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Old 6th September 2007, 11:59 PM   #1
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Maybe I am posting in the wrong section, but I saw that you, Chris , are the lucky owner of a KSA 250 schematics. I am the very bad lucky owner of such a monster, one channel dead. Probably, my IQ is a bit too low, since I had to replace some drivers (MJF15030/15031) and a Zenner 3 times allready, and now, after this third attempt, it worked, finally. Jus for 30 min... Than, 60V DC on my Martin Logan ReQuest. And, to make everything perfect, the protector didn`t kick in. No oscillations, no apparent reason. You, Chris, must know what it means to disassamble EVERYTHING 3 times. Please, pleeeeeaase, send me a copy of the schematics at gsm-club@usa.net Actually, your phone number would be a very big help
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Old 7th September 2007, 01:41 AM   #2
anatech is offline anatech  Canada
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Hi highend-madness,
I started this new thread for you. More chance of others helping out as well.

I replied to you.

-Chris
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Old 7th September 2007, 02:15 AM   #3
PB2 is offline PB2  United States
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Quote:
Originally posted by anatech
Hi highend-madness,
I started this new thread for you. More chance of others helping out as well.

I replied to you.

-Chris
I could use the schematics also, as I know of a basket case KSA-250 in need of repair.

Now this one is worth fixing, needs a power transformer and front panel. I'd guess it was a donor for parts.

Pete B.
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Old 7th September 2007, 04:01 AM   #4
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That was quick! Thanks, Chris!
Now, hello, everybody!
Initial fault, discovered and fixed, on the pream board, under one of the capacitors, the PCB was damaged, maybe because of the glue, as Chris says. Atached, a relevant picture. The capacitor, all of them, actually, are ok. Even checked the ESR, I have a nice gadget doing the job.
Turned on my amp, checked with a scope, no oscillations, the DC offset : 40 mV on the good channel, 75 mV on the one that had a problem. Temperature, identical on both channels, so, lets`s rock!
Half an hour later, my woofer from my ReQuest is gone, 60V DC on the output, protector not doing the job.
Turn it off, turn it on, protector works... A bit late...
So, based on your experience, what could be the cause? Could be:
- a capacitor not working ok at 60-80V?
- a transistor, looking 100% ok on the tests, performed at 3-4V, but not good at full power/voltage?
For those of you who never disasambled a Krell, I am telling you, it`s a lot of fun! I had to bring half of my tools from the garage..
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Old 7th September 2007, 04:49 AM   #5
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Quote:
Originally posted by highend-madness
That was quick! Thanks, Chris!
Now, hello, everybody!
Initial fault, discovered and fixed, on the pream board, under one of the capacitors, the PCB was damaged, maybe because of the glue, as Chris says. Atached, a relevant picture. The capacitor, all of them, actually, are ok. Even checked the ESR, I have a nice gadget doing the job.
Turned on my amp, checked with a scope, no oscillations, the DC offset : 40 mV on the good channel, 75 mV on the one that had a problem. Temperature, identical on both channels, so, lets`s rock!
Half an hour later, my woofer from my ReQuest is gone, 60V DC on the output, protector not doing the job.
Turn it off, turn it on, protector works... A bit late...
So, based on your experience, what could be the cause? Could be:
- a capacitor not working ok at 60-80V?
- a transistor, looking 100% ok on the tests, performed at 3-4V, but not good at full power/voltage?
For those of you who never disasambled a Krell, I am telling you, it`s a lot of fun! I had to bring half of my tools from the garage..

Looks more like spark/fire damagae....
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Old 7th September 2007, 06:51 AM   #6
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Yes, it looks so, but there was no spark or fire. More than that, the capacitor was ok.
I forgot to mention that the PCB has 4 layers...
Still, the cause is not the PCB any longer. Like I said, it worked for a half an hour after cleaning the remaining glue and soldering the capacitor back, after exchanging the faulty parts on the power PCB
Any ideea, what`s the bias setting for this model?
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Old 7th September 2007, 06:47 PM   #7
anatech is offline anatech  Canada
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Hi highend-madness,
It looks as though a capacitor vented at some point in time. Either that or it's a via (yipe), brought out under a component! I can't see well enough.

Quote:
Any ideea, what`s the bias setting for this model?
If it's not listed, just measure the other channel and make it the same. You could always give Krell a ring and ask.

Quote:
So, based on your experience, what could be the cause? Could be:
Nice, 60 VDC offset! Now you know why I am so set on output protection circuits.

A cracked solder joint or trace could do this, they would tend to be intermittent.

-Chris
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Old 7th September 2007, 10:43 PM   #8
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Default schematics

I have checked the pcb`s three times allready. When I work, I do it well. I have cleaned it very well, so, I wouldn`t miss an intrerupted part. Not to mention that they have a lot of solder over the copper..
Thanks for pointing that to me.
I think that beneath that capacitor was some liquid that was spilled over time, I can`t figure it out, but the capacitor measures its marked value, tested with a FLUKE 124 and with another dedicated device, ATLAS ESR. And it shows no signs of leakage...
I will order new ones (wher from? Low ESR, high temp, LOW PRICE

To Audioman and Pete B. :
I got your emails, but I am waiting for an aproval from Chris to forward the info received from him...
I don`t want to upset you, I will be more than happy to help you, I know since when I am looking for it, but also, I don`t want to upset Chris. It is from him, so, I have to have his permission..
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Old 7th September 2007, 11:04 PM   #9
anatech is offline anatech  Canada
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Hi highend-madness,
The point is that a cracked solder joint or foil is next to invisible. Some of the most difficult problems to find. Normally you eliminate all the parts and this may be the only scenario left.

Capacitors age, so are better to be replaced if at all in doubt. They can leak some fluid and still measure okay. If I see any fluid under a capacitor, it's gone. Over here we buy from Digikey, Mouser or Newark (Farnell). The parts are good and the price is not that bad. I never buy from a less expensive place if I have any doubts about where the parts come from. Any caps these outlets sell are probably good. Philips (BC Components) would be the worst (and the least expensive). There are really bad capacitors that you can buy, BC Components are better than those by far. Beware the no name part. All new caps are lower ESR. Use a higher voltage rated part as the new ones are too small and measure less good than the originals. The higher voltage parts are about the same size and measure about the same for ESR and inductance. Hmmm, how about that?

Those diagrams are not for general release, sorry.

-Chris
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Old 7th September 2007, 11:13 PM   #10
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Quote:
Originally posted by anatech
Hi highend-madness,
It looks as though a capacitor vented at some point in time. Either that or it's a via (yipe), brought out under a component! I can't see well enough.

Nice, 60 VDC offset! Now you know why I am so set on output protection circuits.

A cracked solder joint or trace could do this, they would tend to be intermittent.

-Chris
Yes you are probably right about the capacitor Chris. If it was ok it shouldnt look like the picture shows. It has probably vented. Measure the capacitors on a capacitor tester (all the caps.) if they are not "longlife", they tend to only last only 10-15 years or so depending on temp and "on-time". Many to old capacitors will show up as shorted caps when measuring or when driving them in an application. Those bad caps also tends to force semiconductors to failure. Check transistors who are connected to, or in the nearby of the faulty cap (-s).

And cracked solder joints and/or broken traces, (due to heat over time), Oh my god, this is not fun: Intermittent faults/errors occur, , I know after 20 years of audio services.

No matter what you are going to repair highend-madness, NEWER connect the amp to your speaker at the first, instead, drive your amp on a dummy load (approx. a quarter to half output power) as for example on an 8 ohm, 100-200Wresistor and study the offset, and measure the response to audio frequencies. Preferable on an oscilloscope for a period of several hours after the temp has stabilize (after setting the bias to the right value). Also do physical test by bending the printed circuit boars slightly back and forth and also do a "hammer"-tests with the help of a screwdrivers handle (if plastic not metal-handle) on the boards and at the same time look at the oscilloscope for suddenly high DC-offsets. This is standard procedure when done service repairs.
Good luck

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