Amp only has sound when caps are draining

[Stevem]

lift the two red wires that feed each of the 3 diode string and measure the voltage across them, what is that reading?
if it is a correct reading of no ac voltage on the bias feed wire than that PT winding is open, if you still get a good ac reading on the red wires you can tap ac off of that and get a negative bias voltage from that sourse!

[martin manning]

I don't know what to make of the 0 VAC on the bias tap, or how that translates into all the negative voltage on the bias supply, but I'm afraid that the PT was damaged by shorting the bias tap to ground. Is the PT getting hot?

Maybe try disconnecting the red and red/blue leads from the rectifier board and measure the AC voltage on them unloaded. Be careful doing this- best to put them between pieces of masking tape and secure them so they can't touch the chassis.

[blueamberol]

OK

I disconnected and tested PT

across the reds = 715v

bias tap = 356v

[martin manning]

Do you mean bias tap? The red/yellow center tap (CT) should be connected to ground, along with the Green/Yellow filament winding CT.

[blueamberol]

yes, sorry - bias tap

thanks for bearing with me!

[martin manning]

And that bias tap voltage of 356V is from the Red/Blue lead to ground?

What voltage do you get from each of the red leads to ground, individually? They should be the same.

[blueamberol]

that was just across the leads

leads to ground looks strange:

Red #1 = .5

Red #2 = .2

bias tap = 0

[martin manning]

Those voltages are measured with the meter set for AC?

[blueamberol]

yes. on the 200v AC setting on my meter, using alligator clips on the leads and chassis.

[martin manning]

Strange indeed. You're sure your meter is giving you good readings? Are the batteries fresh? What do you get measuring across the primary (incomming line voltage with power on)? If shows 120VAC or so then it doesn't look good for the power transformer.

[Stevem]

If that red / yellow is grounded as it should be than those two red leads tested separate to ground should have over 300 volts
on them since you found 715 volts across them so why are you using a 200 volt meter setting to test them?
If you indeed have over 300 volts on the bias tap than the transformer has a short, but more than that the bias filter which most times is rated for under 75 vdc should have blown wide open on you!!
If this is the case and the bias winding is shorted or not working any more than you can do as I posted and pull ac voltage off of one of the red wires and get a new bias supply up and running by just changing the valve of the bais dropping resistor on the ac side of the bias diode to a larger value.

[martin manning]

If the bias tap is as shown on the schematic (literally a tap on one side of the secondary winding), then grounding the bias tap has likely damaged part of the secondary. I would not be comfortable with continued use of this transformer in that condition.

[JazzGuitarGimp]

If you're measuring 715V between the reds, but nothing from either red to chassis, I would suspect the center tap is not connected to the chassis.

[martin manning]

Try increasing the voltage range setting on the meter. It may show ~zero if the applied voltage is out if range. I think I'd go nuts without an auto-ranging DMM.

[Firestorm]

There's no way you can get -113VDC off a half-wave rectified 50VAC tap. A winding is shorted or the center tap is open. Disconnect the red and red/blue secondaries, leave the red/yellow and green/yellow center taps soldered to the chassis and measure the resistance from each secondary to ground. Lowest resistance setting on your meter, power off of course.