1986 GK 400RB schematic

[mitsu]

Hope someone can help me. I have a 400RB manufactured in Jan of 1987. I have seen some schematics here that call for different bias settings. The 1982 schematic calls for a setting of 10mV across R26 + R27 but the 1992 Schematic calls for 5mV across those same resistors. I was hoping someone can clarify as to which I should go with.

[The Dude]

According to different schematics that I have (not sure if they relate to yours- there are several different versions of this amp), the output circuits are quite different. They have different emitter resistor values, different output transistors, etc. I would say that you need to compare the amp in front of you to the schematics and see which matches yours. Then, set bias accordingly.

[mitsu]

Thank you, I am comparing between page 11 and page 13 of the attached service manual. Output transistors appear the same. I cannot easily spot any other differences but I'm sure there probably is. Can you take a quick look for me please?

[mitsu]

Here is Page 13

[The Dude]

Your link in post #19 doesn't work. It says "invalid file specified"

Edit: In the service manual I have it says to measure 5mV across EACH resistor in the bias procedure instructions. Then on the schematic it shows to read 10mV across BOTH resistors. Possibly this is the confusion? It's pretty much the same procedure either way assuming the amp is working correctly and both sides of the push pull have the same current. It's just that one method measures each emitter resistor separately and the other method measures the voltage drop across both.

[mitsu]

Your link in post #19 doesn't work. It says "invalid file specified"

How about this?

[The Dude]

See my post #20 edit.

[mitsu]

Your link in post #19 doesn't work. It says "invalid file specified"

Edit: In the service manual I have it says to measure 5mV across EACH resistor in the bias procedure instructions. Then on the schematic it shows to read 10mV across BOTH resistors. Possibly this is the confusion? It's pretty much the same procedure either way assuming the amp is working correctly and both sides of the push pull have the same current. It's just that one method measures each emitter resistor separately and the other method measures the voltage drop across both.

I see what you're saying. That makes total sense now. Thank you!!
My problem is sometimes I don't pay attention to those small details. Need to rewire my brain. lol