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Old 2nd Dec 2015, 8:56 pm   #92
David G4EBT
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Join Date: Feb 2003
Location: Cottingham, East Yorkshire, UK.
Posts: 4,099
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):

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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