Help Fixing a Mcintosh mc75

Hello,
I've got a mc75 that was rebuilt but after a few minutes of use i get a terrible hum. For the first 5 min after power up all the voltages are within
3% of the voltages on the schematic but then when the hum happens pin 1 of the 12ax7 (V1) shows 2v instead of 132V. For V2/12au7 pin1 is at 50v and pin 7 at -7v. Pin 1 of V3 12bh7 goes up to 412v. In the power supply 380v drops to 300v and 170v drops to 100v. All the capacitors ,resistors and diodes are new, i have tried new tubes as well. If anyone could  suggest something else to try it would be greatly appreciated.

I don't see any way V2/7 could be negative unless the amp is oscillating. I take it you don't have an oscilloscope? If you don't you can make a detector using a diode and small cap (.1 or something). Connect the anode of the diode to pin 7 and the cathode to the cap. Connect the other end of the cap to ground. Measure the DC voltage across the cap when the amp is humming.

--mark

Thanks Mark i will check

> a mc75 that was rebuilt

Was it rebuilt right? Or has it been bad since the rebuild?

> In the power supply 380v drops to 300v

Gut feeling is that the big bottles are also red-plating. Drawing MUCH more current than they should. Power sags and ripple increases. Usual cause is a fault in the negative bias supply, "-58V", though in a MC75 it could also be around V4, or disconnected R19 R20.

The super-low and negative in the early stages is very odd though. Mark's oscillation theory is more likely to do that than my red-plate idea.

When i got the amp it wasn't working , but all the components were new. It looks like i've found the fault, one of the doubler capacitors. It was looking new but probably sat on a shelve for too long. Since the replacement the amp is behaving properly. Thank you all for your help

i spoke too soon, changing the cap helped for a short while but now the same problem is back.
R27 gets really hot when the problem happens. I'm getting a scope soon

Hi PRR,
It looks like V5 has a problem -58V drops to nothing, the other side of r25 drops a little from -100 to -90. it takes a lot longer than before for the problem to show up but it is still happening. You suggested problem around v4. Could it be C8 ?
Thanks
francois

> -58V drops to nothing

Remove output tubes; does it still drop?

Disconnect one end of C8; does it still drop?

I ended up replacing R11/R13/R14 and it's all good now. Many thanks for your help and suggestions