XPost: sci.electronics.design   
   From: roger@hayter.org   
      
   On 16 Oct 2024 at 16:29:02 BST, "Cursitor Doom"    
   wrote:   
      
   > On 16 Oct 2024 14:39:27 GMT, Roger Hayter wrote:   
   >   
   >> On 16 Oct 2024 at 15:20:53 BST, "Cursitor Doom"   
   >>    
   >> wrote:   
   >>   
   >>> 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.   
   >>   
   >> I've got an R54 somewhere.  I think they are still available as NOS.   
   >   
   > I'm just wondering if a modern bead thermistor would work as well. The   
   > ones in this 56 year old piece of kit are large, glass-encapsulated types,   
   > but maybe that was just the way them made them back then when everything   
   > was bigger. I might try a few bead thermistors in place of the busted one   
   > once I've removed it just out of curiosity.   
      
   The advantage of the vacuum encapsulated ones is that it requires vastly less   
   power to heat them, and thus I doubt an open bead one would easily work at the   
   same signal level.   
      
   --   
      
   Roger Hayter   
      
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