Re: Oscillator Distortion

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