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From: Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net>
Newsgroups: sci.electronics.design,sci.electronics.repair
Subject: Re: Oscillator Distortion
Date: Sun, 13 Oct 2024 20:56:15 -0000 (UTC)
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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 

-- 
Dr Philip C D Hobbs  Principal Consultant  ElectroOptical Innovations LLC /
Hobbs ElectroOptics  Optics, Electro-optics, Photonics, Analog Electronics