Sovtek MIG 100U-needs TLC

[Steve Fundy]

Hello all, It's been some time since I have needed to call on the "Gods of the electrons" but I do need some advice, so here I am. I am trying to revive a Sovtek MIG100. I have very little prior experience with Sovteks. I know they're similar to Marshalls, and the technology isn't rocket science, but....
This amp was deemed to have a bad PT, and it was removed and the bells were removed. Since a replacement is rather pricey (Mercury $377) I decided to reassemble it and test the voltages. All of the voltages actually are there, though the high voltage taps are definitely off, rather low @408V. However, I decided to put it back into the chassis, reconnect and see what happens. After reassembly, I tested it on a light bulb tester, and I have a short. The short occurs when the bias circuit is attached, and goes away when I lift the yellow lead attached to the ground side of the circuit. I have checked, replaced and/or isolated all associated components, but I still cannot determine the reason for the short. I'd really like to revive this amp, which belongs my luthier friend. He's repairing one of my guitars and a trade of services is in the works. Any help greatly appreciated. Many thanks, Steve

[johnnyreece]

Some voltage measurements at different points in the bias circuit would be a good start.  How much bias voltage are you getting at the tubes?  A schematic wouldn't hurt, either. 

[Steve Fundy]

Hi, So unfortunately I haven't been able to measure any voltage on the bias circuit, as when I attach both leads from the PT it goes short. When the leads from the PT are disconnected I read 67VAC across that pair. When I attach the +Ground side of the bias pair, that's when the short occurs. I do have a schematic, and have attached it.
Thanks

[sluckey]

The short occurs when the bias circuit is attached, and goes away when I lift the yellow lead attached to the ground side of the circuit.

Your schematic shows three yellow wires on that PT. Two for the bias winding and third wire for the HV winding that connects to the junction of two series filter caps. If you connect this third yellow wire to chassis you will create a short that will destroy two of the bridge diodes.

Use your ohm meter to be absolutely sure you are connecting the correct yellow wires to the bias circuit.