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From: Cursitor Doom <cd999666@notformail.com>
Newsgroups: sci.electronics.design
Subject: Re: Oscillator Distortion
Date: Sun, 13 Oct 2024 19:31:14 -0000 (UTC)
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On Sun, 13 Oct 2024 15:00:15 -0400, Joe Gwinn wrote:

> On Sun, 13 Oct 2024 17:58:08 -0000 (UTC), Phil Hobbs
> <pcdhSpamMeSenseless@electrooptical.net> 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?
>>
>>And can you get your hands on a schematic?
> 
> My guess was that the light bulb filament broke from the shock.
> 
> It's probably the founding HP 200A Wien Bridge audio oscillator circuit
> from 1939.
> 
> .<https://en.wikipedia.org/wiki/HP_200A>
> 
> Joe Gwinn

Somewhat unusually for me, this is not an HP instrument. It was made by 
some company called Venner in London. It did come with a very helpful 
manual which gives expected waveforms at various key points in the 
circuit. As you might expect, it shows sine waves for the TPs in the osc 
section.