Sweet Linda came home today.

[Oldskoolfan]

I seem to recall a substitution list for the obsolete transistors on the control board. Anyone got a link?

 

[Oldskoolfan]

Well, it seems to be working again. Looking at the schematic, I started with Q7 and all the voltages were unstable. I pulled it and tested it. All seemed good. Powered up and found I had no wild voltages. I noticed the bias transistors were dissimilar so I replaced them with 2n5088s. Put Q7 back in, powered up and....it works. The issue I have now is bias voltage can't be adjusted below 500 millivolts either channel. Do I need to go to a 10K trim pot?

 

[Skratch]

Change D6 to a Bat 41 diode and it will bring your bias into the correct range with plenty of adjustment

 

[Oldskoolfan]

Change D6 to a Bat 41 diode and it will bring your bias into the correct range with plenty of adjustment

I'm on a learning curve here. Why change to a Schottky diode? Also why not D6, D7, and D8?

 

[Skratch]

Lower forward voltage. I had your bias problem and Joe suggested changing D6 to the bat41 and it worked perfectly. My bias pot is in the middle now

 

[Oldskoolfan]

Fair enough!

 

[Skratch]

Watch polarity and only D6

 

[Oldskoolfan]

Watch polarity and only D6

Copy that. Ordered the diodes and some fresh caps today. Been running the amp all day with no pops, clicks, or scratchiness.

 

[Oldskoolfan]

I think I made a mistake. Seems I ordered Nichicon VP non-polar electrolytics to recap the control board instead of FG caps.
Is this going to cause issues? Caps are cheap enough, I can reorder if there is a big issue here.

 

[J!m]

I would expect ANY fresh cap is going to sound better than the best 50-year-old cap.

 

[Skratch]

You should change C6 to 470uf and put back to back diodes across it 1n4148

 

[Oldskoolfan]

You should change C6 to 470uf and put back to back diodes across it 1n4148

This is the mod to prevent the amp from going into oscillation?
I remember reading about this.

 

[Skratch]

Yes, also helps with bass response

 

[Gepetto]

This is the mod to prevent the amp from going into oscillation?
I remember reading about this.

The diodes prevent the early board design from going into latch up.

 

[Oldskoolfan]

You should change C6 to 470uf and put back to back diodes across it 1n4148

Got a pic of how you installed the diodes?
Not a lot of room on the back of the board.

 

[Oldskoolfan]

Well, I ordered the 470uf caps and the 1N4148 diodes. I started reading through the tips and archives. Discovered people are recommending getting the 470uf cap in 50 or 100v rating. I ordered 10v because the drawing shows C6 rated 100uf, 6.3v. Do I need to reorder?