XPost: sci.electronics.design   
   From: pcdhSpamMeSenseless@electrooptical.net   
      
   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?   
      
   And can you get your hands on a schematic?   
      
   Cheers   
      
   Phil Hobbs   
      
   --   
   Dr Philip C D Hobbs  Principal Consultant  ElectroOptical Innovations LLC /   
   Hobbs ElectroOptics  Optics, Electro-optics, Photonics, Analog Electronics   
      
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