KA8100 Repair

[DrFesseden]

When I got this amp it was completely dead. Traced the problem to the control board as signal was getting through both the power amp board and the preamp board. A cap on the control board was why the amp as a whole wasnt functional. Did a Complete recap an all three boards and sounds pretty good now.

Have a problem with one transistor hot to the touch. All other transistors do not get warm besides Qe21. With no signal this transistor us slightly warm, with moderate volume with signal it's hot to the touch. At first I suspected Re73 since it measured 2.2ohm. Once I removed it I realized it appeared to be a replacement and measured exactly as marked. I went ahead and replaced it with a 4.7ohm resistor but that transistor still gets hot.

Any suggestions on where to look next. All voltages seem right on Qe21 and emitter resistor measures fine.

I've attached a picture of the power amp section in question and the schematic.

 

[petehall347]

qe23 cool ?
bias and offset ok ?

 

[audiomaniac2]

Measure voltage across re47 and then re49. should be the same. Measure the voltage across re63 and then re65 should be the same.
It's interesting that schematic reads re63 150 ohms and re65 reads 330 ohms. Seems like they should be the same. Also measure voltage across re73 and re75

 

[gslikker]

Measure voltage across re47 and then re49. should be the same. Measure the voltage across re63 and then re65 should be the same.
It's interesting that schematic reads 150 ohms and re65 reads 330 ohms. Seems like they should be the same. Also measure voltage across re73 and re75

Indeed. Voltage over RE73 and RE75 reveils the current needed for the output transistors. If too much over RE73 then one can conclude HFE of the output transistor QE25 is low or bad (or even has its collector open circuit by loose connection from the power to it)