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Old 28th November 2004, 02:55 AM   #1
Rob M is offline Rob M  United States
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Default NAD 7130 troubleshooting

I'm trying to resurrect this old amp... one channel works fine, but the other distorts badly. I think I've tracked down the problem, but I want to ask for advice before replacing parts in a circuit I don't fully understand. (I don't have an online version of the schematic, but it's essentially the same as the venerable NAD 3020 and the like, as attached.)

After poking around with a scope, I think things are going wrong in Q601 -- everything looks fine on the base and emitter, but the signal at the collector is not good. I only get the lower half of the wave there, and it clips at a very low level as well.

So, am I safe in guessing that Q601 is shot? The voltage offsets everywhere match the notes in the service manual, more or less.
What sort of failure would cause this behavior?
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Old 28th November 2004, 05:16 PM   #2
anatech is offline anatech  Canada
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Hi Rob,
Have a look at C603and C605. They may be leaky. How is your DC offset on that channel?
Check C615 and C703 while you are there, I find 6.3 and 10V caps to be prone to failure. Replace them with 16V values on both channels.
-Chris
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Old 28th November 2004, 08:52 PM   #3
Rob M is offline Rob M  United States
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Quote:
Originally posted by anatech
Hi Rob,
Have a look at C603and C605. They may be leaky.
To the extent that I can test them, they seem fine. I'll try replacing them (not a bad idea anyway, I guess) and see if that improves things.

Quote:
How is your DC offset on that channel?
Offset at the speakers is 22 mV on the bad channel and 24mV on the good one.

Quote:
Check C615 and C703 while you are there, I find 6.3 and 10V caps to be prone to failure. Replace them with 16V values on both channels.
Will do. Thanks for the advice!

-- Rob
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Old 29th November 2004, 06:11 AM   #4
hjelm is offline hjelm  Sweden
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Does the base and emitter voltages track i.e. follow eachother (with gain)?

It seems the collector is the feedback node of the input and therefore it is hard to determine why the signal looks like crap there, a failure in any other transistor following the q601 might cause this.

You are saying that the DC points look fine, have you checked the -26V point. The powersupply seems prone to failure in these.
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Old 30th November 2004, 06:49 AM   #5
Rob M is offline Rob M  United States
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Quote:
Originally posted by hjelm
Does the base and emitter voltages track i.e. follow eachother (with gain)?
Yep.

Quote:
You are saying that the DC points look fine, have you checked the -26V point.
Yep. It's where it should be.

I also looked at base of Q605 and the base of Q607 (top trace is Q607 -- and that's supposed to be a sine wave). Anything else I should check?
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Old 30th November 2004, 08:23 AM   #6
hjelm is offline hjelm  Sweden
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Is there any current across R629?

What levels does the scope image represent, i.e. at what level is the flat line and how high are the peaks, and for which input ac voltage?

Try to adjust the vr601 slightly and see if the signal improves, think it sets the bias for the input. Check if the trimpot is broken.
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Old 1st December 2004, 12:09 AM   #7
anatech is offline anatech  Canada
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By chance is the 1 ohm resistor open in the collector of the MJ2955. I have seen techs forget to resolder the bridge to short this resistor out. The only time the resistor is not shorted is to set bias (with no load). It is a 1/4W device.
-Chris
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Old 4th December 2004, 06:33 AM   #8
Rob M is offline Rob M  United States
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Default More clues

I replaced all the electrolytic caps in the bad channel, just to rule that out as a possibility, but it didn't solve the problem.

I've noticed some very strange symptoms, though. Across R637, the -38V rail (the 7130 is slightly beefier than the 3020) is supposed to drop to about -15V. Instead, on the bad channel it drops to -5V for 1 second, then to -15V for 30 seconds, and then it shifts to -7V and stays there. Across R649, that -15V is supposed to drop to -1.1V at the base of Q613. Instead it goes to +16V for 1 sec, falls to 0V, then it looks like it starts oscillating at 60Hz, followed by gradually increasing oscillation beyond what I can make out on my salvaged-from-the-trash 15MHz scope. After about 30 seconds of that, the DC offset there goes back to +16V. Oh, and, there's no C631 on this particular model.

Any ideas?
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Old 4th December 2004, 03:44 PM   #9
anatech is offline anatech  Canada
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Hi Rob,
Did you check if R659 is shorted by a solder bridge (it should be)? If not, R659 is open and you have no -29V on the collector of Q617.
-Chris
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Old 4th December 2004, 10:05 PM   #10
Rob M is offline Rob M  United States
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Quote:
Originally posted by anatech
Did you check if R659 is shorted by a solder bridge (it should be)?
The idle-current-checking resistor is on the positive rail on this one, and it is shorted. And, I just checked, the collector of Q617 has a nice steady -31V, as it should.

Thanks for the suggestions, though! I can use all the help I can get.
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