Krell KAV 250a deader than a dodo - diyAudio
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Old 11th February 2007, 11:55 PM   #1
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Unhappy Krell KAV 250a deader than a dodo

Hi everyone this is the first time i've posted hope to start an interesting project the kav250a i've recently got has 1 channel working and the other dead. 6 of the krell output transistors have gone! (wasn't me lol) and i've been on google all night looking up this amp and the bjt's which are motorola 9710/9711 (npn/pnp) there are 7 of each type, a darlington driving 5 output devices tied with 5 x 0.33Ω emitter resistors for each half.

Basically this amp didn't cost anything - and i don't have the spare either to do a krell repair so how about using different output devices?

Any info on this would be greatly appreciated since i would rather find equivalents that will "work" plug+play than having to modify the driver board which tested ok (substituted in the other channel) and i'm not too worried either about trying to meet the original spec - the krell devices are high bandwidth is that their only feature?

Look forward to chatting to you knowledgable people i've got a fair amount of experience building high power amps for PA/disco and making speakers but this is probably the nicest bit of kit i'm likely to own!
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Old 12th February 2007, 03:52 AM   #2
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refering to an earlier post...

http://www.diyaudio.com/forums/showt...925#post883925

K-Amps says the output transistors are similar to MJ21195/96 is it alright to replace "all" of them (outputs) using the 4 good krell devices for the driver stage? Thanks a lot !
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Old 12th February 2007, 08:33 AM   #3
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Have you checked with a scope? Does the driver stage give a nice clean sine if you run it with a signal generator? If so, you should be good to swap the outputs. I'd swap the output resistors as well whilst you're at it, the short could have damaged them.
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Old 12th February 2007, 11:04 AM   #4
AndrewT is offline AndrewT  Scotland
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Hi,
MJ21193/94 are almost identical to the 95/96 types.

These are not widebandwidth with Ft=4MHz, but are good for the power they can pass reliably.

Did Krell use a Darlington transistor in the KAV?
or did you mean a darlington connected transistor pair?

When you swapped the board over, did the drivers go with the PCB? and only the output devices and Re were not swapped?
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Old 12th February 2007, 10:19 PM   #5
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Lightbulb update on krell service....

Hi, thanks for the replies and info guys I've been working on the amp this afternoon and had some success...I stripped it right down and cleaned the chassis first then did some minor work on the psu board (new mains fuseholder - someone had mashed it !) and then tested the psu on and off load (88.6 / 91.2VDC) it's ok.

I did some tests on the working channel, DC offset is fine 25mV but there is +/-1.2VDC on the driver outputs is this ok? It is working perfectly tho - had it running for a few hours into a 4Ω bass bin earlier.

The next stage is testing the dead channel, first the driver board like you suggest with a scope, I'm not sure how to go about this I use my PC for measurements normally don't own a scope....can I set up an attenuator from the +/- drive outputs and safely leave the output stage disconnected? I don't want to fry the amp (or my PC!) by overloading the line input......

Yes the drivers are the same as the outputs (9710/9711) and the drivers are in a darlington pair feeding the 5 outputs on each side. I've sketched the output board, the terminals marked "drive +/-" are the test points. According to the datasheet the max gate voltage is 5VDC but what sort of voltage can I expect here for attenuation purposes (i.e. connection to computer for measurement?)

Thanks a lot for your responses here is the sketch....
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Old 12th February 2007, 10:19 PM   #6
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Default error in diagram

doh! just noticed i've just drawn all the pnp transistors as npn's sorry about that
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Old 12th February 2007, 11:33 PM   #7
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Default Re: update on krell service....

Quote:
Originally posted by digitallake
Hi, thanks for the replies and info guys I've been working on the amp this afternoon and had some success...I stripped it right down and cleaned the chassis first then did some minor work on the psu board (new mains fuseholder - someone had mashed it !) and then tested the psu on and off load (88.6 / 91.2VDC) it's ok.

I did some tests on the working channel, DC offset is fine 25mV but there is +/-1.2VDC on the driver outputs is this ok? It is working perfectly tho - had it running for a few hours into a 4 bass bin earlier.

The next stage is testing the dead channel, first the driver board like you suggest with a scope, I'm not sure how to go about this I use my PC for measurements normally don't own a scope....can I set up an attenuator from the +/- drive outputs and safely leave the output stage disconnected? I don't want to fry the amp (or my PC!) by overloading the line input......

Yes the drivers are the same as the outputs (9710/9711) and the drivers are in a darlington pair feeding the 5 outputs on each side. I've sketched the output board, the terminals marked "drive +/-" are the test points. According to the datasheet the max gate voltage is 5VDC but what sort of voltage can I expect here for attenuation purposes (i.e. connection to computer for measurement?)

Thanks a lot for your responses here is the sketch....
I have noticed that some analogue scopes go for peanuts nowdays on ebay. Perhaps it would be worth getting one? Look for one near you so you can pick it up, as they are heavy and can be damaged. Also, you may want to see it in operation before you hand over your money.

Oh, and make sure it comes with probes!

If you are near Oxford you can borrow my spare Philips 25 MHz scope instead.

P.S: Just noticed: Cornwall. Not near Oxford!
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Old 13th February 2007, 11:33 PM   #8
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Default scope testing

Hi, thanks for the advice on getting a scope shame I'm miles from you thanks for the offer of borrowing one though. I've managed however to do some tests today and come to the conclusion that both the driver boards are fine. I've had one running but wanted to make sure it wasn't a fault with the driver section that caused the output devices to go short but all looks good it produces the same results with either board.

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Old 13th February 2007, 11:41 PM   #9
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Excellent. Looks like you're good to go with a new output stage then. Don't forget to change the resistors as well, and get some new isolation pads for the TO3s. Will you be redoing the other side to match?
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Old 14th February 2007, 12:43 AM   #10
K-amps is offline K-amps  United States
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I have just learnt that Krell also used some 484xxx prt numbers that are basically MJ1302a's. It's possible that the 97xx's are also MJ1302a's/pairs. The MJ1302's are 30Mhz parts.

ON does not sell these anymore but sells the TO-264 version MJL1302a/Pairs.

I am very sure that the 21193/4/5/6 will work, though I would keep the driver/ pre-drivers as is 97xx because of their better gain.

I have not seen a pre-driver, driving the next stage via an Inductor (L1/2 in your diagram)... is this because of high FT devices?

Yes I do remember Krell literature reading drivers are OP devices in this amp...

PS: What number Hex key did u use (or was it torx) to open up the heatsink to expose the TO-3's?

PPS: Krell had to buy 1 million OP devices from MOT to get exclusivity.... with those many devices.... one can see them being used a lot.

HTH...
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