HP 6826A Bipolar power supply/amplifier repair - power supply voltage

[tonycox01]

Hi,

I am trying to repair an HP 6826A PSU. It was very "glitchy" - with unstable output and showing rapid swings and jumps in output voltage - especially on the negative side. There was also a small "crackling" noise that sometimes accompanied the glitches. Sounded like a momentary short somewhere.

I checked the main power supply outputs and the +/-15V and 80V were fine but the +/-65v was sitting at ~+-/52V. I decided to recap it as they were all the original components. I replaced the 3000uF with 3300uF and the 200uF with 220uf, and finally the 325uF with 330uF - all the nearest values I source conveniently. Voltage ratings were all fine too.

While the jumps in output are now gone, the supply is super sensitive to voltage adjustment - small voltage adjustments give very wide swings in output current. I think I have made it worse! The main supply output voltage is sitting at +/-77V! So recapping has pulled the voltage up much too high if it is supposed to be +/-65V - and I'm speculating that this is causing the new overly sensitive output problems.

I'm not quite sure how to approach the problem with the supply over-voltage. Putting back the old caps seems to be the wrong approach to me, but any help/suggestions would be greatly appreciated.

thanks

Tony

[goaty]

You say voltage adjustment cause _current_ swings ? You mean output voltage swings, no ?
I´d measure the voltage on the voltage adjustment pots with a scope to see if they are noisy or interrupted.

[PartialDischarge]

You did well in replacing the caps, as the outputs do oscillate when these go bad.
About the high voltages a similar mode I have has exactly the same problem. But I suspect this explained by the fact that the input transformer was designed for 220V whereas now we have 240V.

[tonycox01]

Yes, when adjusting the voltage a small bit either side of zero (it has the 10 turn pot option 1) - less than 1V - I see a big swing in current. I have a 47R power resistor across the output terminals but an adjustment of less than a volt either side of zero sees the *current* go to FSD (>1.2A and current limit kicks in).

thanks
Tony

[tonycox01]

I wondered if this might be a problem -  I was thinking of plugging it in via a variac and pulling down the main 240V supply to the point where the main supply drops to +/-65V and see if that resolves the problems. Not sure what else to do?

thanks

Tony

[goaty]

So how does it behave without load ? Is it voltage regulation that fails or the current limit you set ?