Re: PSU Ripple Update

On Mon, 18 Mar 2024 07:19:08 -0400, legg <legg@nospam.magma.ca> wrote:

On Sun, 17 Mar 2024 17:48:07 +0000, Cursitor Doom <cd@notformail.com>
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.

>
Did you replace the rectifiers, until something (anything) changed?
>
The ripple has changed since your last photo, as have your test
conditions. You still don't indicate a 0V reference, so we can't
tell what the % ripple IS.
>
This waveform shows equal phase peaks at the expected frequency.
>
What is your problem?
>
RL
frequency.

I fell into the same old trap as last time and the time before that
and the time before that....
It was nothing to do with the PSU. I eventually tracked it down to a
coax's shield in the RF section which had come adrift. When
re-grounded, the ripple on the output completely vanished. Must  have
been somehow picking it up from the mains transformer despite all the
screening and compartmentalisation in this device.
All that time I wasted on the PSU - just because ripple *has* to be a
PSU problem, doesn't it. Until it isn't, that is.