PSU Ripple

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

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

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

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

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.

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

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

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.

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. :(

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