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i have a NAd stereo (7220pe) that keeps blowing fuses, does anyone know what this could be??? Power supply, board ground? I was given the stereo from my dad so it has sentimental value and I would like to attempt to bring it back its former life.Much appreciation,
Follow Ups:
Which fuses is it blowing? It has 6.BT
It blows fuses ,501-504 (t4a/125). Not the t1a/125(f505,506)
All four of them huh? Yikes.OK, before you try anything else, find R437 and R438, they are on opposite sides of the big reservoir caps, next to the trimpots farthest out from the heatsink.
Got them? Ok, are they still in one piece? If they are, are they visibly damaged, smoked, value bands burned all the same color? If they are damaged, a few transistors in the power amp section have failed, but off the top of my head, I can't remember which ones, but I think they go to the bases of the driver transistors.
If those resistors have not failed, and you know how to do either of these, try one of them.
You can unsolder and remove the output transistors (completely out!), put in fresh fuses, power it up, and see if the fuses hold. If they do, check the voltages at where the output transistors should connect, with no signal applied (keep a steady hand here). The emitter location should be somehwere around either + or - 29 to 34 Volts. The bases should be 0V, or darn close to it, in the mV range definitely, and the collectors should be 0, period.
If any of the above is different, or if the fuses don't hold without the output transistors, it can't be diagnosed from where I'm sitting. Also, if any of the above is making you go huh? PLEASE don't stick your hand in a powered up amp!
The second thing is to measure the resistance from each pin to the other on the output transistors, in circuit or out. Anything even close to zero, in any direction, is bad. 2N3055s and MJ2955s should measure in the megohms in the direction they are SUPPOSED to conduct, and read open when the probes are reversed.
If nothing at all that I've typed doesn't make any sense, it may just be me, but, for your own safety, and possibly the chance to make me look like a fool in public :), find someone you know that can check the things I just described.
Good luck,
Thanks BT, R438 is burnt, color bands all the same, r437 looks fine, Im going to replace both for good measure. I assume they both have the same values?Also, if anyone knows where to locate said output transistors, I would like to know. If I understand they failed causing the resistor to fail thus blowing the fuses.
I feel confident replacing the components, i just dont know where they all are. Is there anything I should know "not" to do. I know the caps hold juice, is there a way to depower the board?
I can't remember where and when I posted this, but I wrote a long post describing how I upgraded and modified an old NAD 3020. The amplifier circuit is essentially the same. I think I gave transistor substitutions also, so check it out, if you can't find it let me know.As for where, the original output transistors used in that amp are so common, that if you just randomly happened to find a TO3 transistor lying in the middle of a road, chances are it would be one of the two you need! However, if you find the post I had on upgrading the 3020, there are faster substitutes out there, and it does make a difference! I like the MJ15001 and MJ15002 in that amp, and you can get those just about anywhere also.
While you are checking transistors, check all eight of the TO126 transistors (Q408, 410, 412, 414 and their odd-numbered companions in the other channel), it is likely that one or more of those have failed, or are close to it now, plus they are relatively cheap. To save time and headache, and give yourself a bit of "safetly margin", I normally replace all of those with their NTE equivalents. You luck out in this case anyway, because all the NPNs are NTE 373, and all the PNPs are NTE 374, even though all the original part numbers are different. If anything, I think the amp sounds better with those anyway.
Yes, R437 and R438 are identical, just make sure they are mounted up off the board like the originals, they get hot even when everything is working correctly.
The only caps you really need to worry about are the two big ones in the center of the main board. I usually use a 100 Ohm 10W resistor and bridge it across the terminals for a second or two.
Good luck!
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