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NAD 7175PE- access to power amp?

ConradH

ConradH

Lunatic Member
Found a nice 7175PE in the trash. For good reason- the power amp section is dead. Tuner and preamp seem OK. The lower voltage supply (40-something volts) looks good on the positive, but the negative is high, around 70 VDC, and has no ripple at all. Probably something that should be loading it, isn't. So the big question- is there any easy way to get the upper board (tuner) out of the way to work on the thing? Obviously if I disassemble and disconnect enough stuff, sooner or later it will come out. It just seems like it should be easier than it is. I don't see any good way to drop the power board from the bottom. How did they even expect anybody to set the bias, offset and THD adjusts? Any hints from anybody that's worked on these? It looks like a pretty decent receiver if it were working- yes, I've read all the stuff about NAD reliability.
 
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Well, maneuvered the tuner board out of there to look at the amp. Bit of a PITA. Visually near perfect. Nothing burned. The right channel is sitting at -80ish volts. Left channel seems normal. That's why the lower voltage supply is closer to -70 than -45 as it's being pulled by the higher voltage supply. Probably not the root cause of anything. Now it's just a matter of checking all the devices to see what's going on.
 
I only have experience with other PE units, not this one, and this is anecdotal , but it seems the filter caps on the upper supply on the PE units have the habit of going bad. Typically it hums then, but perhaps it is a different fault, such as an internal short. With your mad skills, I'm sure you will get to the bottom of it shortly.
 
Don't tell anybody, but the only real skill I've got is dogged perseverance. It's too bad one can't just probe the schematic, as quickly locating the circuit nodes is a bit of a PITA on this one.
 
I'm working on a Nad 7150 right now. I had to disconnect all the wires on the the top board (tuner board to get to the audio board also). I've allready replaced around 14 defective electrolytic caps. They looked like they got hot and most were bad.Also found the protection relay missing.I know that there are holes in the solder side of the audio board to get to the adjustment pots. I'm wondering if you can run the audio board without hooking up the top tuner board. Also i've ordered more caps that I did'nt have and a relay. The relay I order was one that was mentioned on this sight but it won't drop in without modification.So if anyone knows the right relay please let me know.I'll be amazed If I can get this receiver to work.I'm going to give it a heck of a try. Good luck with yours!
 
Yes, the power amp board seems to run fine without the tuner board in place. I've still got the tuner board attached and it lights the display and works, so has to be insulated while working on it. This NAD has proven me a complete idiot. I've checked about every component on the board and haven't found a thing, save for one tired electrolytic that I replaced. I've even got a good channel to compare to, but no joy. The right channel is turned on to the -80 volt supply, but not sure why. It isn't drawing excessive current. Nothing is getting hot. Semis are good, pots are good, but I'm missing something. It has to be something simple or there would have been smoke. I need a new plan. Time to give it a rest for a day or two.
 
ConradH said:
Yes, the power amp board seems to run fine without the tuner board in place. I've still got the tuner board attached and it lights the display and works, so has to be insulated while working on it. This NAD has proven me a complete idiot. I've checked about every component on the board and haven't found a thing, save for one tired electrolytic that I replaced. I've even got a good channel to compare to, but no joy. The right channel is turned on to the -80 volt supply, but not sure why. It isn't drawing excessive current. Nothing is getting hot. Semis are good, pots are good, but I'm missing something. It has to be something simple or there would have been smoke. I need a new plan. Time to give it a rest for a day or two.
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The 7155 uses one power transformer and an unregulated supply to feed the rails on the parallel push pull output transistors.(4 power transistors per channel).
It's very similar to a nad 7140 I recently worked on. The power supply to the plus and minus power rails for both channels are taped from what looks like the same point on the schematic.Check the voltage on the collectors to chassis ground. If they are not the same voltage follow it back.There is an impedance switch that changes the voltage on the xfmr. secondary, that effects both channels.You may want to disconnect the the power supply and find out why the power supply is so high. That high voltage may damage parts so be careful.
 
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The 7175PE has two unregulated supplies, something around +/-44 VDC and +/-80 VDC. Not sure exactly what they should be. Everything appears normal, save for the right channel being stuck to the negative rail. Obviously it isn't going to come out of protection in that condition. My new plan is to print up a large schematic and fill in every voltage I can measure. That will usually show something that violates expectations and can only be explained by the fault. Or so I keep telling myself.
 
ConradH said:
The 7175PE has two unregulated supplies, something around +/-44 VDC and +/-80 VDC. Not sure exactly what they should be. Everything appears normal, save for the right channel being stuck to the negative rail. Obviously it isn't going to come out of protection in that condition. My new plan is to print up a large schematic and fill in every voltage I can measure. That will usually show something that violates expectations and can only be explained by the fault. Or so I keep telling myself.
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Yes got it mixed up i was looking at the wrong model.
 
ConradH said:
....My new plan is to print up a large schematic and fill in every voltage I can measure. That will usually show something that violates expectations and can only be explained by the fault. Or so I keep telling myself.
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Sounds, and looks, like fun (not !).

I'm still trying to get my head around the operation of the NAD PE set up, so if you do get a bunch of voltages measured, it'd be good to see how they look. I also noticed NAD are using a few fusibles in the 7175PE, I'm not at all sure they'd be the problem, but they're probably old enough to warrant checking.

:lurk:
 
I've been tring to figure it out also.I think the power amps are fed from 1 power supply and when it calls for more power the the other power supply kicks in through q 703 q704 and q 705. on the plus side, and there are 3 other transistors on the neg side. I think that's the power envelope circuit. That secondary power supply is a higher voltage. q703 feeds the collectors of the power amp transistors. q704 and q705 drive q703. I would look at the power envelope circuit, it may be stuck on.I would love to get my hands on the Nad 7175. It is a rare bird.There are no voltage values printed on the schematic which makes it harder to trouble shoot.Also in the parts list it doesn't show the values of the rectifiers and other parts , it just lists there part #.
 
Yeah, I got that far as well. It's more the dynamic control side of it I'm struggling with - it's obviously via the D707 & D710 Zeners, and D709 & D711 diodes.... but I'm not seeing it all yet, specifically, how the 2 different supply rails are interacting. I think they're basically 'balanced' against each other, such that as the (Q617 & Q619) emitter resistor voltages rise, past a certain point, Q705 & Q706 are turned on... and that 'cascades' through Q704 & Q708, to turn Q703 & Q707 on.

It's also the first area I'd start looking at.
 
I'll spend some more time with it later today. The dual supply is a mystery to me at this point. I don't see how the supplies are switched, and I don't think it works like the Carver amps where they anticipate the peaks and raise the voltage, though it could be something similar. Because there are no blown power parts (I think), I'm leaning towards the front end. That could rail the amplifier and not do any damage. If something downstream were blown, I'd be looking at smoke and blown fuses. Naturally my curve tracer failed last night, so I need to fix that too! "We're gonna need a bigger bench."
 
Goldie99 said:
Yeah, I got that far as well. It's more the dynamic control side of it I'm struggling with - it's obviously via the D707 & D710 Zeners, and D709 & D711 diodes.... but I'm not seeing it all yet, specifically, how the 2 different supply rails are interacting. I think they're basically 'balanced' against each other, such that as the (Q617 & Q619) emitter resistor voltages rise, past a certain point, Q705 & Q706 are turned on... and that 'cascades' through Q704 & Q708, to turn Q703 & Q707 on.

It's also the first area I'd start looking at.
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I know it is a complicated circuit for me to. I'm getting a headache trying to figure it out. Maybe someone out there can explain exactly how it works.
 
This thing must be payback for something awful I did. Yes, the fuses are good. No shortage of supply voltages everywhere. I enhanced and printed out an 11x14 schematic of the right channel, correcting all the part numbers by adding 50. NAD decided to save paper by not bothering with a right channel specific diagram. I've been measuring voltages, filling in the schematic and getting a better understanding of how the thing works. I think the negative HV rail switch is stuck on, but I don't see how that should put the amp itself against the rail. Feedback should still correct it. I'm thinking there's a shorted power device I haven't found yet, and I'll probably have to pull some to be sure. It takes me longer than most to fix things because I insist on working out the problem by measurement and not pulling parts unnecessarily, but in this case it may be the fastest route.

Looking at the parts and build quality, I don't see anything that would support the things I've read about problems. It's electrically and mechanically as sound as anything else I've worked on. Maybe the 7175 was better than other models, but it's only the second NAD I've ever messed with. Assuming I ever fix it, I'd expect it to be a fine performer. The fact that something failed doesn't make it any different than all the other more highly touted brands where something failed.
 
Think I found it, but won't know until tonight when I can get back to it. The first things I checked were the pots. Thought they were OK. Apparently I suck at pot checking because the voltage readings across the THD pot didn't make sense. Now the resistance readings show it's high or open. That would prevent the entire positive side of the amp from working. When the amp goes to the negative rail, the HV rail switching does exactly what it's supposed to, it turns on. Thus the thing rails at -80ish rather than -44ish. I've got a much better understanding of how the circuit works now and if I get it working I'll post a few details that could be useful. It's not very complicated as long as you break it down into bite-sized functions. It does in fact just switch the HV supply rails in and out as needed. The control signals to the switches pass through some 1k resistors. The entire HV rail PE thing can be defeated by lifting one leg of those resistors.

BTW, they hide that tiny THD pot under a shaft from a front panel knob, a less than ideal location.
 
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