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Old 1st Dec 2015, 6:26 pm   #81
Herald1360
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Default Re: Test equipment for valve radio repair

Some of the not so recent hand held 'scopes like the Tektronix THS 7xx series are particularly interesting for one capability- the two inputs are both isolated from each other and the DC PSU which makes the 'scope very handy for some jobs. Live chassis electronics is one of them.

They're getting down to affordable-ish now- not much more and than a new Fluke DMM. Battery packs are expensive and difficult to find, but at least they use standard size NiCd cells.

The similar vintage Fluke scopemeters have the isolation too, but I find their lack of low frequency rolling display annoying.
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Old 1st Dec 2015, 9:04 pm   #82
Phil G4SPZ
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Default Re: Test equipment for valve radio repair

I like test equipment - restoring it, using it and just owning it - although I have used my scope only once in the past year. The humble Avometer and a decent signal generator have sufficed for the overwhelming majority of fault-finding and alignment for the past 50 years on all forms of analogue equipment. But I do confess to using the valve tester pretty frequently.
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Old 1st Dec 2015, 9:33 pm   #83
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Default Re: Test equipment for valve radio repair

One thing which often catches out 'newbies' using DMMs is that these have such a high input-impedance (tens/hundreds of megohms per Volt) that they can often 'see' a voltage which isn't really there. Someone I know spent ages failing to identify an open-circuit IF transformer primary because they 'insisted' they were seeing +200-odd volts of HT on the anode-pin of the IF stage (with the valve removed) - what they were seeing was the microamp or so of leakage current through the IFT's tuning-capacitor and across the 60-year-old IFT's insulation.

A 'classic' 20KOhm/volt meter like an AVO8 or a Taylor would not have showed any significant voltage.


The other essential tool I use for valve-equipment repair is a small hammer. Tapping the chassis is a good way to reveal intermittent joints (particularly earth connections), microphonic valves etc.

My current hammer is one of the rubber-faced kind neurologists use to tap your knee to test your reflexes; in the past I used a customer-supplied "Blue Bird toffee" branded hammer to track down a dry joint in the tuner of a 1960s Baird TV; this both impressed and confused the customer....
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Old 1st Dec 2015, 10:32 pm   #84
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Default Re: Test equipment for valve radio repair

Mmmmm.Typical DMM is 10M input resistance. AVO8 on 500Vdc range is also 10M input resistance. Tricky call.

A serious bench DMM like the HP 34401A does have a hi z input (Gohms) selectable on its lower voltage ranges. Useful for measuring voltages across hundreds of Kohm resistors.
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Old 1st Dec 2015, 10:52 pm   #85
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Default Re: Test equipment for valve radio repair

One of the many rows I had with the idiot physics teachers at school started when I pointed out that a typical digital multimeter on the 1kV range took twice the current of a typical analogue multimeter on the 1kV range (the former having an input resistance of 10M, the latter, being 20000 ohms/volt having an input resistance of 20M). Said teacher believed that digital multimeters 'took no current'.

Mind you the same teacher claimed that an oscilloscope input was direct to the Y plates and thus had an infinite impedance. Well, a few special purpose 'scopes have no Y amplifier and direct input to the plates but most don't! And even if you do go straight to the plates the impedance is far from infinite (there is the capacitance between said plates for a start).
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Old 1st Dec 2015, 11:04 pm   #86
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Default Re: Test equipment for valve radio repair

Quote:
Originally Posted by Herald1360 View Post
Mmmmm.Typical DMM is 10M input resistance. AVO8 on 500Vdc range is also 10M input resistance. Tricky call.

A serious bench DMM like the HP 34401A does have a hi z input (Gohms) selectable on its lower voltage ranges. Useful for measuring voltages across hundreds of Kohm resistors.
I never really gave that any thought. So a reading taken across a 1M resistor will be 10% out?
I always took Moving coil measurements with the effect of the shunt resistance across the DUT but have to admit never thought that much about the DMM.
Guess that means the voltages on service sheets taken with AVO8's on the 500V range would tie in pretty well with those taken on a DMM there or thereabout?

Andy.
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Old 1st Dec 2015, 11:49 pm   #87
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Default Re: Test equipment for valve radio repair

Quote:
Originally Posted by G6Tanuki View Post
Someone I know spent ages failing to identify an open-circuit IF transformer primary because they 'insisted' they were seeing +200-odd volts of HT on the anode-pin of the IF stage (with the valve removed)... A 'classic' 20KOhm/volt meter like an AVO8 or a Taylor would not have showed any significant voltage.
I remember a similar situation in an industrial electrical circuit where there was enough leakage through a blown fuse to read 415 volts on an Avo 8, yet the traditional test lamp stayed dark.

The Avo 8 was designed for the emerging solid-state era to enable low voltages in transistor circuits to be measured with minimal loading, and it's probably not the best meter to use with valve equipment, for which the Avo 7 still takes some beating!
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Old 2nd Dec 2015, 9:43 am   #88
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Default Re: Test equipment for valve radio repair

Many years ago we were working on power supplies with a very high internal resistance. At that time AVOs were the general test meter but a valve voltmeter had been produced with an input impedance that was so high we were assured it was almost infinite. A colleague using this was quite surprised to see very wide variations in his voltage measurements even though he made no changes to his set up.

It turned out that a visitor had come in and was walking about the room. He was wearing a trendy nylon or similar suit and generating an electric field, our meter was showing the variations of this. I have not seen the effect since, even though DMMs are claimed to have very high input impedance. It does though make me think when using one, especially when checking circuit voltages against data sheets produced in the 1930s
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Old 2nd Dec 2015, 12:38 pm   #89
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Default Re: Test equipment for valve radio repair

Valve voltmeters have a much higher input impedance than most normal DMMs. Mine gets a fair bit of use in high impedance circuits. I can't remember the exact number I came up with when I tested mine but the impedance was certainly more than the >200Megs that I needed and that on the 1V range.
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Old 2nd Dec 2015, 2:21 pm   #90
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Default Re: Test equipment for valve radio repair

Valve voltmeters such as the Heathkit V7A have an input resistance of 11meg ohms, 10meg in the input attenuator chain and another 1meg in the probe tip to minimise capacitive loading in the circuit under test.

Electrometers, where the input goes directly to the grid of the first valve, can have a much higher input impedance, but I guess they can't handle high voltges

S
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Old 2nd Dec 2015, 3:28 pm   #91
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Default Re: Test equipment for valve radio repair

Many valve voltmeters have resistive range attenuators before the first valve, and so have finite loading resistance on the circuit being tested.

The same goes for transistorised meters.

With modern DMVs there is usually a need to arrange the input range attenuation to allow autorange switching with the electronic switchhes at conveniently low voltages. This usually means having the attenuators designed with constant input resistance, not maximum input resistance.

This is a design style thing, not a valve versus semiconductor thing.

Oh, there is a technique using matched varactor diodes measured with oscillators that allows voltage measurement with only the DC leakave of a reverse biased diode and a couple of capacitors. Solid state electrometers can be made this way.

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Old 2nd Dec 2015, 8:56 pm   #92
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Default Re: Test equipment for valve radio repair

This thread is now some eight years old and the original poster hasn’t been on the forum for more than two years, so inevitably the thread has drifted a bit here and there, but has proved interesting and informative, with lots of useful input.

The original query was about recommendations for equipment to enable a novice to progress further in the hobby. Tek scopes and Fluke meters and other big name complex equipment favoured by more experience restorers are rather wide of the mark when it comes to the basics. One thing that I don't think has been mentioned - which I invariably use to speedily pinpoint faults - is a signal injector/tracer. Circuits for these abound, but there’s the excellent Velleman K7000 kit which can be put together in an hour or so, and just needs a speaker, a project box and a few other bits. (You can actually use the plastic box in which the kit comes):

http://www.velleman.co.uk/contents/en-uk/p261.html

I’ve attached a couple of pics of one I that knocked up, showing the PCB and the finished injector/tracer. I use it a lot and find it invaluable. The least useful items for restoring valve radios, which I use little are a signal generator and scope. Occasionally I’ve had sets that have been tampered with and have needed re-alignment, but generally, I’ve found that with most sets, it’s just needlessly 'going through the motions' and the set isn’t off tune. I also have a wobbulator, which I’ve yet to resort to and I think it falls outside the brief of equipment for aspiring novices.

I think too, that along with equipment goes diagnostic techniques. Many tests can safely be carried out with the set switched off and disconnected from the mains. There are visual checks for such things as perished wiring, signs of scorched components. The continuity tests of valve heaters, dial bulbs, fuses, coil and transformer windings, continuity of ‘switchery’ and so on. Then resistance checks to see if any resistors are way out of tolerance.

The other useful and cheap item that’s become widely available this last couple of years are the digital multi-testers which cover resistance, inductance, capacitance (including ESR), as well as transistor and diode tests. Typically no more than a tenner on ebay. The fourth pic below shows a typical digital display of one that I have - in this instance a capacitor under test.

AVOs keep cropping up as the instrument of choice, which is fine for work on valved equipment with higher voltages, and was invariably what is quoted on maker’s data sheets. But Avo 7s and 8s or similar low input sensitivity meters when measuring low voltages across potential dividers will not produce accurate results and I'm sure most on the forum will know that anyway, but novices may not. True, the meters will accurately measure the actual voltage that they see, but that voltage will be lower than we'd expect to see because the Avo will act like a resistor in parallel with the circuit under test. EG if we have a potential divider using two 100k resistors across a 12 Volt supply, we know from Ohm's Law that the calculated voltage at the junction of the two resistors will be 6 Volts.

An AVO 8 (or any 20,000 Ohms Per Volt meter) on the 10V range will be like plonking a 200K resistor in parallel with the 100K one in circuit, so will produce a reading of 4.8V. However, an FET or Valve Voltmeter (such as the Heathkit V7A which has been mentioned), with an input impedance of 11 Meg Ohm will show 5.97V, close to what we'd expect to see.

Worse still if we apply an AVO 7 (or any 1,000 OPV meter) on the 10 Volt range across the junction as it will be like putting a 10K resistor across the 100K one, so the reading will be 1 Volt - the actual voltage due to the loading of the AVO. Not so important when taking readings of higher Voltages, and there is the merit of using the same meter that was used when measurements were taken on valve radios by the manufacturers and shown on service sheets, so I do understand why many restorers like to use AVOs on old radios and TVs, but the lower the voltage being measured, the more significant this loading effect becomes, and given that this thread was about equipment for novice restorers, I'm not sure in many inexperienced restorers appreciate these limitations - I certainly didn't at the outset. If I need to resort to an analogue meter, I use a homebrew one which featured in PW more years ago than I care to remember.

I'm not knocking AVOs - just saying that users need to be aware of their limitations - as with any 20,000/1,000 OPV meter.

Just my thoughts as a hobbyist restorer and home-brewer since the 60s, with no technical background, more at home with simple home-brew test gear than complex ‘big name’ equipment which I don’t have need of, wouldn’t know how to use, nor would wish to. I'm well enough off to be able to buy equipment which would go far beyond my simple needs - a digital storage scope for example, but why would I want to when it would serve no useful purpose and has no relevance to basic equipment for novices, which I still consider myself to be, albeit of many years standing? In a practical hobby such as ours, to my mind it makes sense, wherever possible, to make my own simple homebrew test gear.
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Old 2nd Dec 2015, 9:12 pm   #93
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Default Re: Test equipment for valve radio repair

I will certainly agree with the last paragraph of David's post. While it can be fun to have high-end test gear (and while collecting and restoring it can be a hobby in itself) it is very often possible to trace faults with very simple test gear, provided you know how to use it properly and interpret what it is telling you.

As a simple example I have an old Tektronix 575 transistor curve tracer. It's a great instrument, it's got the legendary Tektronix build, and so on. And it works well. But I don't think I have ever needed it for faultfinding. Juat about always a faulty transistor has at least one junction open or shorted, that you can pick up with the diode test range of a digital multimeter, an analogue ohmeter, or by seeing crazy voltages in the circuit under test.

I think it is worth remembering just what test equipment is for (in my opinion). What faultfinding _should_ involve (and how I fix classic computers, my main interest) is :

1) Discover what the device on the bench is actually doing. Now the human body can't directly detect and measure electrical signals, it can't determine the timing between pulses, and so on. So you use test gear to show you the signals, or to introduce known signals that you can trace through the circuit, etc

2) Work out what the device should be doing. Read the schematics (and in my case microcode listings), understand them. Look at the signals and voltages given in the service manual.

3) Compare them.

4) When you find something different enough to matter then work out what could be causing that. Do more measurements if necessary, test components, and so on.

IMHO the most important piece of diagnostic equipment is a brain. All the test gear in the world won't help if you don't understand what you are doing with it. Conversely I suspect many here (probably even me) could fix most radios with nothing more than a multimeter (any multimeter, provided you know its characteristics and thus how it will affect the circuit).
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Old 2nd Dec 2015, 11:35 pm   #94
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Default Re: Test equipment for valve radio repair

Quote:
Originally Posted by David G4EBT View Post
...Worse still if we apply an AVO 7 (or any 1,000 OPV meter)...
Don't forget that the Avo 7 is actually 500 OPV until the divide-by-2 button is pressed! The other thing to remember is that the Avometer's scale and movement can be read accurately right down to a couple of percent above zero. Pre-1936 service sheets often refer to voltages being measured with a Universal Avometer on the 1,000 volt range - the Avometer of that era was no better than 167 OPV, but the loading of 167 kilohms on the 1kV range was still light enough to permit meaningful readings of just 20 or 30 volts to be taken and compared with expected results. The 'sensitive' Avo 7 from 1936 onwards must have seemed a real relevation to the service engineers of the day! For an excellent treatment of meter loading (and a lot more) the PW Publication 'Are The Voltages Correct?' is a good source of guidance for the beginner. PW did tell me that they were planning to re-publish this, but I'm not sure if that's happened.

That's a very impressive signal injector/tracer, David. I built one many years ago, nothing like as good as yours of course, but I found I never had cause to use it. For audio, a finger or a screwdriver blade (applied with due care) often injects enough audio to check whether a stage is working, and an old battery-powered PC speaker serves as a workable signal tracer. For RF, I find that a reliable source of modulated RF at a known frequency is almost indispensable, although like many contributors I certainly don't need to use one on a daily basis.

Phil
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Old 3rd Dec 2015, 12:27 am   #95
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Default Re: Test equipment for valve radio repair

In my teens I acquired an avominor which was used for many tests for years (including motorcycle electrics. When I began playing more with electronics, I bought a Grundig Valve voltmeter. It had no case, just the top section. I made a box it would drop into. Very impressive. Shortly after I picked up a Phillips (or Mullard) DC valve voltmeter. It was a non-runner, and I diagnosed it as having all valves with O/C filaments. They were all 1.4v B8G (??) types (EL41?? etc). The input was chopped by a well sealed up aluminium vibrator. Quite different physically to the type used in car radio PSUs. I vaguely recall that its input resistance was supposed to be in the 100's of megohms. I suppose the vibrator was similar to the Honeywell used in some of our potentiometer temperature instruments which required accurate results of around 10mV to a couple of decimals.(almost 50 years ago, so I forget exact details.
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Old 1st May 2016, 3:25 pm   #96
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Default Re: Test equipment for valve radio repair

I use an Avo Model 8 and a CT160 valve tester. New here, hello everyone
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