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NNTP-Posting-Date: Sun, 13 Oct 2024 20:04:28 +0000
From: Joe Gwinn <joegwinn@comcast.net>
Newsgroups: sci.electronics.design
Subject: Re: Oscillator Distortion
Date: Sun, 13 Oct 2024 16:04:28 -0400
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On Sun, 13 Oct 2024 19:31:14 -0000 (UTC), 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.

HP's patent ran out a very long time ago.  The bit about tungsten
filaments was the tell.  Read the HP patent - it's short.

Joe Gwinn