NAD 3020 A - HELP to repair

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#22
To be honest, I did the measurements without a lamp. It's just that at 5 in the morning it was difficult for me to even dig out a lamp.
If I decide to buy 2 pairs of MJ15024/25, can I simply install them in place of the old 3055 and 2955, or do I have to make some changes to the circuit. This worries me a bit because I will have to pay at least 40€ for transistors and things will get worse.
Otherwise, I am in Europe/Bulgaria, so I can relatively easily order from UK
 
#23
wg_ski said:
The voltage at the base of Q601 is too high. That is what is causing it to stick to the rail. Check the input stage bias stack D603 and D601, and all associated resistors for opens or cold solder joints. This is an unusual arrangement - so that a singleton input stage (instead of a diff pair) can be used. It’s touchy, with multiple points of failure. Since the current thru that circuit is limited by a 22k resistor it’s unlikely to have burnt anything out - so an open resistor or diode may look fine. Don’t make that assumption on this amp - think of how a government contractor would bid it and what resistors they’d use.

And as an aside, if you do use MJ15024 outputs on this thing - immediately after fitting them, short Q609 collector to emitter for the first test. The dies in those are larger (which is actually good if you’re looking to improve thermal stability), but that will usually cause the amp to power up OVERBIASED. Not usually a big deal if there is a pot you can set to turn it to the minimum for a first time power up. But they cheaped out and used fixed resistors - which you will have to adjust once you’ve verified that nothing will cause fire. You start with R641 out of the circuit, and trim from high to low value to get the bias right. Or install a cermet pot. With it completely OUT the bias may still be high requiring further adjustment. Technically using the larger, lower Rth outputs is the right thing to do, but they didn’t make it easy.
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Thanks wg_ski
 
#24
indianajo said:
I would pull Q617 & Q615 to take voltages without the problems. These voltages are with the 60 w light bulb in series with the AC line, correct? rail voltages look suspiciously high. New parts could keep blowing if you don't limit the energy of the faults.
Reason for taking voltages without the outputs, you want to buy the other blown parts in the same $10 box as the MJ15024/25 recommended for outputs by wgski.
I would definitely look at voltages both ends of D601, also junction R670&R671, also junction C601C603R607. See if adjusting pot VR605 can pull front end voltages much closer to normal.
I you are in USA I see no reason to import parts from UK, IE cricklewood. Customs sometimes takes 8 weeks. Cricklewood didn't have MJ**** anyway. Newark+digikey+mouser ship real parts, don't stock the fakes.
This is a hobby for me too. Work at your own pace, in the winter I'll be watching here. til May anyway.
Click to expand...
Thanks
 
#28
Regardless of which out you use, the output stage bias must be checked, and usually needs adjustment. See the service manual. If you change transistor types it just needs to be adjusted a little more and may go out of range requiring another resistor to be swapped out besides Rx. A circuit with a pot would always have enough range to accommodate any reasonable type. Do not ”try” one of those other types in 27 unless you just happen to have them on hand. Go straight to the 15024 if you have to go BUY anything new.
 
#32
You look like you have done quite a bit of work to it, and if that was me i would have put 2 new matched pairs back rather than just replace the odd ones

one thing that is ALWAYS faulty with these is the soldering on the regulated supply transistors
if you wobble them ,they are ALWAYS loose
may not be related to your current issue but check them anyway 👍
 
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