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Appreciate that. Matches what I had gleaned. Is there a schematic for the light board?

Everything is proceeding smoothly and then an "Ah Shit". First, I tested the power supply - both B+ & B-, without connecting either the WOPL PL400 backplane or the WOPL PL14-20 control board. It tested fine - about +/- 82 vdc. Then I connected power to both the backplane and the control board...

Nice! How's the condition?

Just did. Painless.

Construction continues - when I get a chance. So far, no serious issues and I've been following the instructions. Here are some notes. 1) As stated earlier, I stripped the chassis down to nothing to treat some beginning corrosion. The transformer had some. I looked at the suggestion to use...

Sorry Peter, you're missing the point. All transformers will exhibit this behavior - it's inherent and is expected, there is nothing wrong with your transformer. If you isolate the signal ground plane (i.e. the secondary center tap from the chassis) from the earth ground of the plug, the...

Peter, in my opinion, the chassis and signal ground should be kept separate. That is, signal ground is not connected to the chassis. The sketch below shows the idea. The signal ground which is connected to the center tap of the secondary is not connected to the chassis. The AC power earth...

I planned to replace the power cord since its condition was looking poor. Initially I thought I'd put on a 3 prong plug with an earth and tie it to the chassis and float the amp's ground from the chassis. This would provide safety and shield the amps innards. But then the amp can't be plugged...

Stripped the chassis down to nothing - cleaned everything. Plan is to build the WOPL PL14/20 card first. Then build up the backplane without the transformer just to make it easier to move around. Then install transformer and rebuild power supply and test it before connecting it to anything...

I would not jump to the conclusion the hum is somehow due to the transformer. Maybe, but keep an open mind. There are other possibilities as suggested above. Is it on both channels? Can you see it on a scope (and hence measure it). Try shorting the inputs to shield - and this has to be a...

Spent a few minutes looking at the schematic. I see what you're saying - no direct connection from the negative speaker terminals to ground. Sorry I can't be of more help.

Beautiful! Glad to hear things have worked out. A nice new year's present.

Peter stated earlier the snubber caps were 0.3 uF.

Question is: Is this the source of your hum. Sounds to me that your signal source and the amp are on two different ground planes. Connecting a jumper from the amp chassis to 120vac outlet ground is putting them on the same ground plane (or nearly so). The coax from the signal generator should...

So that all makes sense if you're getting a bit of leakage current due to the inherent capacitance/inductance between the primary and secondary in the transformer. Making the same measurement with one of those old 1000 ohm/volt VOMs, and the voltage would shrink to nearly zero. This also...

Do you have any idea what the impedance of your meter is on the AC scale? Agree that 42 uA is well below what would trip a GFCI circuit. With a high input impedance meter it's possible the voltage you see is due to stray capacitance/inductance of the transformer.

Just to be sure I understand. This is your situation:

Ha ha. Thought I was going to get a lot of stick for refurbishing a PL400. Your pros and cons are always welcome.