Path: ...!news.mixmin.net!eternal-september.org!feeder3.eternal-september.org!news.eternal-september.org!.POSTED!not-for-mail From: Cursitor Doom Newsgroups: sci.electronics.repair Subject: Re: PSU Ripple Date: Sun, 10 Mar 2024 23:57:47 +0000 Organization: A noiseless patient Spider Lines: 56 Message-ID: References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Injection-Info: dont-email.me; posting-host="842509ddb5b6262d421e7f39189230a8"; logging-data="3400823"; mail-complaints-to="abuse@eternal-september.org"; posting-account="U2FsdGVkX1+g+7pgkZ/39WlQO6XV14k3N9cC3JgHo3s=" User-Agent: ForteAgent/8.00.32.1272 Cancel-Lock: sha1:bTT5y8SwsDAgbu0Ve+4VPGpGGQ8= Bytes: 3425 On Sun, 10 Mar 2024 14:02:14 -0400, ehsjr wrote: >On 3/10/2024 9:35 AM, Cursitor Doom wrote: >> Returning to this Marconi signal generator which has a lot of ripple >> on the (linear) PSU output, I managed to get probes through a barely >> accessible crevice and get a couple of screen shots of the rectifier >> outputs I'm seeing on the scope. >> So there's mains incoming which goes into a toroidal transformer and >> thence to the rectifiers. There are two secondary windings on the >> transformer and they each get their own bridge rectifier. This is the >> waveform that's being applied to the storage caps of the PSU (which >> I've disonnected for testing purposes). >> The outputs of neither rectifier look at all correct to me. What does >> the Panel make of them? >> >> >> https://disk.yandex.com/i/CP8qRMy-QA-fCg >> https://disk.yandex.com/i/ubNazf1pFhuNtg >> >> (probes are on 10x and I did compensate them first) > >If we assume that the probe is (one probe and 1 ground >only!) properly connected across the output of 1 bridge >rectifier ONLY and the load is not connected to that >bridge, you should see an almost perfect waveform. Quite. And that *is* how I've measured it. >Do a google on bridge rectifier waveform to see what a >perfect waveform looks like. That's a big assumption, >based on your description of the accessibility problem. I know what a full-wave rectified signal should look like and what I'm seeing doesn't much look like one! It does look like a diode's gone down in each bridge, but it could conceivably be just one that's failed in certain circumstances I guess. >Assuming all is well with the measurement technique, >those wave forms are wrong. Both bridges look bad - >replace them. > >With the above action in mind, you must disassemble things >enough to replace the bridges --- SO that means you >must have unfetterd access to the bridges before removal >so that you can make measurements and KNOW you have your >probe and ground connected properly. With that much >disassembly you can use your VOM on diode test to check >the bridges. > >To me, it really looks like you _MUST_ do the work, even >though it may be a PITA, to verify that those really >are the waveforms and not a measurements error. I fully agree. It's just a nuisance not having two consecutive hours to get stuck into this job. I'm lucking if I can find a spare hour once a week to work on it. :(