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From: Cursitor Doom <cd@notformail.com>
Newsgroups: sci.electronics.design
Subject: Re: PSU Ripple
Date: Mon, 11 Mar 2024 00:53:31 +0000
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On Sun, 10 Mar 2024 17:25:29 -0700, John Larkin <jl@997PotHill.com>
wrote:

>On Sun, 10 Mar 2024 23:19:33 +0000, Cursitor Doom <cd@notformail.com>
>wrote:
>
>>On Sun, 10 Mar 2024 14:55:36 -0700, John Larkin <jl@997PotHill.com>
>>wrote:
>>
>>>On Sun, 10 Mar 2024 17:28:13 +0000, Cursitor Doom <cd@notformail.com>
>>>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)
>>>
>>>Are the filter caps removed? Is there a load on the rectified DC?
>>
>>There isn't a load other than the probe itself. The downstream storage
>>caps are completely disconnected.
>>
>>>I assume the scope is AC coupled. You may be seeing a lot of
>>>capacitative coupled hi impedance cruft, not necessarily a failure.
>>
>>It is AC coupled, yes. 
>
>So you are seeing capacitively coupled random crud. The diodes aren't
>even conducting. The only thing you can be sure of is that all the
>diodes aren't shorted.

It's the same shape with DC coupling!
I have no idea what the rest of your message means.

>A resistive load and a dc-coupled scope would show you the classic 100
>Hz rectified waveform if the diodes are all good. Try to poke a
>DVM-type probe in there and do that maybe.

On another forum one of the accredited gurus said to do the
measurement *unloaded* which I did. Loaded makes more sense to me,
though. I'll give it a whirl next time I get an hour to spare (that
could be some days away...)

>
>If you suspect a bad part but can't open the box, what next?

I *can* open the box if I have to. But it's quite a big deal so I just
wanted to know there was a fair chance the bridges could be the issue
first, that's all.