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From: Jeroen Belleman <jeroen@nospam.please>
Newsgroups: sci.electronics.repair,sci.electronics.design
Subject: Re: PSU Ripple Update
Date: Sun, 17 Mar 2024 20:20:58 +0100
Organization: A noiseless patient Spider
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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