Re: Oscillator Distortion

On Sun, 13 Oct 2024 17:58:08 -0000 (UTC), Phil Hobbs 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?

That's a bit tricky to ascertain at the moment. I might be able to verify
it later. However, I have managed to get the relevant section of the
schematic. The supply is just under 12V. The nomenclature is a bit odd.
The core of the oscillator is comprised of VT10, VT11 and VT12. All the
other stuff beyond those to the right is for shaping and whatnot.

https://disk.yandex.com/i/dbsB6wiQMDKtWw