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From: piglet <erichpwagner@hotmail.com>
Newsgroups: sci.electronics.design
Subject: Re: Oscillator Distortion
Date: Mon, 14 Oct 2024 06:40:51 -0000 (UTC)
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Cursitor Doom <cd999666@notformail.com> wrote:
> 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.
> 

Venner were famous for their time switches made since forever and used in
street lights before photocells.


-- 
piglet