Re: Oscillator Distortion

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.