XPost: sci.electronics.design   
   From: cd999666@notformail.com   
      
   On Sun, 13 Oct 2024 17:58:08 -0000 (UTC), Phil Hobbs wrote:   
      
   > Cursitor Doom  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   
      
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