Krell KSA250 large channel imbalance

willcycle

Member
2003-10-22 1:51 pm
MD
I measured the right channel of my Krell KSA250 as having a gain of 16 dB when it should be 26. The left channel measures fine. A 1 KHz sine wave going through it seems to look fine. I suspect I should check the feedback resistor but I have no idea which one it is. Anyone know or has any suggestions? I recently performed a recap on it but the problem was there before.

Thanks,
Will
 
pls post schematics

Perhaps it is close to the KSA50? Some indications in another thread I see.

I have no component reference other than a resistor in the area about 12KOhms, and a 470Ohm in series with this ending in a capacitornetwork with two electrolytics in series turned polarities, and a filmcap parallell to theese.

Theese five components should be the feedback.

Values of theese may differ.

But I have no schematics to back it up.
 
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Zero Cool

Member
Paid Member
2004-09-20 6:10 am
MN
yup you have an input stage problem! the protection circuit is probably seeing a dc shift when the input pin is removed. something is drifting and when a large enough DC offset is present the relay kicks in.

My KSA-250 has a faint crackle in one channel as it is warming up from cold so I am probably headed towards the same problem!
 

willcycle

Member
2003-10-22 1:51 pm
MD
The problem was a transistor in the pre-driver stage (WPS42). I replaced all the transistors in this stage including the WPS92s. All is well now.

As the amp warms up I could see some fluctuation in the DC offset on both channels where it would jump momentarily by perhaps 40 mV positive or negative. Both channels exhibit this behavior though not in a synchronized manner. Sometimes one channel will fluctuate, then the other, and sometimes both at the same time. I wonder if anyone else with a version 3 KSA-250 can study the offset over time to see if your amp behaves the same.