Re: PSU Ripple

On Sun, 10 Mar 2024 23:51:43 +0000, Cursitor Doom <cd@notformail.com>
wrote:

On Sun, 10 Mar 2024 14:05:52 -0400, legg <legg@nospam.magma.ca> wrote:
>

On Sun, 10 Mar 2024 13:35:18 +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)

>
Glad to see you tackling something fairly basic, sport.

>
It *would* be extremely basic, if only the manufacturers hadn't gone
out of their way to make these parts as inaccessible as possible.
>

>
Care to mark up your images to include a 0V baseline?

>
I'm only measuring the voltage difference across the bridge output.
>

With the bulk caps removed, I trust you are not applying
full mains voltage - just enough to get a diagnostic waveform,
with a load resistor in place.There will be other cicuitry
going bonkers, if not.

>
Everything downstream has been disconnected. It doesn't seem right
that I'm only seeing 4V peak to peak, though - and that's at the full
230VAC in. The storage caps that waveform is applied to are only rated
at 16VDC, but even so, with no load other than the probe itself, 4V
ain't much.
>

Most technicians in this situation will just check/replace
the bridges. If that doesn't do the job, only then is it time
to speculate.

>
Oh yes. I wouldn't have even bothered to query the waveforms I'm
seeing if only the damn rectifiers were readily accessible.

A linear psu, properly designed, was once considered to be the
last point of failure, except for fuses. Fans were considered
as anathema - they were just not used.

It was with this confidence that the psu could be placed at the
bottom, in the back. First failure mode was predicted to be the
storage capacitors - their mtbf was an order of magnitude
smaller than any other component used.

This isn't the case, today, but your unit has some age to it.

RL