing stereophonic machines in any great detail). The signal, preamplified, goes through a 'Phase Splitter' circuit wherein the signal is split into phases; imagine an AC waveform with one side + and one side -, this means the two phases are then 108deg apart or, if you like, polarised. To provide superior performance (compared to a single valve [single ended] output stage), each phase is seperately amplified and hence the reason for the twin output devices ("Device" is a common term for a valve or a transistor in the context of a circuit).

That's the nutshell description, the reality being rather more complex in that output devices must be balanced (matched, hence the term "Matched Pair" [commonly applied to transistors] which you might have heard of) and biasing circuits must be precisely set.

To learn more, do a web search for "Phase Splitter" and "Push-Pull output stage". Don't worry if most entries concern guitar amplifiers as the principle is broadly the same.

Brief summary

Record player amplifiers do not, largely due to their relative simplicity, give too much trouble. When faults do appear, they can be out of tolerance resistors, leaking capacitors ("Leaking" means passing DC), dirty valve pins and sockets, dirty or otherwise faulty controls, 'dry' soldered joints and/or cracked tracks if a PCB type chassis is used.

Valves, contrary to popular myth, don't give too much trouble beyond the aforementioned dirty (or bent/broken) pins and sockets. There are, however, a small few exceptions but most of these are seldom found in record players.

Finally for this post, the keener eyed among you may have noticed the wire clips securing the valves in position in the above image. These should be handled with care; they're tough and can break the pip on the top of the valve, if not smash the glass envelope completely. Should this occur the valve is, obviously, scrap.

19th Oct 2011, 10:06 pm	#10
Darren-UK Retired Dormant Member Join Date: Apr 2003 Location: Blackpool, Lancashire, UK. Posts: 4,061	Valves, mains power supply and amplifier components. Valves It simply isn't practical here to describe each and every valve (vacuum tube), what it does and how it functions. However, the beginner needs to recognise the handful of types found in the record players which this guide embraces. Take, say, your run-of-the-mill Dansette or Fidelity machine; you may find a solid state rectifier and just one valve (an Output valve driving the loudspeaker), or you may find a rectifier valve and an output valve, or two output valves and an amplifier valve preceding the output valve. You will need to know which is which and, importantly, where they locate on the chassis/PCB. On the latter point, it's not uncommon for the 'phantom twiddler' who doesn't know what he's doing to twiddle about and insert valves in the wrong sockets - then wonder why the machine fails to function. The problem is quite common with radio receivers but isn't unknown with record players. Service data or, often, a diagram inside the cabinet, will show you where the valves are located. To make things easy we'll stick herein to the more familiar valve codes as used by Mullard and a few other manufacturers. Codes can be cross-referenced via valve equivalent books or, perhaps more conveniently, via websites such as http://www.r-type.org In the main, record players designed for AC mains (only) power will use valves with codes beginning with the letter 'E' whereas record players designed for use on AC/DC supplies will use valves with codes beginning 'U'. A few examples follow: EL41 and EL84. These are Output valves for AC supply. UL41 and UL84. As above but for AC/DC supply. EZ40. Rectifier valve for AC supply. UY85. Rectifier valve for AC/DC supply. Note that if the second letter is either a 'Y' or a 'Z' then the valve is a rectifier. Other valves found in record players from the period in question might include: ECL82 ECL86 PCL82 PCL83 UCL82 UCL83 The above are Audio frequency/Audio amplifier valves. The two PCL's are actually, or rather were originally, television valves. Ignoring rectifier valves obviously, you may find your record player contains two identical valves. If this is the case you'll have what is known as a "Push Pull" output stage. However, to further describe this here would be drifting somewhat so we'll elaborate in a later post concerning P-P output stages and stereophonic machines (the two are not necessarily the same thing). Meanwhile, hereinafter, you can assume we're talking about the more common single-ended (ie single output valve) amplifiers. However, before we press on to amplifiers proper further down this post, one further explanation would be in order. Beginners may by now have realised that Output valves are the ones which ultimately drive the loudspeaker. Some confusion may therefore arise when a valve such as an EL-something, or a UL-something is not present, but ECL - or UCL - type(s) are present on its, or their, own. This is because valves, put simply, are in fact two or more functions in one glass envelope - hence diode, triode, pentode and so forth. So if we take, say, an ECL82, this valve on its own acts as a preamplifier and an output valve; E = for AC supply, C = the preamp section and L = the output section. The same applies to, say, a UCL82 with the exception of the "U" which, as you'll recall, signifies use in AC/DC machines. You cannot, therefore, mix E and U series valves on the same chassis irrespective of the remaining letters and numbers of a valve's code. Out of interest, there exists one or two odd expections to this in the radio world but you won't come across it in record players. Valve and valve base faults There exists, among 'Joe Public' an erroneous assumption that almost any fault with any valved equipment has to be down to a faulty valve or valves. This is probably an assumption stemming from the fact that valves are, since 1930-ish anyway, the only components which can be readily unplugged from and reinserted into a chassis. Nevertheless some problems can - and do - occur with valves from time to time, albeit far less so in record players when compared to radios. This is because; record players usually contain less valves, because record players tend to see less use than radios and because record players usually have fewer other components which can instigate valve trouble. The latter, of course, is rather an obvious statement because the number of "other components" is directly related to the number of valves and the function of those valves. Common and easily traced and remedied valve faults include: Identification Perhaps not strictly a fault but a commom problem with valves is the inability to identify them due to the markings on the envelope being invisible. You won't get very far if you can't identify your valves. A common 'cure' is to place the anonymous valve in the fridge for a while, then breathing on it in the hope that the ghost of the markings will show up. If they do, then writing the details on the envelope with a CD marker pen is the usual next step. A quicker and better idea, however, is to shine an LED torch onto the valve. In 99% of cases the markings thus become visible - or at least the imprint of where the markings once were becomes visible. Valve totally dead and not 'warming up'. Assuming none of the faults described below apply, the valve most likely, although not unquestionably, has a failed heater. Substitution will prove or disprove this, but first it's wise to grab your meter and check for voltages at the valve base. To do this you will need to obtain - and understand - valve data. Of course, you can use your meter to check heater continuity by first ascertaining which two pins connect to the heater. Be careful your meter doesn't pump more current into the heater than it's designed to handle though. If you have any uncertainties regarding this and/or any uncertainties about LT and HT supplies you're advised to seek help in the forums. Lost vacuum. A valve (or vacuum tube) consists of a glass envelope, expelled of air, within which the electronics are located. A valve which has lost its vacuum will have a milky on the inside if its envelope. Replacement of the valve is the only solution. Dirty valve pins and/or base sockets. Clean by spraying the base with switchcleaner spray and plugging in/unplugging the valve several times. This MUST be done with the mains supply isolated and do not reconnect the mains until the switchcleaner has dried. Bent or broken valve pins. Assuming the valve to be 'all glass', you can straighten a bent pin be careful bending with your fingernail. This does, however, carry the risk of the glass cracking and the valve thereby losing its vacuum. Purpose designed gadgets for straightening valve pins were made but they're now scarce. Should you fail to remedy the problem, replace the valve. Same for broken pins, replace the valve - unless the broken pin happens to be an unused one. Poor or non-existent connection(s) between valve pin(s) and base sockets(s). Assuming the valve pins to be in good order, this problem is usually the result of the socket(s) for some reason opening up and thus not contacting the pins. You can attempt to close up the affected sockets but they tend to break. If this happens, replace the base. Tracking across valve base. This is the same as the tracking which can occur across the distributor cap of a petrol engine. Sometimes this can be cured simply by thoroughly cleaning the valve base, but sometimes the problem can persist and the easiest solution is to replace the base. Internal short circuit. One or two valves can develop such between the heater and cathode. The most well known valve for this problem is the UL41. Replacement is the only solution although, perhaps fortunately, you're unlikely to find many record players which contain a UL41. Low emission. Put simply, this means a valve is not pumping out what it's supposed to. Like Selenium rectifers, rectifier valves are not immune to the problem either. Replacement is the only solution. Perhaps I should add here that "emission" is a term more often applied to valves other than rectifiers. "Low output" would perhaps be more appropriate for a rectifier valve, but I stuck with "emission" to avoid any confusion with Output valves. In connection with the low emission problem described previously, one point is worth a reminder. This reminder being that a defect with a valve can be, and often is, caused by faulty associated components such as capacitors. This is one reason why old capacitors (and resistors) cannot be ignored. Output valves are especially vulnerable to capacitor problems. Power supplies, progressing onto amplifiers The distinction between the power supply and amplifier if, say, the mains transformer is excluded, can become blurred. For example, some may see a smoothing capacitor and a rectifier as being part of the power supply whereas others may see these components as part of the amplifier. To sidestep this distinction and to assist newcomers in recognising components and their possible faults, this post is component, as opposed to unit, orientated. At this point we are dealing with mains electricity, so if you are unfamiliar with this area, or cannot recognise components and/or understand their function, you are strongly advised to leave well alone and seek advice. Although much of it revolves around radio receivers, reading this and other pages of our main website will help you to understand components and their function. Pay particular, but not exclusive, attention to the capacitor section; illustrations within which will help you identify the different types and also recognise the potential troublemakers. So what, you may be asking, has radio-orientated stuff got to do with record players? A fair amount is the answer; what you'll find inside your run-of-the-mill record player is effectively the power supply as found in a mains powered radio, married to an audio output stage as also found in said radio(s). Indeed, many radio receivers include a facility by which an 'add on' record deck can be connected to - and thus played through - a radio receiver. In such instances the radio receiver is then, for all intents and purposes, serving exactly the same function as a record player amplifier. Therefore, many power unit and amplifier components found in radios will be the same as, or similar to, those found in record players. For the moment, however, we'll stick to mains power supplies. Put simply, this will consist of a mains transformer (if an AC mains only machine) or a dropper resistor (if an AC/DC mains machine), a rectifier (which may be a valve or a Solid State device) and smoothing/reservoir capacitors. In some circumstances the supply for the valve heater(s) may be via a tap off the motor windings. If the motor has three wires connected to it, then this will be the situation. Regarding mains transformers, usually an 'isolating transformer' is used. This type, in simple terms, has two coil windings; one primary and one secondary. The windings are inductively coupled and thus 'isolated' from the mains power source. Beware though, on rare occasions an 'autotransformer' may be used and I've come across at least two record players so fitted (one being an Emerson). Autotransformers are little more than a large wire-wound resistor and do not, therefore, offer the aforementioned advantage of isolation. The mains dropper resistor takes the place of a transformer in AC/DC machines, the latter meaning a machine designed to operate from either Alternating Current or Direct Current mains. There are various reasons for the provision to operate off DC mains but these are beyond the remit of this guide. What you do need to be aware of, however, is that AC/DC equipment (as with equipment using an autotransformer) is potentially more hazardous than AC-only equipment. This is especially so if your record player has a metal chassis (often a PCB is used though). So it will by now be understood that should you need to do any work on your record player's electrics, you need to be armed with knowledge of what in fact you're dealing with - another reason to be familiar with the various components. Otherwise the relevant service data as advised back in post #1 will tell you, as will a little data plate affixed somewhere on the record player (these are not always present though). A vaguely similar power supply arrangement to that of AC machines will be found in mains powered transistorised machines, with the obvious difference that there'll be just a low voltage supply to the amplifier but not necessarily to the motor - so beware. At this point it's convenient to elaborate upon why power supplies and amplifiers are combined in this one post. See the image below. ^ Here's the reason, power supply and amplifier components are mounted upon a common chassis - with the exception of the mains transformer. It is probably, therefore, an idea to seperate the components by explanation. This is a typical arrangement for AC machines although, as mentioned earlier, a PCB (Printed Circuit Board) is often used instead of the far better aluminium, or aluminium alloy, chassis seen here. On AC/DC machines using a dropper resistor, the dropper will likely be mounted on the chassis rather than seperately elsewhere as per the mains transformer. The reason for this will be explained shortly. The image above shows what is, by domestic record player standards, rather a deluxe set-up. There are three valves; a halfwave rectifier (to convert the AC mains to a form of DC), an audio input amplifier and an output valve. That latter is the device which drives, via the output transformer, the loudspeaker. Output transformers may be located on the chassis, as in this case, or on the loudspeaker itself, or on its own somewhere else within the cabinet. If you're a newcomer to this malarky, do not get the mains and output transformers mixed up! Also, do not run the amplifier with the loudspeaker disconnected unless a load such as a test meter is connected to the 'speaker wires. At the bottom left of the image we can see the mains transformer, well away from the chassis. This component is mounted here for two reasons; 1. It facilitates the fitting of different transformers according the which market the machine is intended for, thus allowing the chassis to be of a fairly standard design. 2. Keeping the mains transformer away from amplifier components helps reduce the risk of induced mains hum. This transformer provides LT voltage for the valve heaters (LT = Low Tension) as well as the feed to the rectifier, via a surge limiting resistor, which emerges as the HT (High Tension). Moving to the chassis proper, at its left we can see a pilot lamp which tells us when the power is switched on at the on/off switch. This latter is combined with one of the two potentiometers seen next to the pilot lamp. Next we can see a large cylindrical alloy can. This is the reservoir and smoothing capacitors; the purpose of which is to smooth out the hum caused by alternating current mains. The rectifier alone cannot do this. At the bottom of the chassis can be seen the three valves, one of which is the rectifier (Rectifiers are called 'valves' for convenience. In this case the chassis contains, from the technical viewpoint, two valves and a rectifier. This would commonly be stated on technical data as being "2v + R". Do not confuse "2v" with "2V", the latter meaning "Two Volts"). Behind the chassis and thus out of view in the image lurks a bare minimum of other components, namely two or three capacitors and a couple of resistors. All very simple when compared to, say, a radio receiver. The components described in the above three paragraphs are to do with the power supply and we'll continue with some specific issues in this area. Smoothing 'cans', as they tend to be called, should be treated with respect. Examine them at their solder-tag end; is that end 'pregnant' ie bulging? is there any mould or goo-like substance around the solder-tags (terminals)? If any of these visual faults are present the 'can' must be replaced as it's in a dangerous condition and could literally explode, resulting in at least a vile mess or at worst physical injury to yourself. These 'cans' are another reason you should not apply full mains power to a machine with an unknown history. The failure to smooth out mains hum is another indication of a failed smoothing can, although in some cases the can may just need 'reforming'. Do a search for "Reforming", "Reforming electrolytics" etc. Although it's mentioned elsewhere, it is convenient now to repeat that with many mains powered record players a degree of 'warm' background hum is normal. Get hum thanks to a duff smoothing can and you'll know it - a very loud hum not unlike the sound of muffled, but very rapid, machine gun fire. Another aspect relevant here is 'Ripple Current'; rectifiers leave behind what you could call a 'residue' from the ingoing AC current. To handle this, capacitors have a 'Ripple Current Rating', quoted in mA (milliamps). Determining ripple current is really not something the beginner, so if you find yourself needing to replace some electrolytics it's easiest to select the highest rating for the voltage and capacitance you require. That said, most modern replacements seem to be perfectly adequate regardless so it probably won't be an issue you need to lose sleep over. Finally on the smoothing can subject, another safety issue. When a machine has been powered up, these cans can retain their charge for a while after power has been disconnected. Therefore NEVER touch the terminals of these cans, or any other parts connected to them, for some minutes following power disconnection. A discharge resistor may or may not be present, either way take no chances as belts from these cans are nasty and potentially lethal. ^ A little while ago we talked about mains transformers. With machines designed for AC/DC power supplies (that's AC or DC) you will find not a transformer but a 'Dropper resistor' (simply "dropper" hereinafter). The image on the left shows a typical dropper BUT one removed from a radio receiver. This type is mounted on, and perpendicular to, the chassis/PCB and has connection tags to suit various input (ie mains) voltages. You may find this type of dropper inside an AC/DC record player or you may find a simpler 'fixed input voltage' axial type such as that shown in the image on the right. However, the latter type can be found elsewhere on the chassis for ballasting purposes so don't get confused (Earlier it was mentioned about some more capacitors and resistors being out of sight behind the chassis depicted; one of them is just such a resistor). Droppers/ballast resistors are high power devices and thus run hot, very hot in fact. "High power" refers to their Wattage ie Volts x Amperes = Watts = Power. It is worth adding here that Output and Rectifier valves also run hot and this is normal. Should a dropper fail, and they do fail, you have various options available to you. Should you find yourself in this position, plenty of information exists in our forums and main site. Alternatively and especially if you're unsure of what you're doing, you can create a forum thread to seek advice. ^ In the rectifer department we've so far babbled on mostly about 'valve' rectifiers. Many old record players, and other equipments, will instead make use of a selenium rectifier. A 'Solid State' rectifier in other words. This image shows a typical selenium rectifier which, as it happens, was once installed in a record player (some joke of a low quality 'attache case' effort I believe it was). Other rectifiers (metal oxide) take the form of ganged discs with fins for air cooling, the flat Selenium type as illustrated are affixed to the (metal) chassis which cools by conduction ie the chassis acts as a heatsink. You will therefore not find flat selenium rectifiers PCB mounted. Selenium rectifiers, being solid state, have some advantages over the 'valve' types. Unfortunately they can go wrong - and they do. The symptom of impending failure is low output, meaning the voltage output is lower than what it should be, but if a rectifier has reached this state then it's scrap anyway - impending total failure or not. Total failure cannot be mistaken as very unpleasant odour is emitted - and the fumes are toxic. Do a forum search for "selenium rectifier" and "rotten eggs" and you'll get the idea. Failing or failed selenium rectifiers can easily be dealt with by replacing them with modern, easily obtainable diodes. As aforementioned, searching for "selenium rectifier" will give the details. If your amplifier has poor, or no, output, distorted audio and/or poor bass control (if fitted) one of the things you should check is the rectifier output. Set your meter to a range of at least 250V and check across the rectifier's output. Remember you're dealing with high voltages here, with AC mains in and DC output at, if all is well, a figure (voltage wise) approaching that of the input. If the output is less than 200V, the rectifier is failing. ^ We'll move onto amplifier components now. This image appears earlier in this post but is repeated here to avoid the nuisance of repeated excessive scrolling of a long post. As touched upon earlier, if you take the chassis of a radio receiver and rid it of it's RF and IF stages (look those up if unsure) you're left with just the audio and output stages. In very simple terms, this is what's inside a record player - or at least in the case of the example used in the images here; some really cheap, rubbishy offerings contain little more than an output valve and a few capacitors and resistors. It is impossible to describe here how an amplifier works simply because amplifiers vary. For example, there may or may not be a tone control or there may be seperate bass and treble controls. There might be a single valve or there maybe two or three. The output may be mono or stereo (the latter is somewhat rare where portable domestic record players are concerned, but machines with the facility for an add-on speaker/amp unit for stereo reproduction are quite common). So to simplify things, given too that we have only one amplifier picture to refer to, we'll give just a basic description of how the amplifier depicted functions. Remember that of the three valves seen, only two are part of the actual operation of the amplifier. To begin, we can see a white cable descending to the amplifier. This is the wires from the cartridge, carrying the signal from the record to the amplifier. These wires connect to the volume control which, in this case, also incorporates the on/off switch. Across this is connected the tone control and its attendant 'top cut' capacitor. From here we have the 'input signal'. This signal then passes to the first valve (V1) where it's amplified. This stage is, in effect, a 'preamp'. From here the now-amplified signal passes to the second valve (V2, the output valve) where it is further amplified. The next stage is the 'output transformer'; this can be seen mounted at top right of the chassis. The signal is fed to the primary winding of this transformer, thence to the loudspeaker via the secondary winding. As mentioned earlier, a small few resistors and capacitors also lurk for coupling and other purposes. Capacitors are well documented here and elsewhere but resistors, other than mains droppers (which are wire-wound) are worth a mention. Carbon resistors, especially high value examples, tend to go out of tolerance with age. Specifically they increase in value. This can cause all sorts of apparently strange problems and, as the common-or-garden record player contains only a minimum of components, it's quick and worthwhile to do a check of all resistors. If you can't read resistor colour codes (some have their values actually marked on them) then you'll need the relevant service data, which will tell you values and locations. You can check resistor values in-circuit providing no other components are connected in parallel with them. Once again, service data will give you this information or, if data isn't available, a careful visual examination of the circuit will tell you. This isn't daunting on relatively simple record player circuits (as opposed to the far more complex radio circuits) but be aware that anything connected in parallel with a resistor may not necessarily be located slap bang next to it on the chassis/PCB. If we divert for a moment and go to post #14, take a look at the second image therein which shows part of a transistorised amplifier. At the foreleft of the image can be seen a small low wattage resistor. This is one of those mentioned above which have the value printed upon them, in this case 10K (10,000 Ohms). The main reason for this momentary diversion is to highlight a problem which can occur with this type of resistor; namely the crimped terminals. These can go intermittent or fail completely, the only real cure being to replace the resistor. This type of resistor is common in transistoried equipement - far less so in valved. In fact, off the top of my head, I can't say if they appeared in valved equipment at all. Another not uncommon fault with resistors is a peculiar and otherwise unexplainable, rustling sound at the loudspeaker (Dry solder joints, wiring faults etc usually materialise in the form of louder, harsher, crackles and pops). If you encounter this problem with a resistor, replacement is the cure. The Push-Pull amplifier Generally speaking, the type of record player this guide is concerned with will have a single output valve. Better quality machines may have a pair of output valves of the same type (in transistorised amplifiers a pair of output transistors is virtually the norm, albeit not necessarily of the same type; one PNP and one NPN for example). Sticking with the Push-Pull valved amplifier, the setup is rather more complex (and one reason we're not detailing stereophonic machines in any great detail). The signal, preamplified, goes through a 'Phase Splitter' circuit wherein the signal is split into phases; imagine an AC waveform with one side + and one side -, this means the two phases are then 108deg apart or, if you like, polarised. To provide superior performance (compared to a single valve [single ended] output stage), each phase is seperately amplified and hence the reason for the twin output devices ("Device" is a common term for a valve or a transistor in the context of a circuit). That's the nutshell description, the reality being rather more complex in that output devices must be balanced (matched, hence the term "Matched Pair" [commonly applied to transistors] which you might have heard of) and biasing circuits must be precisely set. To learn more, do a web search for "Phase Splitter" and "Push-Pull output stage". Don't worry if most entries concern guitar amplifiers as the principle is broadly the same. Brief summary Record player amplifiers do not, largely due to their relative simplicity, give too much trouble. When faults do appear, they can be out of tolerance resistors, leaking capacitors ("Leaking" means passing DC), dirty valve pins and sockets, dirty or otherwise faulty controls, 'dry' soldered joints and/or cracked tracks if a PCB type chassis is used. Valves, contrary to popular myth, don't give too much trouble beyond the aforementioned dirty (or bent/broken) pins and sockets. There are, however, a small few exceptions but most of these are seldom found in record players. Finally for this post, the keener eyed among you may have noticed the wire clips securing the valves in position in the above image. These should be handled with care; they're tough and can break the pip on the top of the valve, if not smash the glass envelope completely. Should this occur the valve is, obviously, scrap.