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From: Cursitor Doom <cd@notformail.com>
Newsgroups: sci.electronics.repair,sci.electronics.design
Subject: Re: PSU Ripple Update
Date: Sun, 17 Mar 2024 20:15:31 +0000
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On Sun, 17 Mar 2024 16:05:46 -0400, bitrex <user@example.net> wrote:

>On 3/17/2024 1:48 PM, Cursitor Doom wrote:
>> Gentlemen (and others)
>> 
>> I only get a few spare minutes a week to look into this, hence this
>> update. Hopefully my latest finding might ring a bell for some of you
>> and assist in pinpointing the fault with this (linear) PSU.
>> So, I've carried out a few more tests and discovered that there is a
>> total absence of ripple on the storage caps when all the downstream
>> circuitry has been disconnected. So it's totally fine with no load.
>> However, as I re-connect all those downstream circuits, the ripple
>> commences and the more connectors I re-attach, the worse it gets. This
>> is a screen shot showing over a volt of ripple at only about 66% of
>> the full supply voltage applied:
>> 
>> https://disk.yandex.com/i/vgxfpXgNp-F4Yg
>> 
>> Now I did check to see if there was anything downstream which had
>> shorted or gone low-resistance which could possibly account for this,
>> but found nothing amiss. So the question is:
>> What could cause ripple to arise when even very light loads are
>> applied to the output of a pretty substantial linear PSU?
>
>You sure you got your probe division/scope division configured congruently?

Yes, it's on 1x @ 5ms.
The only thing I can think of is some sort of issue with a rogue
feedback signal which has ripple on it and is causing the PSU to (mis)
regulate accordingly.  But I have no schematic and there's none
available online either, so I can't check for that.