Diagnosing NAD receiver -- unexplained shut-downs

[JordanW]

Everyone -- I am trying to troubleshoot my NAD C740 stereo receiver, which either won't power on or won't consistently stay powered on.

I will start by saying that I am a hobbyist, and while I am decently handy with basic DC stuff, I am more hesitant when it comes to mains-powered equipment (that is, reluctant to poke around with a multimeter when it is powered on)

The central issue is that after a long period of non-use, the radio powers on and all functions seem to behave normally, but after a couple of minutes, it powers itself off.

If I attempt to power the radio on again at this point, nothing happens. But if I allow it to "rest" for at least a few hours, I can power it on again. The amount of time it stays on  after this "rest" seems to depend on how much "rest" it has had (more "rest" = stays on longer)

At first I thought this was a power supply issue, specifically of a relay in the initial power supply stage. The service manual (with schematics) is at https://www.manualslib.com/manual/1254519/Nad-C740.html and in the lower right corner, the initial power supply stage is shown (also highlighted in the attached image). I replaced the relay notated as RL901 in an initial attempt to fix the radio. This doesn't seem to have changed anything.

It seems from the schematic that this relay is in fact energized (through the transistor DTC123JS -- not sure what type that is) by a signal coming from the socket labelled JL951A. I can confirm that socket connects to the "NAD Link" sub-board (presumably used for components to control each other). But that in turn is connected elsewhere in the radio, which I haven't explored.

Presumably that "elsewhere" in the radio turns the DTC123JS transistor on and keeps it on.

I'm really mystified as to what is going on -- the behaviour itself and why it is time-dependent (the "rest" periods). My guess would be that some component is failing as it "warms up" and turns that transistor off.

I should also mention that none of the caps that I can see look obviously damaged or bulging, and the main power fuse is fine.

Has anyone seen behaviour like this in consumer electronics before? What are some other likely culprits?

Any advice would be appreciated, Happy New Year to all.

[MathWizard]

So if I'm seeing that right, it's not a switch mode PSU, otherwise I was going to say see what the standby power was doing.

So after it won't turn on any more, is there no voltage on the big multi-tap transformer ? Assuming that BJT is not just some soft start with some RC circuit to control it or something...yeah find what other chips or circuits, still have voltage, and where the control signal should come from. Still some standby circuits tho.

Perhaps a control circuit could be sensing a problem and shutting down on purpose too.


What are the voltage rails like before it shuts down ?

[JordanW]

So if I'm seeing that right, it's not a switch mode PSU, otherwise I was going to say see what the standby power was doing.

Thanks for your input on this.

On the advice from another forum, I replaced the electrolytic cap labelled C951 in the circuit diagram (it is part of the "standby" power system -- across the discrete-diode bridge rectifier setup right next to the small transformer). After doing so, behaviour seems a bit better -- I can power the receiver on and it'll happily run for hours, but if I then power it off, I can't power it back on again until it has "rested" for a while (hours).

So after it won't turn on any more, is there no voltage on the big multi-tap transformer ?

I haven't actually measured, but current to the multi-tap transformer is controlled by that relay RL901, which (when it won't turn on) doesn't actually engage, so I would guess "no".

Perhaps a control circuit could be sensing a problem and shutting down on purpose too.

That thought did cross my mind, but is now inconsistent with the behaviour where I can have the thing running fine for hours, but when I power off, I can't immediately power back on.

The relay seems ultimately controlled by that transistor (strange symbol) but the base of that transistor in turn is controlled by all kinds of stuff that is hard to trace in the schematic, but seems to go to the main microcontroller (?) of the receiver.

[wasedadoc]

That "strange" transistor symbol means that there are two resistors in the package.  One between the external "base" pin and the base of the "real" transistor inside.  The other resistor is between base and emitter of the real transistor.  These were quite common at one time in Japanese gear.  Just saves the two external resistors that would otherwise be used when feeding a transistor from a logic output.

When it won't turn back on, short the transistor collector to emitter.  If it comes on then the relay and that aux power supply are OK.

[Swake]

I presume there is a jumper over pin 1 and 3 of JL951A ?

[wasedadoc]

I presume there is a jumper over pin 2 and 3 of JL951A ?

Don't know what you mean.  That connector has two power wires going out and a control voltage coming in.

[wasedadoc]

My first suspects would be one of the diodes D951 to D954 and the resistor R951 that feeds them.  Diode going open or the resistor increasing resistance when hot.  Result is reduced current in the relay which is still above "hold-in" current but below "pull-in" current.

BTW there is a small error on the schematic.  There should not be a dot where the vertical line from cathode of D953 to anode of D94 crosses the horizontal line from transformer to the line joining cathode of D952 to anode of D951.

[Swake]

If JL951A is a connector it might be interesting to know what is on the other side as transistor Q951 is driven from pin 3.

You're right about the extra dot error.

[JordanW]

When it won't turn back on, short the transistor collector to emitter.  If it comes on then the relay and that aux power supply are OK.

I will try that tonight.

My first suspects would be one of the diodes D951 to D954 and the resistor R951 that feeds them.  Diode going open or the resistor increasing resistance when hot.  Result is reduced current in the relay which is still above "hold-in" current but below "pull-in" current.

That's a very interesting idea. They should be easy enough to replace (thank goodness this board is all through-hole components and so easy for a newbie like me). I will try to find more about the diodes later (I have a poor understanding of when diodes are "swappable")

D955 is probably not an issue, right? (This is the one that is across the relay coil -- probably used to deal with reverse voltage spikes when the relay coil is de-energized?)

I presume there is a jumper over pin 1 and 3 of JL951A ?

If JL951A is a connector it might be interesting to know what is on the other side as transistor Q951 is driven from pin 3.

There is no jumper -- it is a connector cable that connects the base of the transistor (in a convoluted way) to the main microcontroller of the receiver, elsewhere on the schematic, but the nature of that connection isn't clear to me. The point that it eventually connects to is highlighted in the red box in the attached image.

[wasedadoc]

Pin 41 of the micro, through R733 switches on the transistor to power the relay.  The micro is not powered from the main transformer.

[MathWizard]

OP do u have a soldering iron ? I often solder on little test leads to PCB's, especially around SMD chips, trying to probe that stuff is easy to screw up.

But yeah if it was me, with the circuit un-powered, with the DMM and schematic especially, I'd find some key points of interest, and area's like big supply rail caps, and solder on a few test leads, then power it up and see what the various voltage rails are doing. See how much AC ripple is on those caps after the various rectifier's.

If this is some big spread out, single sided PCB thats mostly through-hole, well then it should be a joy to work., and not that dangerous.

When checking voltages, I pretty much always have an alligator clip on the common ground, and then just probe around with 1 hand/probe. And some stuff I just turn off the power if I need to go moving alligator clips, just mind some caps can still be charged for a long time, most will have plenty of stuff to drain them tho.

[MWH901]

Hello all... waking up this old thread to see if OP was able to find a solution or if anyone else had any ideas. I have the same receiver with the exact same problem. Initial successful power-on, then shut down within 30-60 seconds and total inability to restart for an extended period.

I've hit just about every part of the board with freeze spray just to see if cooling something off would help, but so far no luck.

Thanks in advance...

[JordanW]

Hello all... waking up this old thread to see if OP was able to find a solution or if anyone else had any ideas. I have the same receiver with the exact same problem. Initial successful power-on, then shut down within 30-60 seconds and total inability to restart for an extended period.

Unfortunately, even after replacing a few possibly suspect capacitors, I wasn't able to get mine working. I ended up selling it for parts.
Hopefully you'll have better luck with yours. Please share your progress here!

[MWH901]

Unfortunately, even after replacing a few possibly suspect capacitors, I wasn't able to get mine working. I ended up selling it for parts.
Hopefully you'll have better luck with yours. Please share your progress here!

Thanks for responding! Sorry to hear you weren't successful. I'll certainly post if I can find a solution.

[wasedadoc]

The OP never replied with result of my suggestion to short collector and emitter of the transistor driving the relay coil.

[MWH901]

The OP never replied with result of my suggestion to short collector and emitter of the transistor driving the relay coil.

Thank you for returning to the thread... I shorted C and E of Q951 and the relay did come in, however the unit did not power back up (no green light at power switch and the input selectors were unresponsive).