Sovtek MIG 50 Losing Sound After Standy Off

[Helmholtz]

V1 pin 3 resistance to chassis ground is 4.6 kOhms.

That's too high. Should be 1k according to schematic.
What is the color code of the cathode resistor R19 ( connecting between pin 3 and ground)?

[Coop]

R19 on my amp board is marked 470 ohm. I think V1 cathode resistor is my R3 (yellow, indigo,red) next to the 220uf radial cap. My R3 measures 4.6k across it.

[Helmholtz]

R3 looks like yellow - violet - red, i.e. 4.7k (4.6k = yellow - blue- red would be very unlikely). Don't see where it connects.
Use your Ohmmeter to trace where a component connects. Zero Ohm means connection.

A cathode resistor that high is very uncommon and causes cold biasing (I would use 1k to 1.5k). Maybe not original.

Try to find the plate resistor (connecting from V1 pin 1 to node D). Measure its value - and the voltage drop between its ends when powered.

Again: Does input/channel 2 work?

[Coop]

I changed v1 cathode resistor to 1 kOhm. I now have sound all the time, but it’s low level (weak).

V1 plate load resistor is 100k. And measures close to that , in circuit. It’s voltage drop is 1.7 vdc.

Input 2 has nothing but a low level hum. I’m pretty sure someone has botched the input jacks connections too. This board has been slaughtered!

[Coop]

[Helmholtz]

Ok, now measure resistances from V1 pins 2 and 7 to ground.

[Coop]

V1 resistance to ground
Pin 2 is 316 kOhm
Pin 7 is 0 - so that’s why input 2 doesn’t work!
Guess it’s connected in the wrong spot?

[The Dude]

You should have about 64K from grid to ground with the input jacks empty (shorted). That is the parallel resistance of the 68K and the 1M resistors at the input. Both of your measurements are off with the schematic.

[Helmholtz]

V1 resistance to ground
Pin 2 is 316 kOhm
Pin 7 is 0 - so that’s why input 2 doesn’t work!
Guess it’s connected in the wrong spot?

No, that's not your problem.

With nothing inserted to input jacks, pins 2 and 7 should each read 68k to ground, because the input jacks have shorting contacts.
Make sure those contacts work.
I guess you need to trace out the input jacks' wiring.

I actually expected infinity resistance (meter reading O.L.), as that could explain your problem.
That's still possible if tube pins don't make contact to socket contacts.
Does wiggling V1 in its socket make any difference?

[The Dude]

You should get about 64K resistance from socket grid pin to ground whether the tube is installed or not, if the inputs are wired correctly and resistors are good and values are correct.

[Helmholtz]

You should get about 64K resistance from socket grid pin to ground whether the tube is installed or not, if the inputs are wired correctly and resistors are good and values are correct.

Yes, 68k from each grid socket pin to ground - if jacks are correctly wired and jack switching contacts are good.
But the very low V1a plate current makes me suspect its grid is floating, i.e. has no grid leak path.