Re: Oscillator Distortion

On Mon, 14 Oct 2024 11:41:24 +0100, Liz Tuddenham wrote:

Cursitor Doom <cd999666@notformail.com> wrote:
 

On Sun, 13 Oct 2024 17:39:53 -0700, Dave Platt wrote:
 

In article <veguu6$ofj1$3@dont-email.me>,
Cursitor Doom  <cd999666@notformail.com> wrote:
 

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.

 
Is there a chance that the impact broke one of those fine tungsten
filaments?  Do they read low-Z when cold, as they should?

 
It's one thing I need to look at, if only for the sake of completeness,
next time I have access to it. Since these are so hard to replace, I'd
really rather deal with my original suspicion that something got
shorted out in the fall.

 
Could a pre-set pot have gone open circuit?

I don't think so. TH2 on the schematic appears to have gone open circuit.
Those filaments are pretty fragile! Not sure what they've used - looks
like one of those old dashboard bulbs you see in old cars. I won't be able
to find a direct replacement, but I do have some spare thermistors from
other wein-bridge test gear I've plundered over the years which I dare say
could be pressed into service with a little teak of the biasing. I might
even experiment with some small filament bulbs which are not part of the
WB variety just out of curiosity.