evening.I had just finished repairing one of these today and got it all set up, but when i went to test it, and depressed the SLC switch it all went a bit haywire and the lamp came on relay bright.
so my first thoughts as always was to check the immediate surrounding components but the 4 immediate resitors are all good(circled in red)
Now its only one channel and the lamp is only on with fuse F6 in place
what could have happened here, im not familiar with this feature
whats odd though is i have about +2v on each channel, not just the apparent faulty one.
any help would be great while i plod on.
many thanks
so my first thoughts as always was to check the immediate surrounding components but the 4 immediate resitors are all good(circled in red)
Now its only one channel and the lamp is only on with fuse F6 in place
what could have happened here, im not familiar with this feature
whats odd though is i have about +2v on each channel, not just the apparent faulty one.
any help would be great while i plod on.
many thanks
Attachments
Not another NAD 😉
By lamp do you mean the bulb tester?
Questions to ask yourself are what was the original repair that you did and could the problem be related to that. One thought is that channel/s could be oscillating. Use your scope to check that. AC coupling and look at the output of each power amp.
By lamp do you mean the bulb tester?
Questions to ask yourself are what was the original repair that you did and could the problem be related to that. One thought is that channel/s could be oscillating. Use your scope to check that. AC coupling and look at the output of each power amp.
yes the bulb tester lol, and yep another NAD im affraid.well the original repair was replacement transistors only,main output and driver/vbc.The idle was set exactly to 6mv and the centre ov so not realy much wrong going on anywhere that would have been obvious.
i will get the scope out in the morning and check it out, but engaging that part of the circuit did something that caused a catestrophic event and i want to find out what it was 🙂
Did you have speakers connected when you operated the switch? Only asking because the switch gets a signal from across a 0.1 ohm that is in the earth end of the speaker return. R667 at the speaker socket.
So if you had no speakers connected then its nothing to do with that.
Its still possible the switch could provoke oscillation if something was marginal. I think you have to measure and look with the scope first.
So if you had no speakers connected then its nothing to do with that.
Its still possible the switch could provoke oscillation if something was marginal. I think you have to measure and look with the scope first.
oh no i would never connect a load unitll i had everything set up and left on/monitor the heatsink temp for about an hour standing still, i find you can get alot from it that way without aload.nice and cool after an hour,should be all good.
OK, so that rules out feedback via that route.
As I always suggest, begin by shorting the vbe multiplier to force zero bias and see if the amp is otherwise OK. If the DC offset is not zero (the 2v you mentioned) then you have to look with the scope at the output to make sure it is not oscillating.
If all that is OK then try again with normal bias and monitor the output on the scope.
Oscillation can light the bulb due to cross conduction in the output transistors. That is where the oscillation is so fast (very high frequency) that the output transistors can not turn off quickly enough and overlap in the conduction occurs (so both end up being on and drawing high current).
As I always suggest, begin by shorting the vbe multiplier to force zero bias and see if the amp is otherwise OK. If the DC offset is not zero (the 2v you mentioned) then you have to look with the scope at the output to make sure it is not oscillating.
If all that is OK then try again with normal bias and monitor the output on the scope.
Oscillation can light the bulb due to cross conduction in the output transistors. That is where the oscillation is so fast (very high frequency) that the output transistors can not turn off quickly enough and overlap in the conduction occurs (so both end up being on and drawing high current).
ah right ok, thats something new i have learned.Ive noticed that with some of these older amps with twin transformers the lamp can be on a bit as normal due to the standing current being drawn from them both.
whith the oscillation , will that be obvious on the scope?
il, try this in the morning, time for a bit of chinese now 👍
Oscillation is obvious with a scope like yours. With the scope AC coupled you should see only a straight line trace (nothing there). Of course there is a bit of noise and a small amount of hum and so on and the scope will even show that but oscillation is anything of high frequency and typically that means from a few hundred kHz up to many MHz.
so def a bit with the left channel,but it was only just obvious unless i zoomed in, it was waving about a bit
the drawing is doing my head in at the moement, im sure its wrong, there are components marked up as left when they are on the right.F6 does not feed the side it says on the drawing, which is what has been throwing me out all along i think.
so if F6 is removed and the fault is on that part of the circuit(according to the diagram), it actualy is on the other channel, so throw in that you can hardly read it and its a challenge that im going to come back to a bit later today with a fresh mind, in the mean time i have another easier proposition to finish.
the drawing is doing my head in at the moement, im sure its wrong, there are components marked up as left when they are on the right.F6 does not feed the side it says on the drawing, which is what has been throwing me out all along i think.
so if F6 is removed and the fault is on that part of the circuit(according to the diagram), it actualy is on the other channel, so throw in that you can hardly read it and its a challenge that im going to come back to a bit later today with a fresh mind, in the mean time i have another easier proposition to finish.
so no joy yet, ive started at the beguining again as ive got-14 on the left channel and +2.2v on the other so both channels have an issue
Well start with the basics as we always do. Check the supplies first. There are six for each channel, the -/+50, -/+45 and -/+25. Shorted output stages usually go hard against a rail so these voltage you have could point to something else.
you were right,well there was certainly a fault on the low side- 45/25 rails and that was poor soldered joints on Q610,so thats sorted now, but i still have blackpool iluminations with F6 in,
so the cenario i have now is
Lamp only on if F6 is in, and the lamp is REALY bright so ive not been leaving it in,and thats with all other fuses in.
Im only, for now working on the left channel and i now have pretty much 0v centre, but i just cannot get a current adjusment, with thier method or otherwise
i have no current across the emitter resistor 0.22uf, but that is without F6 in, and that may be why?
and i dont want to put it in for long and blow something else.
so im almost there realy with the left, its just the idle, and im not going to start on the right untill ive finished this side
any ideas about the current? or is it because F6 is out
so the cenario i have now is
Lamp only on if F6 is in, and the lamp is REALY bright so ive not been leaving it in,and thats with all other fuses in.
Im only, for now working on the left channel and i now have pretty much 0v centre, but i just cannot get a current adjusment, with thier method or otherwise
i have no current across the emitter resistor 0.22uf, but that is without F6 in, and that may be why?
and i dont want to put it in for long and blow something else.
so im almost there realy with the left, its just the idle, and im not going to start on the right untill ive finished this side
any ideas about the current? or is it because F6 is out
so this is something i have not come across before.
f4 and f6 fuses out- -45/+45 recifier side of fuse holder
plug the fuses in and it suddenly becomes +3v and -3v
new one for me this one
f4 and f6 fuses out- -45/+45 recifier side of fuse holder
plug the fuses in and it suddenly becomes +3v and -3v
new one for me this one
Last edited:
update on my last post
with F4 in no problerm 42v across the fuse
the moment i put F6 fuse in,this drops it to -1.5v
I would short the vbe multiplier out on the channel that F6 feeds and refit the fuse. When you do that it doesn't matter what any other faults might be, the current flow has to be zero in the output stage. If it is not then you have an issue directly related to the outputs and/or drivers. You can still fault find with the bulb lit to see where the current is going. Look at volt drops over the 0.22 ohm resistors and also see if anything gets warm. The bulb will prevent failure of outputs if they are conducting, the current needed to light the bulb is pretty low.
If the rails are up and the bias current will not set on the other channel then check the voltage across the multiplier is increasing correctly as you turn the preset. The output stage in this NAD is a 'Triple' and needs about 1.4 volts across the multiplier to begin to get current to flow.
If the rails are up and the bias current will not set on the other channel then check the voltage across the multiplier is increasing correctly as you turn the preset. The output stage in this NAD is a 'Triple' and needs about 1.4 volts across the multiplier to begin to get current to flow.
so i shorted it out and blimey it didnt like that, the light was flashing on and off, and with the fuse in ,even brighter
with the short removed, the left channel has a current flow of 260mv, way too high still and it wont adjust.
might have to give this one up for today
might have to give this one up for today
That seems to show you have a problem in the output stage. The theory is sound, short the multiplier and the current should fall to zero.
With Q631 shorted there is no voltage difference between the base of Q619 and Q621. That means then can not turn on together, that bit is important. When they turn on you get a voltage across R647 and R649 (the 330 ohms) and that then turns on the MJE drivers and that in turn develops a voltage across R655 (180 ohm). The voltage across the 180 ohm when it reaches 1.2 volts (the two base/emiiter drops of the output transistors) turns the outputs on.
So you first need to see where the current draw is occurring. Just thinking aloud quickly... if you remove Q615 and Q617 then the bulb should not light and no current flow in the 0.22 ohm resistors.