From: liz@poppyrecords.invalid.invalid   
      
   Cursitor Doom  wrote:   
      
   > On Mon, 14 Oct 2024 11:41:24 +0100, Liz Tuddenham wrote:   
   >   
   > > Cursitor Doom  wrote:   
   > >   
   > >> On Sun, 13 Oct 2024 17:39:53 -0700, Dave Platt wrote:   
   > >>   
   > >> > In article ,   
   > >> > Cursitor Doom   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.   
      
   The circuit, is pretty similar to the Venner TSA 625/2, which was sold   
   as a standalone general purpose oscillator - one of which I have in   
   pieces on the desk in front of me.  The thermistor (which is TH1 in this   
   circuit but more likely to be TH2 in yours) looks like a glass tube,   
   about the size of a DM70 valve (for those who remember them),  with two   
   flying leads.  It is supported in a plastic clip.   
      
   The interior has a pinch with two substantial, longish support wires.   
   Joining the ends of the support wires there is a very fragile wire and   
   suspended by that wire is the thermistor bead.  The idea is that the   
   glass tube is evacuated and there is very little thermal conductivity   
   along the support wires, so the bead is free to self-heat with only a   
   few milliwatts of power.   
      
   If yours is like this, you cannot replace it with a light bulb as the   
   characteristics will be completely different.  The resistance drops as   
   it heats up, where the resistance of a light bulb rises.  The good news   
   is that these are still made (or were until very recently) and you may   
   find the type number of the exact part you need in the parts list for   
   the instrument.   
      
   My guess is that TH1 in your circuit is a slow-acting thermistor that   
   compensates for thermal effects in the transistor biassing and it may   
   look like a black resistor or a small tablet of carborundum mounted on   
   the board with ordinary component lead-out wires.   
      
      
   --   
   ~ Liz Tuddenham ~   
   (Remove the ".invalid"s and add ".co.uk" to reply)   
   www.poppyrecords.co.uk   
      
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    * Origin: you cannot sedate... all the things you hate (1:229/2)