Krell KSA-80B Startup problem

[jhebass]

Hey...

I've got very little experience with solid-state amps, but a fair amount of experience refurbishing and modding tube circuits. Anyway, just looking for some troubleshooting advice for my monstrous Krell amp.

The KSA-80B has a soft-start circuit...basically, you hit the power switch...wait a few seconds and you hear a relay click...at this point the amp is operational. My amp either never trips the relay (thus not working) or trips it, plays for a few minutes, than shuts back down...

Any ideas where to begin? Don't know much about the soft-start circuit...I'm loathe to ship this anywhere, as I can barely lift it and am excited to explore the circuit.

Thanks.

 

[computerwala]

The Krell soft-start circuit is quite simple and can be diagnosed easily. First check out that the Optocouplers (white six pin DIP IC's) and American Zettler Relays DPDT are OK. They should be (if the unit is functional). But do check for dry solders. Then check to see if there is heat damage to the 470uf 100V capacitors sitting next to the brown 700 ohm Power resistors. These should be replaced with high-temp equivalents if defective/browned out. Then test the MJE15030/31 transistors for any dry solder or heat damage. Retighten the heat sink on these if loose. Finally, check out the Diodes and resistors for rating (within 5%) and functionality. That should take care of the soft-start board. If the issue remains, you then need to check the voltages to see what is varying and why. It could be that there is something else tripping the circuit e.g. overheating transistors (due to a loose heat-sink connection).

 

[Dr.H]

Hi computerwalla,
sounds like you know these amps well.
Can you give guidelines on how to convert the ksa80 to a kma160?
Also, if the amp is starting to shut down due to some dc on the output (which occurs anywhere from 5min after switch on to 15 hours after switchon!)
Any good mods you can suggest?
I found that adding a constant current source to the zener diodes (which create 75V Vref) lowered the noise floor considerably!

Thanks for ideas
Ryan