Sovtek MIG 50 Losing Sound After Standy Off

[Coop]

Hello all! I have a Sovtek Mig 50 tube amp, that when I flip the Standby switch off, to play mode, I get good sound for a few seconds and then it quickly fades away. I tested all the tubes, all test good on my Eico 667. Deoxit-ed all pots. Checked all caps and resistors. Deoxit-ed tube sockets, as well as retentioned them. With my DMM negative probe to chassis ground, I get sound back when I touch positive side of C2 (47uf-450v), then it quickly fades away again. Seems like it's missing a ground? I'm kinda stumped here. I'm sure I've overlooked something simple. Not sure if it was working properly before I got it. Looks like it's had some mods done to it. Someone has converted the 120v/220v power switch on the back into a SAG switch. (has "SAG" sticker there") They've replaced the pots and done some other mods that I'm not sure what I'm looking at. Any help will be greatly appreciated! Here's a link to the schematic I'm using. Thanks!!!

https://schematichell.com/newamps/sovtek_mig50.pdf

[Enzo]

Instead of testing all the parts, find out what is wrong wwith the circuit. If touching a cap lead brings it back for a time, then likely a broken solder connection or a cracke copper trace on the board. In any case if you have B+ at the power supply, is it getting to the tube plate resistors?

[Jon Snell]

Your schematic is very inacurate, I think the heaters are not sat at HT some 250volts!
This may help;

I would check the bias voltage.

[Coop]

I checked for broken traces and bad solder joints, The traces are peeling away. Someone has butchered this board too. I done a continuity test, all seems good there. I started taking some voltage readings. I've found no heater voltage on V5, V4, & V3...... V1 & V2 only has 3.2 VAC heater voltage. Looking at the power transformer, V1, V2 has its own winding, V3, V4, V5 has its own winding. Could that last winding be open? And thanks for the proper schematic, and guidance. I'm still green at this stuff.... lol

Enzo, yes, I have 550 vdc on R34, and R35 - Plate load resistors

John, my bias voltage is -57vdc. Also, not sure if this is still the correct schematic. My amp looks exactly like the one Lee Jackson has in his video, where he's changing the pots, jacks, etc. He is measuring the bias at R32,33. on my amp.?

[Coop]

And, what the hell is this mess. Part of that SAG mod someone has done.

[Coop]

Amp view

[Enzo]

When you measure heater voltage, you measure pin to pin, not pin to ground. You have 3v on each side because you are measuring either side of a center tapped heater winding, and the center tap is grounded.

The heaters inside the tubes are either glowing or they are not. If they are glowing, then the amp should be able to produce sound, at least as far as heaters go. If they are dark, no sound. You said the amp works for a short time then goes silent, so are the preamp tubes lit at first and then fade to black?

Pins 1 and 6 are the plates of the 12AX7 tubes. Is there B+ on ALL of them?

[Jon Snell]

And, what the hell is this mess. Part of that SAG mod someone has done.

Back in the 70s that was called a mains dropper. A high power low value resistor usually of a wire wound type with a ceramic former. The steel band is moveable to allow for a change in resistance on the more expensive types.
Old monochrome tv sets had them.

[J M Fahey]

when I flip the Standby switch off, to play mode, I get good sound for a few seconds and then it quickly fades away

Maybe your switch contact is broken and you only make a brief contact while actually pushing the toggle with your finger.
So you charge the capacitor but of course it discharges in seconds.

But the test for that is to measure voltages as Enzo suggested.

[Coop]

When you measure heater voltage, you measure pin to pin, not pin to ground. You have 3v on each side because you are measuring either side of a center tapped heater winding, and the center tap is grounded.

The heaters inside the tubes are either glowing or they are not. If they are glowing, then the amp should be able to produce sound, at least as far as heaters go. If they are dark, no sound. You said the amp works for a short time then goes silent, so are the preamp tubes lit at first and then fade to black?

Pins 1 and 6 are the plates of the 12AX7 tubes. Is there B+ on ALL of them?

Enzo, The 12ax7 tubes, all stay lit...in standby and play mode. I'd forgotten how to measure heater voltage. duh.... Sorry about that. Here are the voltage readings in play mode:

12ax7's
V1 / Pin 1 is 288 vdc
Pin 6 is 181 vdc
Heaters , measuring AC across pins 9, and 4/5 (4 &5 are tied together) 6.4 VAC

V2 / Pin 1 is 170 vdc
Pin 6 is 291 vdc
Heaters 6.4 vac

V3 / Pin 1 is 348 vdc
Pin 6 is 333 vdc
Heaters 7.5 vac

[Coop]

I just discovered that R16 (on my amp board) has 16 vdc on the one side (closest to inside of board) and no voltage on the Other end. Resistance reading across R16 is 4.1kOhm. But it’s marked 39 kOhm. I’m measuring the resistance in circuit with one end of the parallel disc cap lifted.

Update. R16 is fine. Removed it out of circuit. It checked good. 38.8 kOhm

[Coop]

[mozz]

You have heater voltage on the other 2 tubes?

[Jon Snell]

R16 looks fine, no discolouration to the brown paint, however R34 looks very hot and a possible suspect instability.

[Coop]

R16 looks fine, no discolouration to the brown paint, however R34 looks very hot and a possible suspect instability.

R34,R35 are marked 470 ohm, They measure 473 ohm, and 468 ohm. A-ok I think.