Path: ...!eternal-september.org!feeder3.eternal-september.org!news.eternal-september.org!.POSTED!not-for-mail From: Cursitor Doom Newsgroups: sci.electronics.repair,sci.electronics.design Subject: Re: Oscillator Distortion Date: Thu, 17 Oct 2024 13:35:24 -0000 (UTC) Organization: A noiseless patient Spider Lines: 117 Message-ID: References: <1r1f1rf.m9nkf1gcwyvmN%liz@poppyrecords.invalid.invalid> <1r1h7sd.y08lxv1vu2ftiN%liz@poppyrecords.invalid.invalid> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Injection-Date: Thu, 17 Oct 2024 15:35:24 +0200 (CEST) Injection-Info: dont-email.me; posting-host="e4a24edfa1b5e59b50c045b4f2346b49"; logging-data="2882322"; mail-complaints-to="abuse@eternal-september.org"; posting-account="U2FsdGVkX19X/LFewJaeGGX6DvrZ/pDEXtPXyHkCmJo=" User-Agent: Pan/0.149 (Bellevue; 4c157ba) Cancel-Lock: sha1:4yJPS3tnWfv4nIufdsAJTwlU/Eg= Bytes: 7133 On Thu, 17 Oct 2024 13:35:14 +0100, piglet wrote: > On 17/10/2024 12:28 pm, Cursitor Doom wrote: >> On Thu, 17 Oct 2024 11:33:52 +0100, piglet wrote: >> >>> On 16/10/2024 3:20 pm, 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. >>> >>> >>> My guess is type R23 is 2kohm at room temp (the R53/RA53 beloved by >>> hobbyists in the 1960s/70s was 5k). eBay probably has some close >>> enough (1.5k to 3.3k at room temp?) replacements. >>> >>> This link might help you: >>> >>> > Old_STC_Thermistors.pdf? rlkey=jozht29aj1u6ocxnmw8okedrq&st=ohzxz0mw&raw=1> >>> >>> piglet >> >> Thanks for the info, Erich; much useful info in that datasheet. >> Nothing showing up on Ebay at the moment, but I'm making enquiries of >> vintage parts sellers which hopefully might bear some fruit. Failing >> that I'll just have to build a new oscillator stage from scratch using >> a spare vacuum thermistor from my parts bin. > > I saw a seller with Littelfuse GL202F9J which might do electrically if > maybe a bit slow thermally? That part is also available from Mouser. > > piglet According to the datasheet, this device boasts "a fast thermal response time" but doesn't quantify that. It could do the trick with a bit of bias tweaking.