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Old 12th February 2009, 05:12 AM   #1
gto127 is offline gto127  United States
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Default Nakamichi PA7II Problem

Hello everyone I'm starting work on a Nak PA7II which was worked on by someone else. I've made some initial tests & was curious if you guys might could point me in the right direction on this.
On power on protect lights are lit. I tested power transistors & nothing is shorted. R 136 L & R136R are open. These are 1ohm resistors suppling part of the protect circuit. I was just curious what might cause both resistors to go open at the same time. Since the protect circuits are indepentdant L& R I don't beleive this would have caused them both to go out at same time. Any ideas?
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Old 12th February 2009, 09:04 PM   #2
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Did you remove the output devices from the circuit to test them--or at least pull the emitter resistor? I suspect you'll find the corresponding transistors to be roasted.

Grey
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Old 12th February 2009, 09:20 PM   #3
gto127 is offline gto127  United States
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They tested ok . I would have thought that also. The previous owner may have already replaced them. Just can't figure out why they both went at the same time. The only thing abnormal I can find in protect supply is 26v is 25v & 24v is 22v. I have the associated tiny transistor on order but I think it would have shorted to take out those big resistors.
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Old 12th February 2009, 11:32 PM   #4
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It's not the small signal part, it's the output device. At a guess, someone shorted the output. Note that protection circuits...aren't. They give you a good feeling that lasts until the output stage smokes, after which you view them with a bit more skepticism.
As for whether the output devices have already been replaced, whether they're good or bad, whether there are other outputs gone, etc...these things can't be determined at a distance. It's up to you.

Grey
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Old 13th February 2009, 03:42 AM   #5
gto127 is offline gto127  United States
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I spoke with orig owner & he said that the amp originally ran real hot on right side before going out and when it finally bit the dust that it kept eating output transistors on that side within a minute of replacement. It turns out that transistors had been replaced but prev owner was unaware of bad resistors. I'm hoping to figure why it's eating transistors before replacing resistors. Could possibly bad smoothing caps to out transistors cause this? I have esr meter but's it's not very accurate over 100uf. These are 330uf@ 100v. Thanks for replies.
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Old 13th February 2009, 04:02 AM   #6
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Nothing for it: You'll have to start at the front end and test every single transistor, including the front end. (When the amp goes, it can send a surge up the feedback loop and hit the front end, the drivers, etc. can take the hit directly.) I'd suggest a very thorough going over. When you've tracked down the dead stuff and replaced it, bring it up on a Variac. A meter, an oscilloscope, a signal source of some kind...it's a pain in the patootie.
Incidentally, you should back the bias down to minimum before starting the thing up.

Grey
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Old 13th February 2009, 04:41 AM   #7
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Link to the service manual, i case you don't have it
http://www.audio-circuit.dk/images/s...PA7-pwr-sm.pdf
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Free Schematic and Service Manual downloads www.audio-circuit.dk, Company: www.dupont-audio.com, Joint venture: www.DupontMantra.com
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Old 13th February 2009, 04:44 AM   #8
gto127 is offline gto127  United States
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Yes I agree I have my work cut out for me esp with the driver section which I haven't checked. So you think I should abandon the protect circuit/supply with slight voltage off?
Mr Pass you're probably the best amp designer ever but I wish you'd made this one a little simpler. LOL
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Old 13th February 2009, 09:05 PM   #9
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Once upon a time, there was a book called M*A*S*H. (Yes, Virginia, people read in those days...imagine that! It was later made into a movie, then a TV show...but no one I've spoken to remembers the book. No surprise, I guess.) Blake, one of the doctors, was chastised for obsessively rebuilding a patient's hand...while the patient died of other, far more severe wounds. Beautiful reconstructive surgery, mind you, but it was a case of misplaced priorities.
Methinks you have bigger fish to fry.

Grey
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Old 17th February 2009, 03:14 AM   #10
gto127 is offline gto127  United States
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Got a few minutes to work on amp today & noticed main supply to transistors is runnibg 81 volts. Manual states 74v. Does anyone know if manual is correct or if this was misprint?
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