Marantz 2500 UBER HIGH negative dc voltage -14.90 VOLTS ! Help

[xalara76]

I’m working on a Restoration for a Marantz 2500 and I’m basically at the finish line just finalizing calibrations. It came to my surprise that both channels show a very odd -14.9 V, YES volts not millivolts when going to set dc voltage. The unit plays and sounds incredible, but this worries me. I have recapped the entire power supply adjusted to 32Volts as indicated in the service manual, and replaced both main filter caps as they were leaking.

Replaced the power cord as it was chewed up, and fixed the broken scope.

Note, DC offset is not able to be adjusted on either R760. Soft start relay engages instantly and the protection relay comes on about 5-7 seconds later.

Are the TO-3 output transistors suspect ? None of the fuses are blown.
Any help or idea where to troubleshoot will be of benefit, thank you.

 

[ocdecio]

I'm confused. -14V should trip the protection relay and you shouldn't hear anything, good or bad.

 

[xalara76]

I'm confused. -14V should trip the protection relay and you shouldn't hear anything, good or bad.

That’s correct. Which is what puzzles me as well. I should also mention I had to snip the wires for the rectifier diodes in order to pull out the sub chassis completely and do the wiring on the filter caps. I marked these wires and placed them back correctly do the chassis mounted capacitor, but will double check my work again. Could this be the issue ?

If my outputs were bad or shorted, it wouldn’t come out of protection either, nor would it play music.. tricky one here !

 

[Hajidub]

Not very experienced with resto's, but almost every Marantz SM I've downloaded has had errors in the DC offset measuring/adjust section. Don't understand how V's like that wouldn't send the unit into protection.

 

[xalara76]

Not very experienced with resto's, but almost every Marantz SM I've downloaded has had errors in the DC offset measuring/adjust section. Don't understand how V's like that wouldn't send the unit into protection.

I have come across service manual issues as well working on other pieces. In this case though resting dc offset is quite basic on all units. Speaker probes into the terminals, unit set to aux with no load and then adjust with the according trimmer.

I’m thinking of looking at a suspect bridge rectifier which is throwing off the negative and positive rails of the unit perhaps ? Then over compensating and giving me a negative reading

 

[xalara76]

I'm confused. -14V should trip the protection relay and you shouldn't hear anything, good or bad.

Both channels show -15.12 volts. DMM directly into speaker terminals, no load.

Everything’s connected exactly
How the old filter caps were as well as their orientation. Again, relay engages and the unit plays.

I want to sort out this weird negative rail voltage though.