KMA 200 output

[soidog2]

OK, I did something stupid and killed the output of a KMA 200 amplifier.

Need to replace some of the “Krell 484002” transistors.
Did some searching on the forum and found conflicting information.

Some older posts show “Krell 484002 = Onsemi MJ1302A”

Or : “Krell 484002 = MJ15025”

Can somebody let me know which is the correct one ?
Anybody has some for sale ?

Many Thanks

 

[AndrewT]

I think it unlikely that the ONsemi 1302 is correct.

Much more likely is the 15025, also available from ONsemi and their distributors.

Are the parallel output devices matched in any way?
Is there any damage further upstream?

 

[soidog2]

I think it unlikely that the ONsemi 1302 is correct.

Much more likely is the 15025, also available from ONsemi and their distributors.

Are the parallel output devices matched in any way?
Is there any damage further upstream?

Thanks for answering, unfortunately I don’t have the knowledge to answer your question.
Somebody else is doing the work, So far we are replacing the obvious.
Other than a few transistors , one of the two Dale RH-50, 50 watt 5 ohm 1% Power Resistors blew up as well.
The amp is one of four I am using to power a pair of Acoustat #8 speakers.
Can I mix and match output transistors on the damaged amp ? ( original Krell & 15025 ) Or do I need to change all 16 of them ?
Will It change it’s acoustic signature ?
Am I better off sending it to Krell for big bucks repair ?

Thanks Again

 

[wg_ski]

You are always better sending it off to get fixed if you don't know how to do it properly yourself. If you have to ask basic questions you don't know enough yet. That doesn't mean you can't practice on less expensive pieces - just not this one.

Any outputs that are in parallel should be replaced as a group - with devices from the same lot. Any additional matching depends on how anal you want to get about it, but just using the same lot/date coded parts is enough for basic safety.

 

[LineSource]

I was told by Krell service many years ago that a special test spec was used on Motorola MJ15024 & MJ15025 production transistors, which implied that generic MJ15024 & MJ15025 would not be an exact match.

I would:
0) measure the voltage across a couple emitter resistors on the good amp to determine the idle bias current
1) remove a NPN and PNP and make hfe measurements at 25ma and 1A and compare to new MJ15024 & MJ15025 specs / tests
2) if the match is close, I would replace only the defective output transistors with new MJ15024 & MJ15025 that measure close hfe, and leave the bias setting
3) if the match is not at all close, I would replace all output transistors on one channel with new MJ15024 & MJ15025 and reset the bias - same voltage across emitter resistor as-0
4) the Krell schematic shows a 1 Ohm emitter resistor, which is large, and hence allows good current sharing even with fairly large hfe differences
5) if the power supply capacitor date code is over 12 years old, I would accept the high cost to replace them in both amps. Leaky power capacitors often burn out the power-up relay board, which is very expensive.

 

[soidog2]

Thanks you all, you have to learn sometime !

 

[soidog2]

repair advice

I was told by Krell service many years ago that a special test spec was used on Motorola MJ15024 & MJ15025 production transistors, which implied that generic MJ15024 & MJ15025 would not be an exact match.

I would:
0) measure the voltage across a couple emitter resistors on the good amp to determine the idle bias current
1) remove a NPN and PNP and make hfe measurements at 25ma and 1A and compare to new MJ15024 & MJ15025 specs / tests
2) if the match is close, I would replace only the defective output transistors with new MJ15024 & MJ15025 that measure close hfe, and leave the bias setting
3) if the match is not at all close, I would replace all output transistors on one channel with new MJ15024 & MJ15025 and reset the bias - same voltage across emitter resistor as-0
4) the Krell schematic shows a 1 Ohm emitter resistor, which is large, and hence allows good current sharing even with fairly large hfe differences
5) if the power supply capacitor date code is over 12 years old, I would accept the high cost to replace them in both amps. Leaky power capacitors often burn out the power-up relay board, which is very expensive.

I kind of decided, to listen to your advice and take the opportunity to renew the amplifiers.
There are four of them , it’s going to be a considerable expense.

Personally I am over my head in doing it, so I have to look around.
I am curious as to the forums opinion ;

Are the KMA 200 good enough to be worth the trouble ?
I have been listening to them for many years , no complaints.

Can one you kind souls look at this repair/upgrade listing and tell me if any experience with it or what do you think ?

Any recommendations for the work ?

http://cgi.ebay.com/KRELL-MARK-LEVI...4|66:2|65:12|39:1|240:1318|301:1|293:1|294:50

Many Thanks

 

[LineSource]

Re: repair advice

Are the KMA 200 good enough to be worth the trouble ?
I have been listening to them for many years , no complaints.

From my listening experience, the Krell KMA 200 one of the smoothest sounding high current, high power amps. It is one of the best amps to drive very low impedance speakers, stable down to 0.2 ohms with no harsh clipping. Dan at Krell designed it to direct drive the 0.12 ohm Apogee midrange ribbon.

Replacing output transistors and power capacitors is pretty straight forward. A little help from a technical friend should safely get the job done. At 100+ lbs, it is attractive to find a local repair shop/expert. Local service often has better warranty support. Many large amps get damaged in shipping.

 

[soidog2]

Re: Re: repair advice

Dan at Krell designed it to direct drive the 0.12 ohm Apogee midrange ribbon.

Thanks Again !

I guess that's why my Acoustats #8 sound so good !

Regards !