NAD C325BEE Will not power.

[paul_g_787]

Hi Everyone. I have here a NAD C325BEE amplifier which will not power and I am a little stuck.

Service Manual: https://elektrotanya.com/nad_c325bee-sm.pdf/download.html

It was dropped off to me with the report being that it was in use, then started distorting and crackling, then it switched off and had an amber light and would not come back on.

So. I firstly found an issue where the main capacitors for the +46/-46 DC rail were bulging (C517 and C 518) and there was +39V and 0V respectively. As reported the amplifier had an amber power light but would not turn on.

I have replaced these now and I am getting +40V / -40V, which is still a bit low...  Not sure here..... But progress...
I also replaced C523 and C524 as their ESR was pretty high. These are for the +37/-37V rails which measure just fine.

It is my belief these capacitors failed / were in the process of failing as they are exactly next to the main heat-sink... a quick search shows this is a common failure in this model.

So that being sorted (I think) I now have the issue where there is no power light at all. Not even amber. And the amplifier won't respond to the power button.

The service manual check sheet (Page 24) says to check the 5V supply circuit on the power board. However there is nothing labelled 5V at all on the power board schematic. I am assuming this would be the pin labelled "TRIG" ??  Confused here... 

Power supply schematic on page 19 of the SM for those playing along.
I have attached a copy of this with my current readings written on it.

All the bits in green I am sure are good. 

All the red bits are where I am confused. 

The TRIG pin is at 0V
The AC DET pin is at 5V AC and -25V DC
The I-V pin is at 0V

What are these ping exactly. I am not sure.?? 

So my questions are, firstly what should TRIG, AC DET, and I-V be measuring?
And also why would the +46 and - 46V rails only measuring +40 and -40?

Can anyone make any suggestions here?

[up8051]

5V is made by IC45 AMS1117 form 8V (CB53 pin 6,7)

IC45 schematic page 17 right-up corner.

[paul_g_787]

5V is made by IC45 AMS1117 form 8V (CB53 pin 6,7)

IC45 schematic page 17 right-up corner.

Aha well spotted sir!! Even though the SM says "on power board"... No wonder I didn't see that!

I will test that part of the circuit next.

[Kim Christensen]

All the red bits are where I am confused. 

The TRIG pin is at 0V
The AC DET pin is at 5V AC and -25V DC
The I-V pin is at 0V

What are these ping exactly. I am not sure.?? 

So my questions are, firstly what should TRIG, AC DET, and I-V be measuring?
And also why would the +46 and - 46V rails only measuring +40 and -40?

It looks to me like the rails marked 46V are actually capable of being switched between 40V and 51V depending on the voltage level on TRIG. (Voltages approximate)
When TRIG is zero, then the 28.5VAC transformer windings are suppling the power via the D51 bridge rectifier.
But when TRIG goes "logic high" then both the SCRs (D531 and D532) are turned on which connects the D55 bridge rectifier outputs (which is fed from the 36.4VAC windings) to the filter caps (C517 & C518) and thus the voltage on "46V" rails (CB54) increases from 40V to 51V.
I would guess this is supposed to happen when the user turns up the volume and the amplifier needs to put out more power.

[feedback.loop]

All the red bits are where I am confused. 

The TRIG pin is at 0V
The AC DET pin is at 5V AC and -25V DC
The I-V pin is at 0V

What are these ping exactly. I am not sure.?? 

So my questions are, firstly what should TRIG, AC DET, and I-V be measuring?
And also why would the +46 and - 46V rails only measuring +40 and -40?

It looks to me like the rails marked 46V are actually capable of being switched between 40V and 51V depending on the voltage level on TRIG. (Voltages approximate)
When TRIG is zero, then the 28.5VAC transformer windings are suppling the power via the D51 bridge rectifier.
But when TRIG goes "logic high" then both the SCRs (D531 and D532) are turned on which connects the D55 bridge rectifier outputs (which is fed from the 36.4VAC windings) to the filter caps (C517 & C518) and thus the voltage on "46V" rails (CB54) increases from 40V to 51V.
I would guess this is supposed to happen when the user turns up the volume and the amplifier needs to put out more power.

I believe this is so-called NAD PowerDrive. It is supposed to switch from the higher voltage rails to the lower voltage rails when more current is needed.

[paul_g_787]

OK so had a look at the 5V rail and I am getting 0V!!

I measured at the capacitor C444 and I am getting 10.37V DC with no obvious AC ripple. 

The strange thing it that the amp came to life after probing the AMS1117-5.0 at pin 2...... I had 4.96V to GND and the amp switched on and worked! Then after turning the amp off and on it is now dead again and I have 0V.

I am wondering maybe a bad joint on the IC45?

I am going to see if I can get my ESR meter across C445 as well.

[paul_g_787]

OK something now.

I noticed some residual crud.. maybe flux. around IC45's pins. I just cleaned this off with IPA and BAM!!! the 5V is back! And then it went back to 0 again.
So I cleaned it again and now it is back and has stayed back. I have tried prodding the IC and shocking the pcb a little but it appears that whatever it was has now been dislodged. Maybe some solder spatter?

Unfortunately I cannot get to C445 to check it without de-soldering the +/-46V wires which go via tubes in the heat-sink to the power transistors. They are double-sided soldered.... Not sure I want to tamper with them.
However C445 does not appear to have bulged or oozed in any way and as the 5V is reading OK now (and there is no ripple I can see) I think it is fine.

But.... When I press the power button... Nothing. The LED is solid Orange. 
So I checked all my rails and these look OK, however the +/-46 and +/- 27 Rails are reading the same? I checked for direct shorts (with probes both ways) and there are none. But seems suspicious...... 

+37V = +39.3V
-37 = -39.1V
+46V = +39.3V
-46V = -39.1V
+5V = +5.0V

However the 18V rails are measuring:
+18V = 0V
-18V = -1.1V

The next thing the Troubleshooting guide says is "Is the standby signal low level?" I assume this is at R401/C41 in the supply circuit? What voltage is this supposed to read here?

[Kim Christensen]

I noticed some residual crud.. maybe flux. around IC45's pins. I just cleaned this off with IPA and BAM!!! the 5V is back! And then it went back to 0 again.

There could still be a cold solder joint that you just can't see. I would reflow IC45's pins even though the 5V seems OK. It may still be intermittent and causing problems on power up even though you see it stable later when you probe it.

[paul_g_787]

I noticed some residual crud.. maybe flux. around IC45's pins. I just cleaned this off with IPA and BAM!!! the 5V is back! And then it went back to 0 again.

There could still be a cold solder joint that you just can't see. I would reflow IC45's pins even though the 5V seems OK. It may still be intermittent and causing problems on power up even though you see it stable later when you probe it.

Well it was tricky but I managed to wick the old solder off the IC and apply new solder.
(please excuse the cotton swab fibres left on the board, I promise I will clean them off even though they are non conductive).

As you can see it is in a tricky location to get to and there is no way to remove the PCB without de-soldering the red, white and black power wires going via the heat-sink tubes.

I put some Kapton tape on the speaker wires just in case I accidentally contacted them with the iron but I did manage it without making contact.

Unfortunately it has made no difference to the operation but at least we know that IC45 now has good connections.

I have a stable 5V and the amber standby light is on, but pressing the button does nothing.

So it might be something with the standby circuit? I will try and measure the standby signal voltage and see what it reads.

Although the standby is supposed to be on until the button is pressed.. so something is stopping it coming 'out of standby'

[Kim Christensen]

The next thing the Troubleshooting guide says is "Is the standby signal low level?" I assume this is at R401/C41 in the supply circuit? What voltage is this supposed to read here?

When the unit is NOT in standby, I would say the "standby signal" at R401/C41 would be less than 0.6V or so. Once standby goes above apx 1.4V, current will flow through R104 and Q43 will start to turn on more. If the standby voltage goes high enough, then Q43 will saturate and the 18V supplies will turn off.

[paul_g_787]

OK so I took some measurements at connector CB34 which is conveniently labelled.

SOFT CLIPPING is at 0.001V
MUTE is at 0.004V
STANDBY is at 0.001V
PROTECT is at 0.002V
+5V is at 4.99V

But still pressing the standby button does not power on the unit. The Amber LED is lit solid.

[paul_g_787]

According to the troubleshoot in the SM I should check the protect circuit.

[paul_g_787]

OK so quick look at the protect circuit shows something.

Page 17 of the service manual for schematic.

IC44 Pin readings as follows:
1 = 0.01V
2 = 0.01V
3 = 0.01V
4 = -22.5V
5 = 0.00V
6 = -0.35V
7 = 0.15V
8 = 0.15V

Interestingly though IC44 is actually a PCB and not an IC but if you look at Page 37 of the service manual is details changing the IC44 for a protect module and shows the new schematic and some other component changes.

[Kim Christensen]

Page 17 of the service manual for schematic.
IC44 Pin readings as follows:

Unfortunately, due to the changes in "IC44" and other components, you'll have to ignore most of those protection circuit voltages listed on page 17.

A good example would be pin 4 of "IC44": Because D513 has been reversed, now instead of +2V you'll have a negative voltage on the "AC DET" line which is the new normal. So reading -22.5V on pin 4 is probably not unreasonable.

The one that looks wrong to me is pin 8 of "IC44". There should be around 3.6V or so there. Is R441 OK? Also, with power off, what does an ohmmeter and "diode check" show when measuring between pins 5 & 8 of "IC44"?

[paul_g_787]

The one that looks wrong to me is pin 8 of "IC44". There should be around 3.6V or so there. Is R441 OK? Also, with power off, what does an ohmmeter and "diode check" show when measuring between pins 5 & 8 of "IC44"?

R441 measures 4.6k ohms so looks good!

With the meter + on pin 5 and - on pin 8
0.712 in Diode test mode
3.8k ohm amd slowly increasing

With the meter - on pin 5 and + on pin 8
1.68 and slowly increasing in Diode test mode
8.68k ohm and slowly increasing

Why is the value of these readings going up when I leave the meter connected a while? Charging capacitors maybe?

Just powered the amp on again to check pin 8 of "IC44" and it measures 0.15, however... when I press the power button this voltage on pin 8 goes up to 4.68V which then settles to 4.23V after a few seconds.

So before pressing the power button "IC44" Pin readings as follows:
1 = 0.01V
2 = 0.01V
3 = 0.01V
4 = -22.5V
5 = 0.00V
6 = -0.35V
7 = 0.15V
8 = 0.15V

After pressing the power button "IC44" Pin readings as follows:
1 = 0.01V
2 = -0.02V
3 = 0.01V
4 = -22.2V
5 = 0.01V
6 = 0.01V
7 = 1.43V
8 = 4.23V

[paul_g_787]

Just went to take measurements again to confirm and my 5V rail was dead again!!!!   
10.3V at input and 0V at output of IC45 (the 5V regulator)

Again.. I cleaned IC45's pins with IPA and dried it with cotton swabs. Powered it on and it is working.......... This is very puzzling.... 

Anyway I just took the measurements of IC44 again and got the same values before and after power button press, and my 5V rail is working still.

[Kim Christensen]

Just powered theamp on again to check pin 8 of "IC44" and it measures 0.15, however... when I press the power button this voltage on pin 8 goes up to 4.68V!!

I wonder if D431 (3.6V zener) is actually missing since the voltage rises to 4.68V... This would make sense because there is a 4.7V zener (D13) on the "IC44" PCB which is effectively in parallel with D431.
It is possible that Q412 is being turned on by the standby input via R437 or maybe the overtemp sensor (R438/439 combo). If Q412 is on, this would drop the voltage at pin 8 of "IC44" to near zero.

[Kim Christensen]

Again.. I cleaned IC45's pins with IPA and dried it with cotton swabs. Powered it on and it is working.......... This is very puzzling...

It might be that the cooling effect of evaporating alcohol is making that regulator work properly until it heats up and fails.

[paul_g_787]

Again.. I cleaned IC45's pins with IPA and dried it with cotton swabs. Powered it on and it is working.......... This is very puzzling...

It might be that the cooling effect of evaporating alcohol is making that regulator work properly until it heats up and fails.

Good thought, however I left the amplifier over-night (approx 11 hours) and the next day it still worked fine. Would the alcohol not have evaporated overnight? 

Perhaps whatever the fault elsewhere is, is drawing too much current from the regulator and it is getting hot and switching off?

If this happens again I will power it off at the mains for a while and refrain from cleaning it with IPA and see if it "cools down" and works again to confirm if IC45 is overheating.

[paul_g_787]

Just powered theamp on again to check pin 8 of "IC44" and it measures 0.15, however... when I press the power button this voltage on pin 8 goes up to 4.68V!!

I wonder if D431 (3.6V zener) is actually missing since the voltage rises to 4.68V... This would make sense because there is a 4.7V zener (D13) on the "IC44" PCB which is effectively in parallel with D431.
It is possible that Q412 is being turned on by the standby input via R437 or maybe the overtemp sensor (R438/439 combo). If Q412 is on, this would drop the voltage at pin 8 of "IC44" to near zero.

Interestingly D431 is present on the schematic but not on the PCB layout on page 20 in the service manual....?
I can't see this Diode on the real PCB but not sure where to look.

[Kim Christensen]

Perhaps whatever the fault elsewhere is, is drawing too much current from the regulator and it is getting hot and switching off?

Could be. Though putting your finger on it would probably confirm overcurrent fairly easily. You could put a clampon ammeter (Or break the lead with regular VOM) in series with the 8 VAC wire that feeds D430 (wires coming off CB47) and see what changes when you lose the 5V.

Interestingly D431 is present on the schematic but not on the PCB layout on page 20 in the service manual....?
I can't see this Diode on the real PCB but not sure where to look.

Looks like a "typo" in their documentation. It wouldn't make sense to have both zeners installed.

[paul_g_787]

Perhaps whatever the fault elsewhere is, is drawing too much current from the regulator and it is getting hot and switching off?

Could be. Though putting your finger on it would probably confirm overcurrent fairly easily. You could put a clampon ammeter (Or break the lead with regular VOM) in series with the 8 VAC wire that feeds D430 (wires coming off CB47) and see what changes when you lose the 5V.

Interestingly D431 is present on the schematic but not on the PCB layout on page 20 in the service manual....?
I can't see this Diode on the real PCB but not sure where to look.

Looks like a "typo" in their documentation. It wouldn't make sense to have both zeners installed.

Yes OK if the 5V fails again I will check if the regulator IC45 is hot, then let it cool off, then see if it works again after.

And yes I am suspecting that the Zener is not actually there either.

So what is the next step? I can't get to the IC45 to remove it at all without causing a lot of work!

Perhaps there is a short circuit somewhere from the 5V rail which is causing both my faults. - overheating (possibly) IC45 and also the protect kicking in? maybe?

Ouch my brain on this one hehe

[Kim Christensen]

So what is the next step? I can't get to the IC45 to remove it at all without causing a lot of work!
Perhaps there is a short circuit somewhere from the 5V rail which is causing both my faults. - overheating (possibly) IC45 and also the protect kicking in? maybe?

It looks like IC45 is the only thing fed from the 8VAC winding on the transformer. (via D420 and C444)... The good news is, there is a fuse (F55) on the 8 VAC side of D420 on the power board. If you have a 2nd meter, you could flip this fuse up and insert the meter as an AC milliammeter and note what the current is during normal operation. Then wait and see how the current changes when you lose the 5VDC.
If the current dramatically increases, then you've most likely got a short on the 5V output of IC45. (Tiny chance IC45 is bad. More likely some capacitor on the 5V line somewhere else)
But if current drops almost to zero, then the regulator is going open and is faulty.
If you don't have a 2nd meter, measure the AC voltage drop across the fuse (will be in AC millivolts) during normal operation (write it down) and compare to when the fault shows up. ie: Use the fuse as a pseudo "current" shunt.

I had a look at the AMS1117-5.0 datasheet and it is rated for 1A. F55 is rated at 0.5A so you'd think that if the output gets shorted that the fuse would blow. Though IC45 has thermal limiting so current may be less than 1A if it gets very hot.

[paul_g_787]

So what is the next step? I can't get to the IC45 to remove it at all without causing a lot of work!
Perhaps there is a short circuit somewhere from the 5V rail which is causing both my faults. - overheating (possibly) IC45 and also the protect kicking in? maybe?

It looks like IC45 is the only thing fed from the 8VAC winding on the transformer. (via D420 and C444)... The good news is, there is a fuse (F55) on the 8 VAC side of D420 on the power board. If you have a 2nd meter, you could flip this fuse up and insert the meter as an AC milliammeter and note what the current is during normal operation. Then wait and see how the current changes when you lose the 5VDC.
If the current dramatically increases, then you've most likely got a short on the 5V output of IC45. (Tiny chance IC45 is bad. More likely some capacitor on the 5V line somewhere else)
But if current drops almost to zero, then the regulator is going open and is faulty.
If you don't have a 2nd meter, measure the AC voltage drop across the fuse (will be in AC millivolts) during normal operation (write it down) and compare to when the fault shows up. ie: Use the fuse as a pseudo "current" shunt.

I had a look at the AMS1117-5.0 datasheet and it is rated for 1A. F55 is rated at 0.5A so you'd think that if the output gets shorted that the fuse would blow. Though IC45 has thermal limiting so current may be less than 1A if it gets very hot.

I did exactly that. Removed F55 and connected my ammeter to the Fuse holder.

On power up from mains it draws 44mA of power.
After pressing the power button this goes up to 46mA.

Left it running for 10 mins with no issue. Still at 46mA. Haven't lost the 5V again yet.

[paul_g_787]

OK, well I haven't been able to lose the 5V rail again yet so don't know what's happening with that. it has been on for a few hours today with no problem.

Still not sure what to do next with "IC44" and the protect circuit.

What would trigger the protect circuit in this amplifier?