I wonder if any of you kind folks might be able to help me debug a DC offset problem with a NAD 3225PE?
It belongs to a friend, and one day it issued forth magic smoke. The output stage on one channel had failed catastrophically - some passive components badly charred, and dead shorts on the 2N3055 and MJ2955 output transistors.
I replaced all the transistors in the stage, together with the damaged passive components. I also replaced the 2N3055 and MJ2955 in the other channel to keep them matching. And I did a recap of the whole thing while I was in there.
It's almost all working again now, but with one problem. I've got a ~1.9V DC offset on the original blown channel. I can adjust the idling current (on both channels) just fine, and I have ~1.5mV DC offset on the good channel.
I've attached the schematic, plus a closeup of the output stage, and the alignment instructions. I've circled R444 (idling current adjust) and R412 (DC offset adjust) in red. I've also marked R456 (the checking point for idling current adjust). Point "Y" is the signal from the pre-amp out. No matter what I do with R412, I can only get the DC offset down to ~1.9V - if I short R416 next to it, it drops to ~1.7V.
I'd be very grateful if anyone can point me in the right direction.
It belongs to a friend, and one day it issued forth magic smoke. The output stage on one channel had failed catastrophically - some passive components badly charred, and dead shorts on the 2N3055 and MJ2955 output transistors.
I replaced all the transistors in the stage, together with the damaged passive components. I also replaced the 2N3055 and MJ2955 in the other channel to keep them matching. And I did a recap of the whole thing while I was in there.
It's almost all working again now, but with one problem. I've got a ~1.9V DC offset on the original blown channel. I can adjust the idling current (on both channels) just fine, and I have ~1.5mV DC offset on the good channel.
I've attached the schematic, plus a closeup of the output stage, and the alignment instructions. I've circled R444 (idling current adjust) and R412 (DC offset adjust) in red. I've also marked R456 (the checking point for idling current adjust). Point "Y" is the signal from the pre-amp out. No matter what I do with R412, I can only get the DC offset down to ~1.9V - if I short R416 next to it, it drops to ~1.7V.
I'd be very grateful if anyone can point me in the right direction.
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if you have not already done so check the diodes, as often when there are fried passives, one or more diodes often have failed.
Oh yes, sorry, I meant to say I've checked the diodes - well, a simple multimeter check. It might be an idea to just replace them anyway, thanks.
and I meant to ask- are all the voltages shown on the schematic correct- especially those associated with the negative rail (which usually is the problem I have found)?
and yes I would replace the diode as well
and yes I would replace the diode as well
Yes, I checked the voltages during an earlier debug (I'd forgotten, one of my batch of replacement transistors turned out to be faulty and had to be replaced again, and I tracked that down by following incorrect voltages.)
I will replace the diodes and then check all the voltages again.
I will replace the diodes and then check all the voltages again.
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where does the other side of R412 go to?
also try taking out the feedback resistor and see if that makes a difference
and check the components around R412
the circuit is not so easy to read
so you have a scope?
also try taking out the feedback resistor and see if that makes a difference
and check the components around R412
the circuit is not so easy to read
so you have a scope?
Late last year I fixed NAD 3120. Left channel is high on of-set around 680 mV. compared to right channel only 5 mV. It's play music normally but output transistor is hotter than right channel, cannot adjust of-set to lower what so ever. checked transistor and diode are normal. Check voltage every spot seem to higher than right channel. I found C415 or C416 not sure but it's on left channel is dried, I put a new 1000 microfarad 6.3 v with 1000/25 v. in, a-ha! done! I can turn the of-set on left channel down to 5-6 mV. Both channel now run lightly warm same temp. You can check on its if not found yet.
I'll trace out where the other side of R412 goes, thanks.
And yes, I do have a scope.
Interesting, thanks. I have replaced all the electrolytic caps so I should be OK there, but it's something I can check.