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Old 9th Nov 2011, 1:34 am   #14
Darren-UK
Retired Dormant Member
 
Join Date: Apr 2003
Location: Blackpool, Lancashire, UK.
Posts: 4,061
Default Transistorised record players.

This post effectively and conveniently follows on from #13 above.

Valved record players and, as an aside, televisions, survived in production rather longer than valved radios broadly speaking. Transistorised record players (or, more correctly, record players with transistorised amplifiers) became commonplace from the mid 1960's although the earliest examples started to appear rather earlier.

As this post conveniently follows on from the previous post concerning mains/battery machines, we're concerning ourselves only with transistorised amplifiers here.

As with valved record player amplifiers, the transistorised amp' will consist of what, in a radio, would be the Audio and Output stages together with the associated controls and other gubbins such as resistors and capacitors. Other than valves being replaced by germanium or silicon devices, the essential differences between valved and transistorised amplifiers are twofold in respect of the latter:

a) All components run cold (or should do, see later).

b) Voltages are low, typically but not necessarily exclusively, 9V.

Safety warning. On point b) above, remember that if your transistorised amplifier is mains powered, low voltages are only present after the mains PSU (Power Supply unit).

Remember, too, that although the amplifier is low voltage, if the machine is mains powered the deck motor will still be powered directly from the mains supply.

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^ The above image shows the essentials of a transistoried amplifier. As this guide is aimed at the newcomer to vintage record players, in this post we're concentrating only of the fundamentals and areas most likely to give problems.

The image actually shows part of a Roberts radio chassis as this was convenient to photograph. Don't be put off though, for what you see in the image is little different to what you'd see on the amplifier of a transistorised record player. Note that this is a 'transformerless' output stage; an arrangement you may come across in a record player.

Looking at the numbered items on the image, 1 is the audio driver transistor which in this case is an OC75. This is, in a nutshell, a preamp and if on a record player this would be where the signal from the cartridge would enter the amplifier - having first faffed about around the volume and tone (if fitted) controls en route.

At 2 we can see the brass heatsink which, in turn, is clamped to 3, the chassis frame. The heatsink is holding the output transistors, in this case an AC187 and an AC188. Put simply, these transistors are what actually drives the loudspeaker. The heatsink is not there to keep the transistors nice and tidy, it's there to conduct away heat. Like output valves, output transistors run hot - or rather they would do if the heatsink wasn't present.

Even, however, with the heatsink in place and holding the transistors things can - and do - go wrong. To enable the amplifier to do its job properly and for the output transistors to run at a temperature the heatsink can cope with, the biasing must be set accurately. At 4 we can see an adjustable bias resistor; there's actually a pair of these, the second being out of sight in the image, and they're preset at the factory. You must not twiddle or otherwise interfere with these resistors or damage may result to fixed resistors as well as the output transistors. If for any reason the biasing does need a reset, you will need a milliammeter and the service data relevant to your record player in order to deal with it. On some amplifiers even the smallest error in biasing can cause a (fixed) resistor to overheat and for some mysterious reason this seems more common on RBM (Rank Bush Murphy) amplifiers than others. Don't be complacent though, correct biasing is important on any amplifier regardless of brand.

The adjustable biasing resistors are merely circular carbon tracks with a wiper contact, not too unlike the basics of a volume control. They can, therefore, over time, develop poor contact mainly through a build up of muck. If cleaning is thought to be necessary remember, before attempting anything, that you will need a milliammeter and relevant service data to reset them correctly.

Heatsinks and biasing aside, output transistors can develop other faults. On rare occasions a short circuit can occur between one, or both, transistors and the heatsink. When this happens, the amplifier will go totally dead. Sometimes you can trace the fault by removing each transistor in turn from the heatsink, then switching on to see if the amplifier has come back to life. Only do this for a couple of seconds though, just enough time to conduct the test, and do not leave power applied whilst you fiddle with the heatsink.

If this fails, you will next need to grab your meter and service data and then start checking back through the amplifier. If unsure, you're advised to create a forum thread for further assistance. This is not least because amplifier designs can and do vary widely; for example there may or may not be transformers present.

Otherwise, if you're certain the fault lies with the output transistors and the 'removal from heatsink' check fails then it's most probably time to start replacing transistors and "matched pair" then very likely comes into the equation. You are advised to seek additional information on this if you're unsure.

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^ For comparison with the previous image, this one shows a transformer coupled output stage - or rather the essential components of one. The transformer at the rear is the 'driver' transformer (effectively a premap transformer) whilst that at the front is the output transformer. Note how the output transistor heatsinks have utilised the frame of the transformer. This is because in this case there's no metal chassis frame for them to be attached to and thus aid the dissipation of heat. Cheapness in construction, in other words.

This particular arrangement is more usually found in transistorised radio receivers, but it's relevant here as this unit was removed from a Perdiogram 'record player with radio'. More on these in a later post. As well as the possibility of faults and failures with output transistors, the above type of output transformer can be prone to failure. Usually the failure is in the primary (ie input) winding and tends to be more common with mains/battery machines.

So if you have nothing at the loudspeaker and you've eliminated o/p transistor and other possible faults, suspect the transformer. A quick check with a meter will soon confirm; with your meter set on a resistance range, touching the probes on the primary terminals will give a 'click' at the loudspeaker, for example, if both transformer windings are OK. If no 'click' then you'll need to do a continuity check, same method, on both windings but suspect the primary first. If no meter, you can do the 'click' test using a small (no more than 9V) battery and a couple of bits of wire.

The above also applies to transformers of mains (only) machines. In either instance power (to the amplifier) must be off and the position of the volume control is irrelevant. One handy point concerning output transformers is that they're easy to replace; anything of as near as possible the same physical size and which will fit the chassis/PCB or loudspeaker frame in the context of mounting points and wiring/soldering points will usually do the job.

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^ This image, courtesy of Mike Phelan who clambered into his loft to take it, shows a general side-on view of the amplifier of a Bush SRP51 record player. Although not clear (no offence Mike!) it shows the output transistors (the 'output pair'). You can't see them? Well, they're hiding in the gap below the escutcheon beneath the three controls and the chassis itself. These transistors are a matched pair AD161/AD162 and in appearance are unlike most other transistors. The image below of an AD161 will illustrate this.

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^ This image shows extracts from the Bush SRP41 service data; note that's SRP41 and not SRP51 (I have original Bush SRP41 data but not that for the SRP51, hence the cack-handedness).

First looking at the plan on the bottom of the image, the locations of the output transistors are shown - as are the three controls; RV1, RV2 and RV3. This should help clarify the haziness of the earlier SRP51 image, the two machines being practically identical in this respect.

One importance difference, it will be seen, is the use of two identical transistors on the SRP41; both being AD162 types.

Another point worthy of note is that some of these machines have a pilot lamp which also acts as an internal fuse. In other cases, usually on later machines, a 'proper' fuse of the glass type was fitted.

Looking at the top part of the image we see the circuit at the output stage end. We can see the pair of AD162's and the loudspeaker, marked LS1. At this point it's appropriate to mention that SRP stands for Stereophonic Record Player. So, if these machines are stereo, why is there only one loudspeaker? Because for stereo reproduction a matching, add-on, amplifier and loudspeaker unit is required. That for the SRP41 was the AU41 and for the SRP51, the AU51. If you understand circuit diagrams you will see that the output stage is another transformerless example, as mentioned earlier.

We will touch upon stereo machines in a later post, but the Bush SRP types have been deliberately chosen for discussion here because they're a) very common, b) very popular and c) damn good machines - far more so than many (but not all, see a later post) of the rubbish Dansette offerings which people rave over for nostalgia reasons.

The AF117 transistor should be mentioned now. These can appear in, obviously, transistorised record players and especially in the already mentioned 'record player with radio' types where a standard transistor radio chassis was utilised. The AF117 topic and, indeed, any transistor in the AF11x series, has been flogged to death many times in our forums so we're only devoting this one paragraph to it here. To find out all about it see, to give just one example, this thread.

On our main site can be found further information on transistors including testing. However, with regard to AF11x types, testing isn't the usual recourse as experience will teach you that faulty examples are quite readily apparent.
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