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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: piglet <erichpwagner@hotmail.com> Newsgroups: sci.electronics.design Subject: Re: Oscillator Distortion Date: Mon, 14 Oct 2024 06:40:51 -0000 (UTC) Organization: A noiseless patient Spider Lines: 74 Message-ID: <veiedj$13lib$1@dont-email.me> References: <veguu6$ofj1$3@dont-email.me> <vegv1v$ofj1$4@dont-email.me> <veh1nf$p9u1$1@dont-email.me> <ir5ogjlfjsv80ugtdlhk7vopmrcnfua9lk@4ax.com> <veh762$ofj1$8@dont-email.me> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Injection-Date: Mon, 14 Oct 2024 08:40:51 +0200 (CEST) Injection-Info: dont-email.me; posting-host="a1c7ce2d1919c3cfa11b5a26b69b5573"; logging-data="1168971"; mail-complaints-to="abuse@eternal-september.org"; posting-account="U2FsdGVkX18zyJrRxDwzks5Ne8KS5aFE" User-Agent: NewsTap/5.5 (iPhone/iPod Touch) Cancel-Lock: sha1:v263iErnuSQtgYWdd7qnM+M0GX8= sha1:WQh0F+KZHYXTWcGIMTPFqpUDEoU= Bytes: 3991 Cursitor Doom <cd999666@notformail.com> wrote: > On Sun, 13 Oct 2024 15:00:15 -0400, Joe Gwinn wrote: > >> On Sun, 13 Oct 2024 17:58:08 -0000 (UTC), Phil Hobbs >> <pcdhSpamMeSenseless@electrooptical.net> wrote: >> >>> Cursitor Doom <cd999666@notformail.com> 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. >> >> .<https://en.wikipedia.org/wiki/HP_200A> >> >> 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. > Venner were famous for their time switches made since forever and used in street lights before photocells. -- piglet