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From: Cursitor Doom <cd999666@notformail.com>
Newsgroups: sci.electronics.design,sci.electronics.repair
Subject: Re: Oscillator Distortion
Date: Sun, 13 Oct 2024 23:27:10 -0000 (UTC)
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On Sun, 13 Oct 2024 20:56:15 -0000 (UTC), Phil Hobbs wrote:

> john larkin <JL@gct.com> wrote:
>> On Sun, 13 Oct 2024 17:10:30 -0000 (UTC), Cursitor Doom
>> <cd999666@notformail.com> wrote:
>> 
>>> Gentlemen,
>>> 
>>> Last week I got an old (1968) pulse generator out of mothballs and
>>> managed to get it fully functional again. However, before replacing
>>> the case, I (true to form) dropped it on the bench and something on
>>> the PCB must have shorted out against the metal tools it fell on,
>>> because it no longer works properly.
>>> I've found an issue with the principal oscillator. It's generating
>>> distorted sine waves. It's a wien bridge type using BJTs as the gain
>>> element and fine tungsten filaments as thermistors, so should produce
>>> near perfect sine waves before they're chopped and shaped by
>>> subsequent circuitry, but since the fall, it's not.
>>> 
>>> Here's the oscillator output:
>>> https://disk.yandex.com/i/eKAe95xMsiIvNA
>>> 
>>> I found some weird periodic spikes on the power supply rails in the
>>> oscillator stage. They are actually present on the rail, not just
>>> picked up by the ground lead of the scope out of the ether, as I used
>>> a short ground clip in this instance. I'm not sure if these could
>>> cause the distortion or not.
>>> https://disk.yandex.com/i/eKAe95xMsiIvNA
>>> 
>>> I'm out of ideas. What could cause such distortion if the PS rail
>>> isn't responsible?
>>> 
>>> Your pal,
>>> 
>>> CD.
>>> 
>>> 
>> Maybe the light bulb broke.
>> 
>> 
>> 
> It would have to have failed short-circuit for the gain to be too high,
> and that’s not the usual failure mode.
> 
> Cheers
> 
> Phil Hobbs

Anyway, it looks like the gain's too high, then. I don't have access to 
the generator again for a day or two, but I will focus on that aspect and 
hope to identify the precise issue through that.
Thanks, Phil; you're a pal. ;-)

CD