Re: Oscillator Distortion

On Sun, 13 Oct 2024 20:56:15 -0000 (UTC), Phil Hobbs wrote:

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

Anyway, it looks like the gain's too high, then. I don't have access to
the generator again for a day or two, but I will focus on that aspect and
hope to identify the precise issue through that.
Thanks, Phil; you're a pal. ;-)

CD