super high plate voltage

[dmartn149]

I bought this old Harman Kardon ca75 PA amp at a local auction. I've replaced all the e caps and the amp works, but I measured the DC at pin 3 of the power tubes (8417) and it's reading OL on my Fluke, so I tried my old analog meter set on 500-1000 and it pegged the meter. Now I don't know what to do.
I should mention that I don't have a schematic, but it has a 12au7 a 12ax7 a pair of 8417s and a solid state rectifier.

[DrGonz78]

Do you have a variac? Try bringing up the voltage slowly to watch plate voltage rise up. At that point you might be only feeding 50-75vac but you might be able to measure what the plate voltage is...??? So what caps are in this thing and where is the schematic? I mean if you truly think there is 1000vdc or more then I would think those caps would be pretty much goners. In fact what is the state of this amp? Does it have 3 prong power cable? If no 3 prong is there a virtual ground cap (death cap) connected? Perhaps if there is a virtual ground cap on a 2 prong power cable then maybe there is major leakage on the amp causing silly results on testing voltages with a meter.

[dmartn149]

I haven't been able to find a schematic. I haven't replaced the power chord yet. I'll do that before I power it on again. I'm not understanding the power supply either. There is one red lead coming from the PT going through 2 large metal diodes then to the OT. Please excuse my ignorance but isn't there usually 2 red leads from the PT?

[The Dude]

I used to deal HK and that model doesn't ring any bells. Can you post some pics?

[Enzo]

Try measuring the screen voltage instead. Also try measuring the B+ at the supply. Sometimes OT inductance reacts with your meter.

[Jazz P Bass]

Uhmm.
You're not measuring the voltage with the amp passing a signal are you?

Ideally, Vdc measurements are made with no signal.

[g1]

Oscillation is also a possibility.

[dmartn149]

Sorry, I had to step out for a while. I'm taking measurements without a signal but with speaker. I don't have a variac. Here is a picture.

[dmartn149]

Try measuring the screen voltage instead. Also try measuring the B+ at the supply. Sometimes OT inductance reacts with your meter.

grid 1 is -18v
grid 2 is 352v
B+ at the SS rectifier is OL
HV at the PT is 258v ac

[J M Fahey]

OL on WHICH scale?

[dmartn149]

OL on WHICH scale?

DC (auto range)

[Mick Bailey]

....I haven't replaced the power chord yet....

Neither has Angus Young and he built a career on them.

Sorry, couldn't help it with my (still) juvenile brain.

[J M Fahey]

DC (auto range)

Ok, what is the DC auto range "official maximum" ?
Meaning I am trying to get some number to work with

As in: if maximum DC autorange value is, say, 500VDC, then I know I have more than 500VDC, if rated maximum is 1000V, I know I have more than 1000VDC and so on; plain unqualified OL does not tell me much.
Nor does "autorange" if I donīt know your particular meter specs

Check the user manual, it must be stated there.

FWIW

HV at the PT is 258v ac

means you should have +V around 364VDC , VERY readable by any plain vanilla multimeter.

FWIW2:

I'm not understanding the power supply either. There is one red lead coming from the PT going through 2 large metal diodes then to the OT. Please excuse my ignorance but isn't there usually 2 red leads from the PT?

You might have a voltage doubler there, which woud explain many things but must be confirmed:

1) in that case you would have a scary +V of over 700V DC, a forbidden area (for operator safety) in most meters and would explain the OL display

2) you might have something similar to this,

notice it uses only 2 diodes in series, only one wire goes to the center of them, and the other HV wire goes to the center point of two series main capacitors.
One of them goes from ground to +360V, the other floats from +360V to +720V (approximate values)
Meaning its can is not ground but +360V

Be careful , that supply is even deadlier than usual ones.

[dmartn149]

Thanks Juan. That's exactly what I've got.
My fluke tops out at 600v but my old analog radio shack MM is supposed to measure to 1000v it was topped out.

Edit
I had the RS MM hooked up wrong. The + lead plugs into another receptacle for readings over 500v. Now I'm measuring about 750v with the analog MM.

[dmartn149]

So that's still pretty high. Would that be a result of today's higher voltage at the wall?
Thoughts on bringing it down?