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NAD Gone BAD (noob seeks help)

Hey Skywatcher! Sounds like you have first hand experience with this amp. As I think I've mentioned, my background is as an Engineering tech in the Medical Imaging industry - mostly MRI. While I think I can trouble shoot with the best of them, I do have the disadvantage of not having much hands on experience with consumer audio equipment. I always appreciate when someone with more consumer audio experience adds their expertise.
 
Ylli is probably your best bet to figure this one out. But for the heck of it, check C403. It's an X type safety cap that fails short circuit. If it is shorted, it bypasses R401 (68k/1W) and IC401 could be shutting down the 12v, or be over volt and the system is protecting itself by shutting down the power relay.
 
Ylli said:
Hey Skywatcher! Sounds like you have first hand experience with this amp. As I think I've mentioned, my background is as an Engineering tech in the Medical Imaging industry - mostly MRI. While I think I can trouble shoot with the best of them, I do have the disadvantage of not having much hands on experience with consumer audio equipment. I always appreciate when someone with more consumer audio experience adds their expertise.
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Thanks! But I'm no expert on this unit whatsoever. Things like this can be a nightmare to get a handle on.
 
Fred Weather said:
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.
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You shorted IC205 pin 6 to ground and it still seemed to go into a protection fault? By grounding pin 6, you totally eliminated the protection circuitry. Pin 6 grounded connects to P211A pin 3 which loops through at the left amplifier and comes back on pin 4. It then loops through the right amplifier on P212 pins 3 and 4 and should turn on Q208. Q208 provides power to the speaker relays and exerts a high on the 'PROT' line. (High must be out of protection). There is a thermal switch on each power amp assembly in that pin3/pin4 loop. With power off, ground IC205 pin 6 and check that the short propagates to D216 cathode. Then using your meter diode function (to forward bias the diode) check for that ground on R295.

Backing up to the middle of your quote: The power to the speaker relays is coupled through D211 and becomes "PROT". "PROT is presented on P205B pin 6 where it is routed to the Display Board P503A pin 6. From there it the MCU, IC502, pin 1. (on P503A pin 6, the signal is called "Protect", but I'm confident it is the same signal).
 
Fred Weather said:
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.
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If you do bypass the protection, be sure to be powering it up with a DBT. If the power stays on, you'll be able to see via the DBT if there is a short somewhere. Make sure it is not in bridged mode to simplify things a little. The protection circuit probably monitors more than just a simple DC out to the speakers. I'm guessing it also monitors other voltages/current as well. I'll see how you are doing tomorrow. I'm hitting the hay myself. :boring:
 
If you work in that input power supply (R401, C403) be extra careful!!!! This section is isolated from ground is at line potential or higher. Check those parts with power off, and DO NOT connect your scope ground to that isolated ground symbol with the 'F' by it.
 
Ylli said:
If you work in that input power supply (R401, C403) be extra careful!!!! This section is isolated from ground is at line potential or higher. Check those parts with power off, and DO NOT connect your scope ground to that isolated ground symbol with the 'F' by it.
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Yes! SAFETY first!
 
The protection circuit, IC205, is monitoring amplifier output offset at pin 2. The left and right channels are summed via R265 and R266. If pin 2 is >+0.5 V or more negative than -0.1 V, then the chip flags a fault. It monitors the current in one of the output transistors at pin 1. It also monitors the AC input power at pin 4, and incorporates a power up time delay via pin 7.

Interesting.... According to the uPC1237 data sheet, pin 4 needs a small positive voltage to detect 'power on'. On the amplifier schematic, D210/C259 appear to be backwards giving a negative voltage at pin 4. Wonder if there is a difference between the uPC1237 and the HS1237.
 
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If the unit still shuts itself down even with the protection circuitry disabled (IC205 pin 6 grounded) then the problem must be elsewhere. Just some brainfarting....er, brainstorming.

In regards to the 4.65 volt/5.2 volt power generated on the Standby board, across C410 (+5.2VD): This point sits at +4.65 during standby, and transitions to +5.2 during operated. Would be a good idea to make sure that transition is clean, and there is no substantial glitch as it makes that transition.

The MCU needs to be powered as long as the unit is plugged in, whether it is in standby or operate. There is some inconsistency in the signal naming on the schematic. The standby power starts out as "+5.2VD" generated by the Standby board, it exits the Standby board on P402B pin 3 as "+5.2V", and enters the Display board on P502B pin 3 as "+5VS". It passes through a diode on the Display board and becomes "+5VD".

"+5VD" is not shown entering the bus but is shown as breaking out of the bus in a number of places - including as power for the MCU. To cut to the chase, you might want to scope the "+5VD" as close as possible to the MCU to be sure that voltage is not glitching during the standby/operate changeover.

Another possibility is that the MCU clock drops out as the power switches from standby to operate. To scope that you need to couple *very* lightly to either end of the crystal (X501). Perhaps couple through a 1 meg resistor or a 1 - 2.2 pF cap.

Might also want to scope the MCU RST input (pin 4) and be sure it doesn't glitch during the standby/operate transition.

I have not been able to identify any other pins on the MCU that would affect the issue.
 
Ylli,

I can't thank you enough for putting in so much effort on this. I hope you work for cheap like me ;-)

In the notes for the updates, D501 was replaced with a jumper. I was about to tear the front panel apart when I noticed the change page. So +5VS is +5VD.

There is no standby transition to 5.2v, it's stuck at 4.65v.

I got a short email back from NAD support: It is the standby board that gives power supply to keyboard malfunction. In most case is the standby voltage output is too low.�� Also possibly the micro on keyboard is defective.

That would mean board replacement because of the EEPROM.
 
Fred Weather said:
Ylli,

I can't thank you enough for putting in so much effort on this. I hope you work for cheap like me ;-)

In the notes for the updates, D501 was replaced with a jumper. I was about to tear the front panel apart when I noticed the change page. So +5VS is +5VD.

There is no standby transition to 5.2v, it's stuck at 4.65v.

I got a short email back from NAD support: It is the standby board that gives power supply to keyboard malfunction. In most case is the standby voltage output is too low.�� Also possibly the micro on keyboard is defective.

That would mean board replacement because of the EEPROM.
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The 5.2v is generated at the standby board and if it is low there, it would seem that there is a problem there. That is a fairly simple design. With the power disconnected, pull and check every component. You may find the LM7806 regulator to be a problem. They are robust, but I have seen a few cause problems. And of course check any E capacitors for excessive ESR, and the film caps for value. Pretty much after that it's all diodes.The bridge rectifier could also have one diode out inside it too.
 
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The switching supply made up of IC401/TR402 and associated components generates a 4.65 volt output to the +5.2VD line. At start up, when the other power supplies come up and +12 V appears at P402B pin 1, power is supplied to IC402 (an LM7806) and the output of that coupled through a diode to +5.2VD. At the same time, current from that +12 V supply is applied to the LED of the photocoupler and effectively shuts down the switching supply.

So you should see the +5.2VD point sitting at 4.65 volts during standby, and when you turn the amplifier on, the +12 volts should come up, making the 7806 active, and increasing the voltage at +5.2VD to +5.2 volts.

Are you sure the +12 volts comes up when you attempt to power up? If so, does that +12 volts make it to the 7806? Does the voltage at +5.2VD smoothly increase from 4.65 to 5.2 volts before it shuts back down?

Or is it shutting down before the +12 volts from P402B comes up (I think we checked this earlier)? Or doesn't the +12 volts have enough time to propagate to the input of the 7806 and out to the +5.2V point? Follow that +12 volt line through during the turn on attempt. I'm wondering if the 4.65 volts from the switching supply is dropping out before the output of the LM7806 is sufficient. That would cause a momentary drop in the power to the MCU and probably an MCU reset.

As a test - do you have a power supply that will supply 9-12 volts maybe 100 mA? If so, you could try this.... Connect the supply negative to chassis ground. Connect the supply positive through a diode (1N400x would be fine) to the input of the LM7806. Plug in the amp, power up the external power supply, and see if the amp will power up.
 
Fred Weather said:
I got a short email back from NAD support: It is the standby board that gives power supply to keyboard malfunction. In most case is the standby voltage output is too low.�� Also possibly the micro on keyboard is defective.
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Well, you measured the standby voltage at 4.65 volts, which is spec. MCU *could* be bad, but it wouldn't be my guess.

I know you have been using the scope for some of these measurements - I assume if there was excessive ripple on any of the supplies you would notice.
 
If anyone is reading, I managed to get the amp working just fine, all commands seem to be active except remote (no reaction whatsoever), and I have sound on all inputs/outputs. To get to this working state I disconnected the +12v wire from connector P402B on the standby circuit. As soon as I plug this back in the amp shuts down. I don't really see what this 12v is doing here. Also on the display circuit on the same connector the schematic indicates only 8v going out :dunno:.
 
Do you still want to play with this a bit? Is IC506 installed? Looks like it is likely a 7808 three terminal regulator.
 
I will check that tomorrow, I haven't removed the faceplate yet so I don't know. Do you have an explanation for the presence of the 12 (or 8V) on the standby board as it works fine when disconnected ?
 
Yes, without fully reviewing the thread, I believe I described what is happening earlier. Yup, here it is: "The switching supply made up of IC401/TR402 and associated components generates a 4.65 volt output to the +5.2VD line. At start up, when the other power supplies come up and +12 V appears at P402B pin 1, power is supplied to IC402 (an LM7806) and the output of that coupled through a diode to +5.2VD. At the same time, current from that +12 V supply is applied to the LED of the photocoupler and effectively shuts down the switching supply."

Switching power suppliers can be a bit noisy, so in normal operation it is advantageous to shut it down. The circuitry driven by pin 1 of P402B takes care of this. That "+12 volts" shown on P402B pin 1 may be either +12 or +8 depending on whether IC506 is installed or not.
 
depaj said:
Do you have an explanation for the presence of the 12 (or 8V) on the standby board as it works fine when disconnected ?
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When the wire on P402B pin is disconnected, the standby switching power supply is never shut down and it supplies power to that +5.6VD" line. A running switching power supply can also couple excess noise into other parts of the amplifier and reduce the net S/N ratio.
 
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