XPost: sci.electronics.design   
   From: cd999666@notformail.com   
      
   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.   
      
   --- SoupGate-Win32 v1.05   
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