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Old 1st Dec 2015, 9:33 pm   #83
G6Tanuki
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Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 8,630
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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