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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 19:28:33 -0000 (UTC)
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On Sun, 13 Oct 2024 17:58:08 -0000 (UTC), Phil Hobbs wrote:

> Cursitor Doom <cd999666@notformail.com> wrote:
>> On Sun, 13 Oct 2024 17:10:30 -0000 (UTC), Cursitor Doom 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.
>>> [removed]
>>> 
>>> I'm out of ideas. What could cause such distortion if the PS rail
>>> isn't responsible?
>>> 
>>> Your pal,
>>> 
>>> CD.
>> 
>> Sorry, the trace of the 'ripple' should have been this link:
>> https://disk.yandex.com/i/P7AIraCaJybIMw
>> 
>> 
> Looks like an AC-coupled square wave with a too-short time constant,
> which probably means that either the oscillator gain is running wide
> open, or the second stage gain is too high, due e.g. to its feedback
> loop being open.
> 
> Is the output amplitude close to the knob setting, or is it way off?

That's a bit tricky to ascertain at the moment. I might be able to verify 
it later. However, I have managed to get the relevant section of the 
schematic. The supply is just under 12V. The nomenclature is a bit odd. 
The core of the oscillator is comprised of VT10, VT11 and VT12. All the 
other stuff beyond those to the right is for shaping and whatnot.

https://disk.yandex.com/i/dbsB6wiQMDKtWw