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From: Cursitor Doom <cd@notformail.com>
Newsgroups: sci.electronics.design
Subject: Re: PSU Ripple
Date: Mon, 11 Mar 2024 09:36:02 +0000
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On Sun, 10 Mar 2024 20:49:12 -0700, John Larkin <jl@997PotHill.com>
wrote:

>On Mon, 11 Mar 2024 00:53:31 +0000, Cursitor Doom <cd@notformail.com>
>wrote:
>
>>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.
>
>Something has to pull the rectified node down, to discharge the stray
>capacitance and junk coupled into the transformer windings, to make
>the diodes actually rectify.

I did have a light bulb moment at about 3am and came to this
conclusion. I think of these signals as 'ghost voltages' and they're
the same thing that cause electricians to get false positives for
mains voltage on cabling that isn't live when they use their hi-z neon
screwdrivers. It's the same thing we're just calling different names
if I understand you correctly - which I believe I do.

>>>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...)
>
>Add a few Kohms to ground and then scope the waveform.

I have a hunch it will just make it all the more obvious there's
something amiss with the rectifiers.

>>>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.