Krell KSA 80 Repair

[KweeSong Lim]

Last month, received another amplifier repair from another friend of a friend.

Coincidentally it’s another Krell Amplifier. This time it is a KSA80.

The owner reported smoke coming out from one channel.

 

[KweeSong Lim]

The date code 8852 on the capacitor suggests that it is manufactured in 1988 so amplifier could be as old as 36 years.

 

[KweeSong Lim]

The right channel is faulty and it can be seen the driver transistor module appears burnt

 

[KweeSong Lim]

Both driver modules from the right channels are burnt

 

[KweeSong Lim]

Emitter resistors of the driver transistors are burnt which usually means the driver transistors are damaged, likely shorted which will translate also to likely output transistors shorted which will result in driver transistors burnt.

 

[KweeSong Lim]

Testing the driver transistors confirmed they were shorted.

 

[KweeSong Lim]

Damaged original driver transistor, MJE15030 and MJE15031. A picture of the new transistors now it’s ON semi as Motorola Semi parts are discontinued.

 

[KweeSong Lim]

The damaged and the Replacment transistor

 

[KweeSong Lim]

Dismantle the heatsink assembly to remove the output transistors for testing. The transistors’s metal casing are mostly rusted due to Singapore humid weather and also probably the amp was left idle for many years since becoming faulty

 

[KweeSong Lim]

Those marked with a cross are damaged (shorted) that resulted in blowing up the driver transistors and their emitter resistors

 

[KweeSong Lim]

Total 6 (50%) of the 12 output transistors were shorted.

 

[KweeSong Lim]

This is the layout of the damaged driver board. Part of the traces was completely fused so jumpers need to be installed in place to get it working without having to fabricate new PCB which would be a lot more costly approach.

 

[KweeSong Lim]

All four driver transistors were blown and replaced

 

[KweeSong Lim]

Jumper trace installed I place of fused copper trace

 

[KweeSong Lim]

Installed just a single good output pair of transistors to test out if the repaired driver boards will fix the problem

 

[KweeSong Lim]

The repaired driver boards did not fix the problem. It was still drawing very high output current. I powered up the amp through a variac transformer and could see the amp drawing excessive current even before it reaches maximum line voltage showing there is still a fault.

 

[KweeSong Lim]

Problem had to be further up the signal path. Likely in the output bias current section. Here is traced out the bias current circuit.

 

[KweeSong Lim]

Found three more damaged components in this section. Q20,Q21 and R37 (trim pot).

 

[KweeSong Lim]

Faulty transistors and trim pot replaced

 

[KweeSong Lim]

These faulty parts replacement fixed the problem. The output assembly is populated with new output transistors to replace the faulty ones. I did advise the owner to replace all the output transistors but the owner did not want to spend additional parts cost to replace the rusty old parts as they are still working.

I actually advised him that it’s better to replace with all new matched transistors rather than just replacing the faulty ones which will not result in matched output transistors.