krell KMA 200 amp

[technoman69]

Have a kma 200 krell amp running cool. It has 2 trim pots at the back left side. One for dc offset and the other for biasing current. Don't know which one does the bias setting? Original owner. Amp was in storage for 18 years. The other kma 200 amp runs warm. ( normal ).sounds ok but don't know if it's running in class a. I don't have a scope. Every thing looks OK no leakage or swelling inside. But I think the bias trim pot needs adjusting. Don't have any service manual. Have contacted krell service center. No reply yet. Any suggestions as to how I should proceed?

 

[rayma]

Have a kma 200 krell amp running cool. I think the bias trim pot needs adjusting.

The schematic seems hard to find for this amp. Try measuring the voltage across the emitter resistors
in the warm amp, and see if you can adjust the cool amp to be the same.

 

[technoman69]

Ok, thanks I will try that.replying to rayma suggestion.

 

[AVWERK]

Put 2 DVM,s side by side if you don't know which TP is what and adjust one of them slowly and notice the reaction.
One connected to any emitter and the other to the output terminals
I would replace all the electrolytics
DCO can hunt around sometimes and swing -to + near 0 mv

 

[AVWERK]

The DCO trim will probably be near discreet NEC j44/163 trannies if I'm not mistaken on this model

 

[technoman69]

Reply to avwerk. Thank you so much. Very helpfull. Will take your advice.
regards.

 

[technoman69]

Update on cool running kma 200 krell Amp. Adjusted bias voltage across emittor resistors to 260 milli volts there are 4 on each output board total of 8 - 1 ohm. Could run it up to 650 milli volts but not on such a vintage amp with out extensive servicing. Amp is running very warm not hot. Sounds ok. Sound stage and presence good. Dco almost perfect. Bias trim pot is facing up in middle of left driver board.no problem to adjust. 1/4 turn to left then right. Check voltage across resistor and then adjust to right another 1/4 turn.
checked voltage holding at 260 millivolts.

 

[technoman69]

Editing last post. The KMA 200 has 16 output transitors . 8 ea on each heat sink.

 

[RobertS61]

Krell KMA-200

Update on cool running kma 200 krell Amp. Adjusted bias voltage across emittor resistors to 260 milli volts there are 4 on each output board total of 8 - 1 ohm. Could run it up to 650 milli volts but not on such a vintage amp with out extensive servicing. Amp is running very warm not hot. Sounds ok. Sound stage and presence good. Dco almost perfect. Bias trim pot is facing up in middle of left driver board.no problem to adjust. 1/4 turn to left then right. Check voltage across resistor and then adjust to right another 1/4 turn.
checked voltage holding at 260 millivolts.

I've had a pair of Krell KMA-200 Mk2's in the nineties and just like the Rowland Model 7's I once had these amps seem to never run out of power and always control the loudspeaker no matter what the SPL.

With 260 mV over the 1 Ohm emitters you no longer have a 200 Watts Class A amp but a (16 x .260A)².8Ohm/2 = 70 Watts Class A amp.
Nothing wrong with that I did the same with the KSA-100 Mk 2 I have acquired recently, turn down the bias to a 30 Watts Class A value.

Great amp though maybe you can share some highres pictures of its innerlife.
Audio- and Regulatorboard would be interesting.

 

[technoman69]

KMA 200 Krell amp biasing

Thanks for that info. I knew I had them detuned , just wasn't sure about what equation to use to calculate final output.

 

[RobertS61]

Thanks for the pics!
I know that you have to unscrew and screw in 20 screws and the screwdriver seems to slim away easily. Took me about 20 minutes to get the hood on and off again.
I see you have one of the Mark 1 versions with the fans blowing downwards but with a Avel Lindberg toroid. The first vesions had 'E-Core' transformers like these.http://www.diyaudio.com/forums/attachment.php?attachmentid=599058&stc=1&d=1486998116
Final output will still be same around 280 Watts @ 8 Ohms but the 'Class A' portion will be lower then initial that's all.
I have a Krell brochure called 'The mono group' which features also the KMA-200 in the incarnation you have. If you do not have it yet I can make scans for you if your interested.

 

[ticknpop]

Looks like the KMA 200 capacitors are original. When it's time for a recap (soon I suspect) consider changing the bridges to IXSY soft recovery diode bridge with a C and RC snubber across the AC connection to the bridge. I got rid of the original style filter caps , went to Panasonics and got 700,000 uf in an LCL confuration in the same space - about 1/10 the original ripple voltage.

Sure do like the KMA 200s though, I 've got KSA 100, KMA 100 , and KSA 250, KST 100, KBL, etc etc etc. Old Krells never die, they just end up at my house...........................

 

[RobertS61]

Looks like the KMA 200 capacitors are original. When it's time for a recap (soon I suspect) consider changing the bridges to IXSY soft recovery diode bridge with a C and RC snubber across the AC connection to the bridge. I got rid of the original style filter caps , went to Panasonics and got 700,000 uf in an LCL confuration in the same space - about 1/10 the original ripple voltage.

Sure do like the KMA 200s though, I 've got KSA 100, KMA 100 , and KSA 250, KST 100, KBL, etc etc etc. Old Krells never die, they just end up at my house...........................

So, give me back my KMA-100's!

Love to see some highres pics of your KSA-100 or KMA-100 new PSU with the 700mF and snubbers.
I have a Krell KSA-100 Mk2 and just ordered some old style RIFA PEH200 63V and 0,1F per cap. Only 400mF 300 short...