XPost: sci.electronics.design   
   From: cd999666@notformail.com   
      
   On Tue, 15 Oct 2024 16:14:21 +0100, Liz Tuddenham wrote:   
      
   > 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.   
      
   This one's the TSA628. I have the service manual for it, but it only   
   quotes Venner part numbers for all devices listed and "R23" for the type/   
   value of this thermistor. I don't think either of those numbers would be   
   of much use today. TH1 is "Type R52" so while they appear identical, it   
   seems they aren't.   
   TH1 and TH2 both *appear* identical: glass encapsulations about an inch   
   long by 3/8 wide at a guess. They're juxtaposed together on the board. I'm   
   guessing - it is only a guess - that TH2 (the failed one) functioned as   
   some sort of AGC to stabilise the amplitude of the oscillator and the TH1   
   was the actual Wein-Bridge element as is commonly understood in this type   
   of oscillator. That would account for why - as Phil Hobbs observed - the   
   gain has gone up enough to run the output into the supply rails and give   
   rise to the distortion I'm seeing.   
      
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