Reverse engineering Krell KMA 160

[jwb]

I was able to acquire a dead Krell KMA-160 for $0. The driver board of the amplifier seems to have set itself on fire. All the components in the vicinity of the last driver stage have been destroyed, including the transistors, several caps, and a few resistors, one of which is merely scorched, the other if which actually exploded. In addition, a diode in a power supply snubber -- the driver board has a power supply separate from the main power supply -- has exploded. Scary stuff indeed.

What's particularly interesting is there's nothing special about this amp. The topology is unremarkable and the quality of construction screams DIY. In fact the build quality of my Aleph-X monoblocks is, IMHO, quite a lot better than this thing.

Of course, the Krell precedes my efforts by 15 years

I hope to get photos and schematics up soon. So far I don't see any exotic parts. There's plenty of MJE 15030/15031 driver transistors, as well as A968/C2238 (these are not marked 2SA/2SC but they do have a great big capital "T" printed on the case. Are they Toshiba?) The small-signal BJTs are A970 & C2240 like you might expect. The power supply seems really basic, but I'll report more after I've diagrammed the whole thing.

Oh, and on the back of the board is the gross rework hack. Eww.

Anyone else have experience repairing, or improving, this unit?

[jwb]

PS, the output stage consists of 24 MJ 15024 transistors. I hate metal cans, it feels like working on a first-generation C-130.

[X.G.]

Originally posted by jwb
There's plenty of MJE 15030/15031 driver transistors, as well as A968/C2238 (these are not marked 2SA/2SC but they do have a great big capital "T" printed on the case. Are they Toshiba?)

Yes.One type of Janpanese transistor generally only produced by one Janpanese manufactory ,unlike in USA,Europe...etc

[Zero Cool]

I am VERY interested in seeing the final schematic and any photos you take.

Yes these early Krells like the 80's the 100's and 160's were all hand built if i am not mistaken. Krell was very new at that time. 1984 i think. but then again, look at the very early Levinson stuff as well, they had MFG problems as well.

BUT, Dan D Agostino is still the king of SS amps in my book! I dream of owning a KSA-250 some daY! Droool!!!


Zero

[anatech]

Hi jwb,
There is nothing wrong with the type of outputs used. Just make sure they are matched.
X.G. is correct in stating that Japanese transistors only have one manufacturer per type number. Use real parts, do not go anywhere near ECG's & NTE's. At least you will know what is in your hand for sure. Also, don't use any sub manuals for cross referencing to a different part #. The ECG/NTE manual is known as "the book of lies" up here. For good reason.
The fire is normal when the unit has been struck by technician. Sometimes, amplifiers just die that way. I had a Bryston 4B that went that way, burned a hole in the PCB. The guys at Bryston said this was impossible, their amps never fail. Right.
So, take your time and be careful. Use a variac so you don't lose anything else.

-Chris

[jwb]

Okay, now that I've figured out which transistors are B-C-E and which are E-C-B I can get to drawing the schematic

I think the power supply could use some improvement. Diode technology has truly improved in the last decades.

[jwb]

I'm almost done with the schematics. Here's a picture I took of the damaged board. Everything to the right of the molex connectors is the regulated power supply. There's two molex connectors because this same board is flipped and installed on the other side of the amp in the stereo model. For the same reason there are empty pads for a trimmer.

Note the scorched transistor and the molten capacitor. I think the upper transistor is a 2SA968, but I can't tell. I'm guessing because it's complementary to the 2SC2238 below it.

The input comes in on the left from the XLR on the rear panel. Note how the amp is DC-coupled and there is no input protection whatsoever. The gates of those poor FETs (2sk163, anybody have a datasheet?) are just asking to get blown up.

[Zero Cool]

I sure wish i could "Aquire" some dead Krell amps!!!! what a find! I sure hope you bought the person that you got those amps from at least a case or 2 of guiness!!!!

Anyone else want to off load any old Krell, Levinson, Threshold, Classe, Bedini, Sumo, etc etc. power amps for "$0.00" Dollars or very little dollars PLEASE let me know!!!

I will make sure your favorite beverage truck makes a stop by your house!!!!!

And Mondo thanks to JWB for enlightening us with this privvied info! Now if i could just get the schematics for a KSA-250, i would be in heaven!!!!

JWB, please keep going!!! im dying to see those schematics!!! Thanks for the photo. What a great piece of art, you should call it "Amp PCB scorched by fire"

And Please tell me that you plan on rebuilding these babies...right?

Zero

[dimitri]

2SK163 N-FET 50V 0.03A 0.4W
J113 is full analog
can find it on
www.semiconductors.philips.com
www.fairchildsemi.com
www.onsemi.com

[jwb]

OK, thanks dimitri, this is the datasheet I've been using. I wasn't sure if the sheet for k163 was still available.

PS What's with all these websites that want you to pay for old datasheets? It's a scam!