Need help - bias on vintage amplifier SAE Mark IV DM

[JHint]

Hello amazing community, I’m posting today with a request for help with my SAE Mark IV DM Amplifier -

Won at an auction with the matching pre-amp and a dead right channel. The dead channel was solved by identifying FIVE of the eight fuse connectors being loose or broken, and I replaced all the electrolytic capacitors while inside.

I would like to set the bias but don’t even know where to start - I can’t find a manual online but do have an oscilloscope (that I’ve never used).

Is there a way to hook the scope up for this, if so where would I do that? How do I know what to set it to? Up to this point I’ve only used manuals with clear instructions on calibrating bias and offset

Many thanks for any help -

 

[hamrules]

DC Offset closest to 0.
Quiescent Current to 1.24volts across the 62ohm resistor (R24/R25).
Bias to 2.1volts from collector to collector of Q9 & Q10.

This is from the service manual. Also to note it calls out for the QC to be 1.7volts but that's not correct as the schematic shows it being 1.24volts. There have been a few errors in the SAE "manuals".

 

[JHint]

DC Offset closest to 0.
Quiescent Current to 1.24volts across the 62ohm resistor (R24/R25).
Bias to 2.1volts from collector to collector of Q9 & Q10.

This is from the service manual. Also to note it calls out for the QC to be 1.7volts but that's not correct as the schematic shows it being 1.24volts. There have been a few errors in the SAE "manuals".

This is incredible - thank you!! Where did you find the service manual? I might not even need it anymore with this great help - I’ll update again when I finish rebuilding the pre-amp and have the pair ready to go together again!

 

[llwhtt]

I have the SM if you want it. Start a conversation with me and include your email.

Craig

 

[ivandezande]

Hello amazing community, I’m posting today with a request for help with my SAE Mark IV DM Amplifier -

Won at an auction with the matching pre-amp and a dead right channel. The dead channel was solved by identifying FIVE of the eight fuse connectors being loose or broken, and I replaced all the electrolytic capacitors while inside.

I would like to set the bias but don’t even know where to start - I can’t find a manual online but do have an oscilloscope (that I’ve never used).

Is there a way to hook the scope up for this, if so where would I do that? How do I know what to set it to? Up to this point I’ve only used manuals with clear instructions on calibrating bias and offset

Many thanks for any help -

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I’m worried about the Sharpie marks and what the fuses look like….did you make sure they’re the correct rating? I’d put all new fast acting fuses in there. Acetone will take off the Sharpie.

 

[JHint]

I’m worried about the Sharpie marks and what the fuses look like….did you make sure they’re the correct rating? I’d put all new fast acting fuses in there. Acetone will take off the Sharpie.

I added the sharpie myself as a backup to reattaching them 1-4 down on each side to correspond with a before picture, to ensure I hooked it back up correctly - other than looks, is there are reason I should remove them now that the new caps are in and it’s all reconnected and working right?

 

[JHint]

I have the SM if you want it. Start a conversation with me and include your email.

Craig

Will do! Many thanks -

 

[ivandezande]

I added the sharpie myself as a backup to reattaching them 1-4 down on each side to correspond with a before picture, to ensure I hooked it back up correctly - other than looks, is there are reason I should remove them now that the new caps are in and it’s all reconnected and working right?

Why? As long as red goes to red and gray to gray, it doesn’t matter.
Yes, I’d say it’s important to have the right type of fuses installed. Unless you like house fires.