Re: Oscillator Distortion

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?

Cheers

Phil Hobbs

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