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Old 13th Nov 2007, 10:45 am   #1
Vespaman
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Default Test equipment for valve radio repair

I need some advice from you experts out there. I have a small collection of valve radios most of which work although a few are a little 'sick'. My cornucopia of repair equipment is very basic (soldering iron, keen eyes, simple meter for volts,amps, continuity etc.). I need to move on and be more scientific to check caps, resistors, valve voltages and the like. What equipment should I obtain; where can I get it and will it cost me an arm and a leg? And don't worry folks I know of all the dangers of working with steam radios!

Richard
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Old 13th Nov 2007, 10:56 am   #2
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Default Re: Test equipment for valve radio repair

You've probably got enough there to make a start. Resistors and coils can normally be checked with a simple meter, capacitors are best checked by replacement, valves can be evaluated by measuring the voltages around them or by substitution.

A decent RF/IF signal generator would be the next thing to consider, then you can start with alignment (though this is not always necessary if the sets have not been "got at"). A mains isolation transformer would be advisable to go with this.

A 'scope or a valve tester are a luxuries for domestic radio repair and you don't really need them to begin with.
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Old 13th Nov 2007, 10:57 am   #3
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Default Re: Test equipment for valve radio repair

Just about any fault on a radio can be diagnosed using a multimeter, either analogue or digital as preferred. DMMs can be bought exceedingly cheaply these days.

As for testing capacitors just forget it. Replace waxies and Hunts on sight. If you don't want to do that remember that the failure mode of these is to leak electrically and behave like resistors. This will result in incorrect voltage readings. For example low screen grid voltage or positive voltage on a control grid.

If you want more test gear invest in a signal generator. As for valve testers, scopes etc. forget them. They rarely need to be used.
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Old 13th Nov 2007, 10:58 am   #4
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Default Re: Test equipment for valve radio repair

Studio263 and I posted at the same time and came up with the same answers!
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Old 13th Nov 2007, 11:03 am   #5
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Default Re: Test equipment for valve radio repair

A cheap DMM is really all you need. This will measure voltage, current and resistance. You can't test valve radio capacitors easily even with sophisticated capacitance meters because of their typical failure modes, so the best method of finding bad ones is to look for voltages where there shouldn't be any with the DMM.

An RF signal generator is useful for AM alignment, but you can do just as good a job with care using off-air signals. A wobbulator and scope is really needed for accurate FM alignment but it's possible to check the ratio discriminator alignment with just the DMM - many professionals wouldn't have owned a wobbulator back in the 50s, and that's how they would have done it.

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Old 13th Nov 2007, 11:34 am   #6
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Default Re: Test equipment for valve radio repair

If you get to aligning sets, then invest in an analogue multimeter (a second-hand 'AVO' 8 is ideal). Observing peaks, dips, and rate-of-change is far more easily undertaken with a pointer than a digital display.

Some circuits specify the voltages with regard to a particular meter loading (eg: the cct diagram may say: 'all voltages maesured using 20,000 Ohms/Volt meter'). Using a DMM is perfectly acceptable providing you accept that a reading may be higher than specified.
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Old 13th Nov 2007, 1:55 pm   #7
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Default Re: Test equipment for valve radio repair

Hi Richard,

Whilst I agree with the other contributors about 'scopes and valve testers being luxuries, you will find that a 'scope will give you much greater insight into what is going on, and unlike valve testers, 'scopes are dirt cheap, especially as you don't need anything esoteric by way of bandwidth for most radio use.

Any mains driven test gear will of course demand greater respect in regard to connections made to radios that don't have isolating mains transformers.

Good luck,

Peter
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Old 13th Nov 2007, 4:05 pm   #8
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Default Re: Test equipment for valve radio repair

Quote:
Originally Posted by Station X View Post
If you want more test gear invest in a signal generator. As for valve testers, scopes etc. forget them. They rarely need to be used.
Hmmm... but the OP was about 'getting more scientific.' Arguably, a 'scope is an inevitable essential if you want to do this.
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Old 13th Nov 2007, 5:06 pm   #9
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Default Re: Test equipment for valve radio repair

Quote:
I need to move on and be more scientific to check caps, resistors, valve voltages and the like.
That screams out multimeter to me. Why use a sledge hammer to crack a nut?
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Old 13th Nov 2007, 5:12 pm   #10
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Default Re: Test equipment for valve radio repair

I forked out on a valve tester fairly early on, its been quitr fun using it but I wouldn't bother. I'd agree most work can be done well with just a meter (DMM or analogue) but if you're feeling your way there's nothing like seeing a trace on the 'scope. Mines a 90quid crappy one from Maplins. Dual trace isn't hugely useful except for telly work.

Isolating transformer and variac have proved very useful as has a decent soldering iron I've been poking away with a 15W antex for years and not really understood just how much nicer a proper heft voltage controlled one is - they're not that expensive either.

I picked up a nice sig gen for a tenner at Harpenden and that's been useful, both in diagnosing and aligning but also in aiding my understanding of various things. Also its got nixie tube read out of the frequency which is cool!...
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Old 13th Nov 2007, 5:14 pm   #11
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Default Re: Test equipment for valve radio repair

My three ha'porth,
1) An AVO is more use than a DVM as the voltages in the service data that we are generally working to will have been determined using analogue meters, often a mk7 or 8 avo and so will be more meaningful on similar kit.

2) It is actually useful to have a couple of AVOs as you can check conditions simultaneously around a complicated circuit - as they are dirt cheap, there is no problem with having several.

3) A reasonably accurate RF signal generator. It doesn't have to be an all singing all dancing one but something that will give you the common IF frequencies and the MW/LW bands with AM modulation will do all of the older sets.

The above will do nearly all AM domestic sets.

If you are going to set up more sophisticated sets, especially communication sets (AR88 etc) then a good wobbulator and a scope is pretty essential (anything where you want to accurately set up filters where the skirt shape is important). Much as scopes are nice toys and I have to admit that I have far more than I should, unless you are going to be doing this sort of stuff then they are not really necessary.

Once you go beyond this then it just depends how many toys your budget can extend to. I have a valve tester and it is immensely helpful. I also have a very good Marconi comms set analyser which is way way beyond the sort of gear that is really necessary but it does make some things very easy. Other useful items are valve volt meters when you can find them. I also have a megger which gets used for testing the mains side for insulation.

The thing to remember is that test gear comes under the heading of nice to haves and I can just do X better if I have a thingy and it only costs xxxx I'm sure I can find a home for it.

All of us can do most of what we do with 1, 2 and 3 above. It is rare that I actually break out the full tool set and only when I want to do something insane like setting up a complete RA17.

Robin
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Old 13th Nov 2007, 5:21 pm   #12
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Default Re: Test equipment for valve radio repair

If I'm ever forced to get rid of test gear the last four items I'd get rid of would be Valve tester, 'scope, Signal Generator and Multimeter in that order. I probably got them in the reverse order.
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Old 13th Nov 2007, 5:48 pm   #13
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Default Re: Test equipment for valve radio repair

Quote:
Originally Posted by dominicbeesley View Post
I picked up a nice sig gen for a tenner at Harpenden and that's been useful, both in diagnosing and aligning but also in aiding my understanding of various things. Also its got nixie tube read out of the frequency which is cool!
I seem to remember that there used to be a pen-type probe device on the market that contained a square-wave audio generator rich in harmonics. it could be applied to any stage in an RF/AF circuit to obtain an output (of sorts) if the stage was operating correctly. I haven't seen such-like for years, but it wouldn't be difficult to make one.

A 100W incandescent lamp is a useful tool, connected - initially - in series with your device under test, and will limit current nicely if you haven't got a variac.

A 'Megger' (available on the surplus market) is also useful for testing transformers and insulation resistance, and will give an indication of leakage when applied to a capacitor of appropriately rated voltage.
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Old 13th Nov 2007, 6:13 pm   #14
Brian R Pateman
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Default Re: Test equipment for valve radio repair

It all depends what you want to do and how far you want to go!

If Richard has been bitten by the electronics bug I'd recommend a good multimeter, I'd go for the AVO 8 but that is my bias. The majority of valve receiver manuals will have the voltages measured on a sample set using an AVO so you can expect to get reasonably close using one. They are also excellent for the beginner as they are fairly robust and have the trip button for the times when you forget to change range.

I would advise building your own test gear too. A simple signal injector (valve or transistor) is a simple project and building test gear is a good way to learn how circuits work.

There are lots of constructional articles in the old magazines like Radio Constructor or Practical Wireless. PM me and I'll send you some suitable articles.

You can certainly have a great deal of fun and learn a lot with simple equipment.

Regards,
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Old 13th Nov 2007, 7:40 pm   #15
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Default Re: Test equipment for valve radio repair

Looking back to when I just had a DMM, I would say go for an analogue meter for sure. AVO's can be had at resonable prices.

As for other items which I now wonder how I did without :
SIG GEN - GET ONE ! I really wonder how I managed before, especially when a nice FM set arrives and you need to align.
PEAK ATLAS LCR meter - Not expensive and VERY useful when you have dodgy mica caps to contend with ( thanks Ferranti ! )
Hi voltage PSU - Dead useful for reforming caps, powering up AC sets without the rectifier in, and other valve experiments. Bit of 'nice to have' but if it saves a few hours, it goes in the workshop !

Not strictly TEST GEAR , but...
Extra Hand Tools : These make any job easier and are worth their weight in gold !

One handed automatic strippers.
Bench magnifier/light
Nut holder
BA sockets/nutspinners
Screw starter
set of needle files

If you have to pick just one ( sounding like desert island discs !)
Go for the SIG GEN. You'll really wonder how you managed without ....
Andy
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Old 13th Nov 2007, 8:01 pm   #16
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Default Re: Test equipment for valve radio repair

As others have said, you need very little to carry out most fault diagnosis and repair. However my experience has been that in setting myself up for this type of work I have developed an interest in collecting the test equipment itself. Professional laboratory instruments, by makers such as Marconi, HP, Avo and Tektronix, that cost many hundreds of pounds thirty or more years ago can now be picked up for a song. There is also much cheaper but good equipment around such as that by Heathkit which will still provide good service. In the case of Heathkit, because it was designed for home assembly, it is particularly easy to maintain. Such equipment turns up on auction sites or at the BVWS events such as Harpenden. It may even be offered on this site from time to time.

I admit that the equipment is usually far more highly specified than is required for our purposes but there is satisfaction in using such beautifully made equipment and keeping it working long after the end of its intended life.

Edward
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Old 13th Nov 2007, 8:02 pm   #17
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Default Re: Test equipment for valve radio repair

Here's my brief suggestions: Build yourself a safety series lamp for testing sets, if you haven't already done so. IMHO the test lamp is superior to the much vaunted variac and it is certainly much cheaper. Ideally, both a good analogue meter - say, a model 8 Avo - and a DMM both have their uses. The former is good for watching changes in voltage or current, such as when aligning for signal strength (the reading on the DMM tends to jump about) but the DMM is quick and accurate and some can test capacitors - I use a capacitor test on new components before I fit them and use the Avo on high ohms to check old caps for leakage. A reasonable sig gen is very handy, too, but I use my scope only occasionally for radio work, though it was used a lot on colour TV servicing (years back!)
-Tony
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Old 13th Nov 2007, 9:32 pm   #18
Andy Day
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Default Re: Test equipment for valve radio repair

Having come from a digital electronics background - way back fixing and modding ttl circuits, latterly switch mode supplies, monitors and laptops, I started driven by the need to search for a measurable fault on radios I worked on. In my naive fashion I assumed there would be "a fault" which when fixed would make the set burst into life. Poor fool!

The reality is that many components will have become leaky, changed in value or indeed in antiquity have been replaced with something unsuitable!

With this in mind, and as others have suggested, a blunderbuss approach is often more successful, and a simple dmm is the most useful tool in this, but NOT with caps! Firstly, mistrust all of these, especially Hunts and waxed paper, also most electrolytics, though you may be lucky with these. Then resistors, in many sets you will measure these and find the values all over the place, usually high. In spite of this they may not materially alter the operation of the set due to the tolerant operating parameters of hot bottles. If in doubt don't try to intellectualise the problem, change the component!

I've got all the kit apart from an FM sig genny. Mostly by a huge margin I use my 13 Dmm and AVO 8 I've had 37 years, but most useful is the Soldering Iron!
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Old 14th Nov 2007, 10:12 am   #19
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Default Re: Test equipment for valve radio repair

What a response! Many thanks to all you electronic geniuses out there; and so much other useful info as well. I think I shall treat myself to an AVO 8 meter to go with my ancient bakelite megger in it's beautiful metal box. But where to obtain such a gorgeous thing? Any ideas?

Richard
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Old 14th Nov 2007, 10:24 am   #20
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Default Re: Test equipment for valve radio repair

Richard, you are not a million miles away from Wooton Bassett. The next WB event (for BVWS members) is on Sunday 2nd of December. If you are not a member, you could probably join on the day. I am sure that there will be many stallholders selling AVOs on the day.
Neil
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