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Old 2nd Dec 2015, 9:12 pm   #93
TonyDuell
Dekatron
 
Join Date: Jun 2015
Location: Biggin Hill, London, UK.
Posts: 5,190
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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