Model 1200 - bias doesn't go above 0mV

[ahistoricist]

I just finished rebuilding the left amplifier channel of a 1200. I checked voltages connected to a DBT, and they all seem to be pretty close. Light dims, and when I connect it to a signal generator I get a clear, amplified wave in the output. DC is close to 0mv.

However no matter what I do I can't get a bias above 0.00mv across R531 and R532, when I should be looking at 12mv ideally. I know I am connected to the DBT but I expected at least something.

I checked the epoxy transistor, it seems OK with a hFE of 500. The germanium 1N542 diode is brand new.

I am at a loss. What could be preventing it from getting a bias?

View attachment 2198438

Voltages: at (4) +0.6V; (5) -0.595V

View attachment 2198447

Did you ever figure this one out?

I have a 250 on the bench with the exact same issue - everything seems to check out fine but I get 0.0mv bias. Spooky, and frustrating!

 

[ocdecio]

My 1200 had a destroyed board. When I implemented all service bulletin changes including replacing the bias potentiometer it finally worked.
I doubt yours would be that extreme.

One thing I would do is confirm the diode and epoxy transistors are good, and make sure the pot is working properly.

 

[ahistoricist]

My 1200 had a destroyed board. When I implemented all service bulletin changes including replacing the bias potentiometer it finally worked.
I doubt yours would be that extreme.

One thing I would do is confirm the diode and epoxy transistors are good, and make sure the pot is working properly.

Sure enough - I had replaced the potted heatsink bias transistor and swapped the blue and green wires.

Sometimes all I need is to take a break for a day or two and come back to see what was right under my nose all along.

These amps are so sweet - cheers to a job well done.

 

[ocdecio]

Awesome! The 1200/1200B are the best sounding amps I have.

Well done!

 

[steve_it]

My 1200 had a destroyed board. When I implemented all service bulletin changes including replacing the bias potentiometer it finally worked.
I doubt yours would be that extreme.

One thing I would do is confirm the diode and epoxy transistors are good, and make sure the pot is working properly.

Hi, sorry if I write in this old post, in my 1200 in the sm as bias trimmer it reports 1k, but if I remove the wire trimmer I measure about 300 ohms at the ends. Do you know what the right value is? also, they are wire rheostats, what did you replace them with?