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From: Cursitor Doom <cd@notformail.com>
Newsgroups: sci.electronics.repair
Subject: Re: PSU Ripple
Date: Sun, 10 Mar 2024 23:57:47 +0000
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On Sun, 10 Mar 2024 14:02:14 -0400, ehsjr <ehsjr@verizon.net> 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. :(