PSU Ripple

[Cursitor Doom]

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)

[ehsjr]

Post by Cursitor Doom
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)

If we assume that the probe is (one probe and 1 ground
only!) properly connected across the output of 1 bridge
rectifier ONLY and the load is not connected to that
bridge, you should see an almost perfect waveform.
Do a google on bridge rectifier waveform to see what a
perfect waveform looks like. That's a big assumption,
based on your description of the accessibility problem.

Assuming all is well with the measurement technique,
those wave forms are wrong. Both bridges look bad -
replace them.

With the above action in mind, you must disassemble things
enough to replace the bridges --- SO that means you
must have unfetterd access to the bridges before removal
so that you can make measurements and KNOW you have your
probe and ground connected properly. With that much
disassembly you can use your VOM on diode test to check
the bridges.

To me, it really looks like you _MUST_ do the work, even
though it may be a PITA, to verify that those really
are the waveforms and not a measurements error.

I feel the pain! I had to repair a PW1 linear amplifier
that got "eaten" by lightning. To get to the power supply
parts I had to remove over 140 screws one hundred and
forty! just to gain the needed access.

Good luck.
Ed

[Cursitor Doom]

Post by ehsjr

Post by Cursitor Doom
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)

If we assume that the probe is (one probe and 1 ground
only!) properly connected across the output of 1 bridge
rectifier ONLY and the load is not connected to that
bridge, you should see an almost perfect waveform.

Quite. And that *is* how I've measured it.

Post by ehsjr
Do a google on bridge rectifier waveform to see what a
perfect waveform looks like. That's a big assumption,
based on your description of the accessibility problem.

I know what a full-wave rectified signal should look like and what I'm
seeing doesn't much look like one! It does look like a diode's gone
down in each bridge, but it could conceivably be just one that's
failed in certain circumstances I guess.

Post by ehsjr
Assuming all is well with the measurement technique,
those wave forms are wrong. Both bridges look bad -
replace them.
With the above action in mind, you must disassemble things
enough to replace the bridges --- SO that means you
must have unfetterd access to the bridges before removal
so that you can make measurements and KNOW you have your
probe and ground connected properly. With that much
disassembly you can use your VOM on diode test to check
the bridges.
To me, it really looks like you _MUST_ do the work, even
though it may be a PITA, to verify that those really
are the waveforms and not a measurements error.

I fully agree. It's just a nuisance not having two consecutive hours
to get stuck into this job. I'm lucking if I can find a spare hour
once a week to work on it. :(

[legg]

Post by Cursitor Doom
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.

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

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.

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.

RL

[Cursitor Doom]

Post by legg

Post by Cursitor Doom
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.

Post by legg
Care to mark up your images to include a 0V baseline?

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

Post by legg
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.

Post by legg
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.

[legg]

Post by Cursitor Doom

Post by legg

Post by Cursitor Doom
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.

Post by legg
Care to mark up your images to include a 0V baseline?

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

Post by legg
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.

Post by legg
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