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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 19:47:52 +0000
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On Sun, 17 Mar 2024 20:20:58 +0100, Jeroen Belleman
<jeroen@nospam.please> wrote:

>On 3/17/24 18:48, 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?
>> 
>> BTW, the bridge rectifiers were fine and have been exonerated from any
>> culpability in this fault.
>
>Isn't that what you'd expect? The storage capacitor gets topped
>up twice per mains period. In between top-ups, it's the sole source
>of the output current, so its voltage drops until the next top-up.
>
>You could measure the rate of voltage drop and check if it has the
>expected slope for the output current and the storage cap's value:
>dV/dt = -I/C.
>
>You could also check if the pass transistors still have enough
>voltage across them at maximum current and just before the next
>top-up. Do you see ripple on the regulated output?
>
>Jeroen Belleman

Not only am I seeing ripple on the regulated output, I'm seeing it on
the final output of the signal generator this PSU powers. 
It's quite a beefy PSU, but as little as 90mA draw gives rise to an
unacceptable level of ripple which permeates through the whole of the
downstream circuitry.