• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

NAD Gone BAD (noob seeks help)

F

Fred Weather

New Member
Hi all,

Hope I'm in the right place.

I picked up a C375BEE integrated amp that has an issue with turning on. Spent some time on the net and found others have had the same problem but no one has posted any fix yet. Service manual is available with a Google search. If someone with experience on this amp could help, I'd be happy to pay for positive input.

When pressing the front panel power button, it clicks the standby relay, all the front panel lights flash and then immediately shuts off. I know there's a lot of NAD amps that have the 'protection mode' problem but I'm not sure this is one of them.

Things I've tried:
Disconnecting all connections to the power amp boards still kicks the standby relay off as above. But, doing that disconnects some of the protection chip circuit (IC205/upc1237)
Thermal switches on power amp boards are fine (NC).
Disconnecting connector P207B on the main board allows the standby relay to stay on but then nothing on the front panel works. I assume something downstream from that power source (80v &15v) is the culprit. Possibly a shorted power amp transistor. (can you tell I don't know what I'm doing?)

Questions:
Does IC205 only switch off the speaker relays or is it also tied to Standby Board relay RL401?
Connector P403B, is it a thermal switch in the main transformer?
I don't understand the relationship on connector 402B with thermal protect and standby to the transformer.

Thanks for any input.
 
Register to hide this ad
Hmmm.....I recently bought ea “relative” of that component, a Proton D940 receiver off eBay. Seller claimed everything worked, and packed it well. When I went to fire it up: nothing at first, then it started a similar clicking cycle and would not come on. After some cussing, in a long shot I thought I’d try another source just in case the notoriously dirty switching might have something to do with it (was trying to use the tuner). As I plugged a CD player’s AC line cord into the “unswitched” receptacle in the back of the receiver, the tuner started working, and everything’s started okay since!
 
Quite a complex amplifier there. Who ever designed this must have stock in a relay manufacturer. Certainly can offer no guarantees, but I'll give it a shot.

You say when you disconnect the power amps, the unit still shuts down. Are you disconnecting the power amps by unplugging the three connectors for each? What if you disconnect P206B - the other output line from the transformer?

The standby power supply should be operating when the unit is plugged in and turned off. In that condition, do you see the +5.6 volt output? Try turning it on, does the 5.6 V remain with minimal or no glitch when the relay pulls in? (Looks like the standby supply is disabled when it sees +12 volts from the main power supply. But that +12 volts from the main supply also feeds the +5.6V regulator, so the +5.6 should remain.)
 
Thanks for the reply. Helps to have another set of eyes look at it. Forgot to mention, this is the newer amp, using the schematics on pages 55-59.

I hadn't tried turning it on with P206B disconnected. Now, when I pull P206B (with P207B plugged in), it shuts off immediately, as usual.

Checking pins 2 & 3 at P402B (standby board) has 4.65v (back panel power switch on.) (the schematic shows 4.65v in standby mode) and there's a slight glitch (lower) when pressing the power button but returns to 4.65v.

There is no 12v at pins 1 & 2, even the slightest when pressing the power button.
 
The +12 appears to be coming from the attached (PSU board). You indicate that there is no hint of +12 volts entering the standby board when you attempt to power up. Can you look at the +18 V and +26 V to see if either of these attempts to come up?

upload_2019-9-30_20-58-50.png
 
I previously checked all four of the voltages from that section. 80v, 36v, 26v and 15v. The voltages looked like they are there but only briefly when the relay is connected.

Do you know what is coming from the transformer on P403B? (standby board). Is it a thermal cutout device or power?
 
What about the +18? That comes a different area (circuit with the MOSFET between the two sets of bridge rectifiers - don't think I understand what they are trying to do with that MOSFET circuit).
 
Yes, there is 18v present.

No transformer to be found on the schematics. Only the outputs at connectors.

P403B from the transformer is a dead short (unplugged from standby board). That's why I was asking if it could be a thermal device. If it is, and it were to be defective, could it open immediately with current flow? (I don't have a clue about these types of things) If that were the case and it is defective, could I short the two pins on the standby board without consequences?

As to your question about the MOSFET. I have a friend who's much more knowledgeable about this stuff and he had asked the same thing about that.
 
Fred Weather said:
P403B from the transformer is a dead short (unplugged from standby board). That's why I was asking if it could be a thermal device. If it is, and it were to be defective, could it open immediately with current flow? (I don't have a clue about these types of things) If that were the case and it is defective, could I short the two pins on the standby board without consequences?
Click to expand...
Yes you could. I doubt it is the issue, but worth a try.

As to your question about the MOSFET. I have a friend who's much more knowledgeable about this stuff and he had asked the same thing about that.
Click to expand...
I think I will set it up in LTSpice and see what it is doing.
Yes, there is 18v present.
Click to expand...
If +18 and +26 are coming up, then +12 should come up. If not, check Q206, Q207, and C272.
 
I posted the following below about an hour ago but I guess I didn't hit reply.

Ok, I will take back the comment about the 12v on the standby board. I put a scope on it and there is a quick shot of 12v there. I did not see it at all with the multimeter.
 
So it sounds like the power supplies are coming up, something else must be telling it to shut back down. I gotta do some studying......
 
Just going to babble a bit...
Standby (P402B-pin5) needs to be low for the unit to be in 'operate'. Standby comes from the display board (P502A-pin5) which is the collector of Q533. For the collector of Q533 to be low, the base of Q502 must be low. Signal name P0.0 connects the base of Q502 to a pullup resistor at R510, and the MCU pin 37.

So there must be something input to the MCU that is telling it to drop out of operate and go back to standby.
 
Glad you got a scope, might be needing it. After you turn the unit on, how long does it stay on before it is forced back off? - how long do we have to make measurements in the 'operate' mode?
 
It shuts off in 1/2 second.

My friend who's helping here says the same thing about IC502.

I'm going to be away for a few hours. Will get back later.

Thanks again for digging into this.
 
Going through all the pins on the MCU (IC502). Most of them are outputs that drive the various modes, there are some inputs from the push buttons mode selects, power, ground, RS232 interface, IR remote interface, volume motor drive, etc. Only a couple I see that I would expect might tell the MCU to turn things off.

Pin 1 - Protect input
Pin 4 - RST

Could be missing something, but I see only a pull-down on the RST pin (R508). That leaves the Protect input. So we move on to IC205.

Scope pin 1 - anything over about +0.5 V would indicate an output amp overcurrent.
Scope pin 2 - anything above +0.5/-0.1 V will indicate an offset error.
 
Last edited:
Fred Weather said:
Hi all,

Hope I'm in the right place.

I picked up a C375BEE integrated amp that has an issue with turning on. Spent some time on the net and found others have had the same problem but no one has posted any fix yet. Service manual is available with a Google search. If someone with experience on this amp could help, I'd be happy to pay for positive input.

When pressing the front panel power button, it clicks the standby relay, all the front panel lights flash and then immediately shuts off. I know there's a lot of NAD amps that have the 'protection mode' problem but I'm not sure this is one of them.

Things I've tried:
Disconnecting all connections to the power amp boards still kicks the standby relay off as above. But, doing that disconnects some of the protection chip circuit (IC205/upc1237)
Thermal switches on power amp boards are fine (NC).
Disconnecting connector P207B on the main board allows the standby relay to stay on but then nothing on the front panel works. I assume something downstream from that power source (80v &15v) is the culprit. Possibly a shorted power amp transistor. (can you tell I don't know what I'm doing?)

Questions:
Does IC205 only switch off the speaker relays or is it also tied to Standby Board relay RL401?
Connector P403B, is it a thermal switch in the main transformer?
I don't understand the relationship on connector 402B with thermal protect and standby to the transformer.

Thanks for any input.
Click to expand...
Yes, this is one of "them" with protection problems. And nobody seems to be able to come up with a fix. At least none that I have seen yet in my searches. So let's start from the basics... Do you have speakers hooked to it? If so, disconnect them to take them out of the equation. And if you do have speakers now, try it after disconnecting them. A shorted voice coil can trigger protection mode.
Since it wasn't working when you picked it up, it is possible someone else has already tried, and you were the "lucky one" who got it. :biggrin: However this is a possibly because there have been a few (not NAD) AVRs across my bench that turned out to be a bad film cap in the power supply. Yes, a bad FILM cap was triggering the protection circuit. It was out of tolerance and was throwing the MPU into a tizzy. I don't recall all the details, but I stumbled upon it checking everything and when I replaced it, bingo! It was part of the stand-by section of the main AC power board in a Denon AVR. I can't remember the value, but it was something in the range of (but not sure) .047uf/250v red colored cap about "mid" physical size compared to the rest of them. It may, or may not be your issue though.
 
Skywatcher... Very first thing I tried. With or without speakers, same result.

Whoever had this amp before me, gave up early. None of the connectors were ever pulled off. I broke the glue on every one. The amp smells and looks new, like it's never been run (clue?).
We spent about six hours on this today and got nowhere (I'm fried). All voltages needed seem present, albeit for a fraction of a second (using the scope). Tried shorting pins 5&6 on IC205, nothing.
Is it possible to remove the protect circuit to test?

I'd sure like to know how the protect IC works. Is it tied to the MCU or just there to disconnect the speaker relays? I have the datasheet for the upc1237. The one on the amp is not an IC, it's actually a stand off component board.

I will scope IC205 tomorrow.
 
You must log in or register to reply here.
Back
Top Bottom