Path: ...!2.eu.feeder.erje.net!feeder.erje.net!eternal-september.org!feeder3.eternal-september.org!news.eternal-september.org!.POSTED!not-for-mail From: Cursitor Doom Newsgroups: sci.electronics.design Subject: Re: Oscillator Distortion Date: Sun, 13 Oct 2024 19:31:14 -0000 (UTC) Organization: A noiseless patient Spider Lines: 65 Message-ID: References: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Injection-Date: Sun, 13 Oct 2024 21:31:15 +0200 (CEST) Injection-Info: dont-email.me; posting-host="55a35ec646eedbfc987c24eb757f565a"; logging-data="802401"; mail-complaints-to="abuse@eternal-september.org"; posting-account="U2FsdGVkX195ppLCpoboKuBikkKtak6ku+tmT1WZGGU=" User-Agent: Pan/0.149 (Bellevue; 4c157ba) Cancel-Lock: sha1:SPHXZUCuJJE34vLNm0HqrKB0AyY= Bytes: 3671 On Sun, 13 Oct 2024 15:00:15 -0400, Joe Gwinn wrote: > On Sun, 13 Oct 2024 17:58:08 -0000 (UTC), Phil Hobbs > wrote: > >>Cursitor Doom wrote: >>> On Sun, 13 Oct 2024 17:10:30 -0000 (UTC), Cursitor Doom wrote: >>> >>>> Gentlemen, >>>> >>>> Last week I got an old (1968) pulse generator out of mothballs and >>>> managed to get it fully functional again. However, before replacing >>>> the case, I (true to form) dropped it on the bench and something on >>>> the PCB must have shorted out against the metal tools it fell on, >>>> because it no longer works properly. >>>> 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. >>>> >>>> Here's the oscillator output: >>>> https://disk.yandex.com/i/eKAe95xMsiIvNA >>>> >>>> I found some weird periodic spikes on the power supply rails in the >>>> oscillator stage. They are actually present on the rail, not just >>>> picked up by the ground lead of the scope out of the ether, as I used >>>> a short ground clip in this instance. I'm not sure if these could >>>> cause the distortion or not. >>>> [removed] >>>> >>>> I'm out of ideas. What could cause such distortion if the PS rail >>>> isn't responsible? >>>> >>>> Your pal, >>>> >>>> CD. >>> >>> Sorry, the trace of the 'ripple' should have been this link: >>> https://disk.yandex.com/i/P7AIraCaJybIMw >>> >>> >>Looks like an AC-coupled square wave with a too-short time constant, >>which probably means that either the oscillator gain is running wide >>open, or the second stage gain is too high, due e.g. to its feedback >>loop being open. >> >>Is the output amplitude close to the knob setting, or is it way off? >> >>And can you get your hands on a schematic? > > My guess was that the light bulb filament broke from the shock. > > It's probably the founding HP 200A Wien Bridge audio oscillator circuit > from 1939. > > . > > Joe Gwinn Somewhat unusually for me, this is not an HP instrument. It was made by some company called Venner in London. It did come with a very helpful manual which gives expected waveforms at various key points in the circuit. As you might expect, it shows sine waves for the TPs in the osc section.