From: cd6699@notformail.com
On Wed, 3 Sep 2025 16:55:15 +0100, Adrian Caspersz
wrote:
>On 03/09/2025 14:20, Cursitor Doom wrote:
>> Gentlemen,
>>
>> I have an early AVO valve tester (twin panel type, Mk1 or Mk2) which
>> I'm trying to get fully working. Anyway, I have a problem with it
>> which I've never encountered before. I set the panels up to test a
>> valve, set the anode and heater voltages etc, the valves are glowing
>> just like they should, but when I apply the prods of a DVM across the
>> heater pins to check if they're correct WRT the switch setting, that
>> tiny extra load completely collapses the supply to the heaters! The
>> meter reads 0V (or close enough) and the heater glow dims to nothing.
>> Any idea WTF's going on here?
>
>If the circuit has any sort of gain, are you creating a feedback path
>with noise radiating cables that oscillates and overwhelms the power
>delivery of the supply? Try the meter switched to AC, or an
>oscilloscope, you may still see something.
I only wish I could. The AVO has thus declined to generate any more
heater supply. This *has* happened before. I suspect there's a thermal
cutout which operates when an overload condition arises. This will be
some primitive mechanical device no doubt (we're talking early 1940s
here). Effective, but it takes an *age* to reset and I don't have an
age to F about with it. I did check the DVM though, just to ensure it
wasn't faulty and drawing too much current. It was fine, however. Even
on a sig gen with 600 ohm impedance it didn't detectably reduce the
amplitude of the test waveform at 50Hz. So it remains a mystery I'm
afraid - and likely to continue to be so - at least until someone more
gifted than I in 80 year old gear unravels it. :(
--- SoupGate-Win32 v1.05
* Origin: you cannot sedate... all the things you hate (1:229/2)
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