Author Topic: NAD C325BEE Will not power.  (Read 5875 times)

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Offline Kim Christensen

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Re: NAD C325BEE Will not power.
« Reply #25 on: January 06, 2022, 11:25:12 pm »
What would trigger the protect circuit in this amplifier?

Overheating (R439)
Standby being active (via R437)
Output overcurrent. DC bias not being zero or failing to get to zero soon enough after power on. (Q12/1, Q11, Q21, etc)
 

Offline paul_g_787

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Re: NAD C325BEE Will not power.
« Reply #26 on: January 08, 2022, 04:39:02 pm »
What would trigger the protect circuit in this amplifier?

Overheating (R439)
Standby being active (via R437)
Output overcurrent. DC bias not being zero or failing to get to zero soon enough after power on. (Q12/1, Q11, Q21, etc)

OK thanks I will take some measurements at these components and see what I find.
 

Offline paul_g_787

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Re: NAD C325BEE Will not power.
« Reply #27 on: January 12, 2022, 04:04:30 pm »
What would trigger the protect circuit in this amplifier?

Overheating (R439)
Standby being active (via R437)
Output overcurrent. DC bias not being zero or failing to get to zero soon enough after power on. (Q12/1, Q11, Q21, etc)

Thanks for pointing me in this direction. I took some measurements at the input of the protect circuit I measured the Temperature and Standby Signals at the anodes of D42 and D43.

D42 (Temperature) Is at 0.001V and does not changed after the standby / power button is pressed.
D43 (standby is at 1.2V and changes to 0.000V after the standby / power switch is pressed.

The amplifier however still has an amber LED and does not power on. :--

So I believe that this proves the standby circuit is working fine and that it is not overheating.  :-+

I also checked the power amp output before the relay (RL12/1 pins 5 and 3) and these are both at -1.4V DC  :-//

I think you may be thinking along the right lines of the DC bias but my knowledge in this area is limited.  :-//
I am a little confused how can I check this on this amplifier?

My +12V, +18V and -18V power supply rails are all still at 0V. Would I be right to assume that these are turned on by the standby circuit but the protect is keeping them turned off?

Edit: I left an important note out. The output relay switch is not switching on.  I notice the protect circuit taps from after the relay switch via R446 and R445, so this must be where the protect circuit "checks" for clipping and DC offset right?
« Last Edit: January 12, 2022, 05:39:30 pm by paul_g_787 »
 

Offline paul_g_787

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Re: NAD C325BEE Will not power.
« Reply #28 on: January 12, 2022, 07:04:35 pm »
OK so I found something!!



Read the comments, the chap here found that his R45 and R46 were open circuit.

So I checked mine and R46 is fine, 32.8Ω but R45 is open circuit! The pads underneath are a little blackened too.
Edit: I de-soldered R45 and the pads are obliterated! They have de-laminated and disintegrated! It must have got very very hot!
I will have to run some nice neat jumper wires over to these spots but for now I have just tacked the replacement on with some wires hanging out the side of the amp for "testing purposes".


On the schematic it is labelled 33Ω 0.5W FS. What does the FS mean? Is it a fusible resistor I assume?
The parts list specifies 33R, 1/2W, ±5%, FS
« Last Edit: January 12, 2022, 10:25:24 pm by paul_g_787 »
 

Offline paul_g_787

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Re: NAD C325BEE Will not power.
« Reply #29 on: January 12, 2022, 08:52:25 pm »
OK so I just replaced R45 with a new 33Ω 0.5W Metal Film resistor. Now the problem has changed.

The amplifier turns on after pressing the power button but the power LED is Red and soon after the output relay turns off!  :-- The user manual says this is the protect circuit.

I just had a check of D42 and D43 again (Temperature and Standby Signal). These both stay at 0V still so it is not those.

So I measured the output on both channels before the protect relay and the Left channel has -12.4V DC on it! This must be the fault.

Edit: Was just taking the measurements again and the new R45 fusible resistor blew again! R46 is still fine. So there might be a short somewhere on the left channel causing the -12.4V. But how does R45 blow from this?
Edit 2: Consulting the Troubleshooting guide says if the output DC level is not normal to "check the servo circuit". There is no other word "servo" in the service manual. What is a servo circuit? I haven't come across this term with amplifiers before. (You can tell I am new to this lol).
Edit 3: I just measured the idling current on both channels between TP1/TP2 (Left Channel) and TP3/TP4 (Right Channel). The service manual says it should be 4mV, however the left channel has 14.8mV and the right channel has 9.2mV. Related perhaps?
« Last Edit: January 12, 2022, 09:51:33 pm by paul_g_787 »
 

Offline Kim Christensen

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Re: NAD C325BEE Will not power.
« Reply #30 on: January 13, 2022, 03:38:40 am »
Quote
the Left channel has -12.4V DC on it! This must be the fault.
Yup. That should trigger the protect circuit!

Quote
Edit: Was just taking the measurements again and the new R45 fusible resistor blew again! R46 is still fine. So there might be a short somewhere on the left channel causing the -12.4V. But how does R45 blow from this?

R45 is blowing because of over current on the +18V rail or a fault with the +18V regulator circuitry itself. (Q41, Q43, D49, etc)
With the power off, try ohm metering the +18V rail (across C49 or other convenient points). Hard to say exactly what you should see, but a short would be an obvious fault. Or a reading of around 10 ohms would point toward a shorted C13 or C23. Losing the +18V rail would definitely unbalance the amplifier and cause these types of issues. The +18V goes many places. Even the pre-amp.
If you end up replacing R45 again, put a voltmeter on the +18V rail before powering it on. When you power it on, see if the +18V comes up immediately. If not, power it off again right away. (Or if there is +18V but R45 is getting hot)

Quote
Edit 2: Consulting the Troubleshooting guide says if the output DC level is not normal to "check the servo circuit". There is no other word "servo" in the service manual. What is a servo circuit? I haven't come across this term with amplifiers before. (You can tell I am new to this lol).

I would guess that they mean the L&R power amplifiers feedback paths / circuits.

 

Offline paul_g_787

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Re: NAD C325BEE Will not power.
« Reply #31 on: January 13, 2022, 11:31:27 pm »
Alright I was thinking it would be a short on the +18V rail. Glad I am thinking along the right lines and thank you, your explanation has made it clearer what is going on here and how the DC is being generated at the output. I am still learning and this is super helpful!

I only had one 33 ohm 0.5W metal film resistor, so I have ordered some more of those.

In the meantime I will perform the checks you suggested that I can do now (and the rest of them when my resistors arrive). :)
 

Offline paul_g_787

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Re: NAD C325BEE Will not power.
« Reply #32 on: January 14, 2022, 08:51:21 pm »
R45 is blowing because of over current on the +18V rail or a fault with the +18V regulator circuitry itself. (Q41, Q43, D49, etc)
With the power off, try ohm metering the +18V rail (across C49 or other convenient points). Hard to say exactly what you should see, but a short would be an obvious fault. Or a reading of around 10 ohms would point toward a shorted C13 or C23. Losing the +18V rail would definitely unbalance the amplifier and cause these types of issues. The +18V goes many places. Even the pre-amp.
If you end up replacing R45 again, put a voltmeter on the +18V rail before powering it on. When you power it on, see if the +18V comes up immediately. If not, power it off again right away. (Or if there is +18V but R45 is getting hot)

Just took a measurement across C49 with my ohm meter and it is 349Ω. Is this particularly low?

I also just measured across C410 (-18V rail) as a comparison and this is 4.48kΩ

I might add that this is while the amplifier is powered off of course.
 

Offline Kim Christensen

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Re: NAD C325BEE Will not power.
« Reply #33 on: January 15, 2022, 04:53:39 am »
Just took a measurement across C49 with my ohm meter and it is 349Ω. Is this particularly low?

Unfortunately that isn't very conclusive... But, since we know that R45 is a 33ohm 0.5W resistor we can conclude that if we pass more than 123mA through it we'll exceed it's power rating. Therefore we know that the current supplied by the +18v regulator will be quite a bit less than 123mA.
So if you have a small bench power supply with current limiting you could feed the +18V regulator with that and set it's current limiting to 60mA or so. See diagram attached.
Then you'd power on your amplifier and then power on the bench power supply and slowly increase it's voltage while keeping an eye on the current draw (And the +18V rail voltage) as you raise the voltage towards +37V. This way you can limit the current and have time to find the fault without things burning up. There are a few resistors that you can measure the voltage across to figure out how much current is going through them: R254, R154, R616, R615
 
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Offline paul_g_787

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Re: NAD C325BEE Will not power.
« Reply #34 on: January 19, 2022, 07:53:22 pm »
But, since we know that R45 is a 33ohm 0.5W resistor we can conclude that if we pass more than 123mA through it we'll exceed it's power rating.
Oh yes I see: √(0.5W÷33Ω) = 0.1231A Or 123mA

So if you have a small bench power supply with current limiting...
Unfortunately I do not have a power supply with current limiting (it is on my list to get one). Only an old ATX PC power supply I converted into a very basic variable voltage power supply.

I just received my new 33Ω 0.5W Metal Film Resistors today!  :-+

I don't mind sacrificing one or two resistors, they cost pennies. Hopefully nothing else blows up in the meantime though.


So I replaced R45 (and also R46 because I sacrificed R46 in R45's place earlier and it is in resistor heaven) with brand new parts.

I measured the voltage across the resistors in standby, protection mode (for a few seconds at power on) and also at power on state.

As you can R45 (on the +18V rail) is indeed drawing more than it's power rating by quite a bit, but only after the protection turns off.
Although it does appear to be drawing a bit too much current for my liking even in protection mode!

R45 = 32.97Ω
STANDBY  0.00V (0mA / 0mW)
PROTECT  3.76V (114mA / 432mW)
ON             5.23V (158mA / 826mW)


Although R46 (on the -18V rail) is looking good to me.  :-+

R46 = 32.78 Ω
STANDBY  0.10V (3mA / 0.3mW)
PROTECT  2.90V (89mA / 258mW)
ON             2.91V (89mA / 259mW)

The good news is that with the two resistors in circuit I have no DC on the speaker outputs and the protect mode does indeed switch off when it is supposed to.

There are a few resistors that you can measure the voltage across to figure out how much current is going through them: R254, R154, R616, R615

I will check these next and see what the current is.
 

Offline paul_g_787

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Re: NAD C325BEE Will not power.
« Reply #35 on: January 21, 2022, 03:35:14 pm »
Just had a measure of R615 and R616 on both CB32 (In Module) and CB33 (Out Module)

These resistors are specified as 33Ω 1/6W (0.167W)
So I calculate these would max out at 71mA (a max 2.34V across the resistor)

All four of these resistors measures 0.49V across them which I calculate as approx 15mA or 7.3mW.
So looking good here.

So I next measured R154 on CB11. This is measuring 10Ω.
Then I measured R254 on CB21. This is measuring 67Ω!!! (mwah mwah mwaaahhh) And it is a little blackened too. Finally a clue!

So the short is likely to be in this module CB21??

I just measured Q22 and Q23, these do not appear to be shorted.
« Last Edit: January 21, 2022, 03:57:13 pm by paul_g_787 »
 

Offline paul_g_787

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Re: NAD C325BEE Will not power.
« Reply #36 on: January 21, 2022, 04:24:53 pm »
OK so my 5V rail disappeared again while I was checking the voltage across R254. It happened about 3 seconds after I had removed my probes from R254 and was writing the voltage down.

I tried feeling IC45 immediately and it is not even warm. I probed it with the multi-meter temperature probe and it is at 17.1°C and the ambient room temperature is 16.9°C.

So I just cleaned very carefully the pins of IC45 again and made sure the alcohol was dry thoroughly. Powered it on and 5V is back!! This is completely confusing me......


Anyway, I measured 17.87V across R254, which is still measuring 67Ω which is 4.76W!! That is a lot.
This would be 31.93W across the un-damaged rating of 10Ω!!

I tested Q201 and If I put the + multimeter probe on the Collector and the - probe on the Base I get a reading of 0.6V. I repeated this test on Q101 and I get the same measurement.
If I swap the probes so the + is on the Base and the - on the collector I get a reading of 0.2V This seems wrong to me... I repeated this test on Q101 and I get an infinite measurement here.
« Last Edit: January 21, 2022, 04:38:08 pm by paul_g_787 »
 

Offline paul_g_787

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Re: NAD C325BEE Will not power.
« Reply #37 on: January 21, 2022, 06:55:29 pm »
I managed to remove the whole module. Here is a close up photo of it.

A couple of spots look like they got pretty hot. R219 doesn't look too good (under capacitor top right), neither does R205 (under capacitor top left) and of course R254 (middle right).
 

Offline paul_g_787

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Re: NAD C325BEE Will not power.
« Reply #38 on: January 21, 2022, 07:56:15 pm »
Just tested the components on this module.

Green components appear to be fine.
Red are not testing as they should be.
Orange components are ones I was getting inconclusive measurements on in circuit.
 

Offline paul_g_787

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Re: NAD C325BEE Will not power.
« Reply #39 on: January 21, 2022, 08:03:55 pm »
Just found this thread. Interestingly the last poster is also having issues with the same module as mine.
https://www.diyaudio.com/community/threads/nad-325bee-will-not-turn-on.245053/
 

Offline Per Hansson

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Re: NAD C325BEE Will not power.
« Reply #40 on: January 21, 2022, 08:14:11 pm »
Maybe it's not of great help but I repaired this exact model of unit a few months back.
C110, C111,C210,C211 where completely open, they sit right behind the module you have removed between some large power resistors that run around 100°C
Also the SMD caps on those modules where completely shot too, so they where replaced also.
The actual main fault of my unit was that Q213 KSA1220 and Q214 KSA1943 where shorted.

One thought that occurred to me when you bypassed that fuse and the unit worked fine:
Have you ruled out that the fuse F55 is not actually intermittent?
Maybe the wire inside does not make contact when it gets hot and that is why your 5v fails?
 

Offline paul_g_787

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Re: NAD C325BEE Will not power.
« Reply #41 on: January 21, 2022, 09:04:53 pm »
Maybe it's not of great help but I repaired this exact model of unit a few months back.
C110, C111,C210,C211 where completely open, they sit right behind the module you have removed between some large power resistors that run around 100°C

Just removed these four capacitors and tested them on my capacitance and ESR meter and they are perfectly OK on my unit.

Also the SMD caps on those modules where completely shot too, so they where replaced also.

The 22µF ones on both modules measure just fine in circuit.
The 10µF ones I can't get a good reading in circuit but all four are behaving the same so these are possibly OK.

The actual main fault of my unit was that Q213 KSA1220 and Q214 KSA1943 where shorted.
I just tested the continuity on these transistors and they appear alright on my unit. No direct shorts between the pins.

One thought that occurred to me when you bypassed that fuse and the unit worked fine:
Have you ruled out that the fuse F55 is not actually intermittent?
Maybe the wire inside does not make contact when it gets hot and that is why your 5v fails?

This is a good point and I could order up some to test, however it would not explain the common solution is to clean the IC45 pins every time and this makes it work fine.
I am wondering if somehow a short is occurring between the pads but I cannot see anything likely in my magnifier.

I am genuinely really stuck on what to try with this amplifier next.
 

Offline Kim Christensen

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Re: NAD C325BEE Will not power.
« Reply #42 on: January 21, 2022, 09:19:58 pm »
Just tested the components on this module.
Green components appear to be fine.

When you are checking the transistors, I assume you are using the diode check function on your multimeter. If the BE and BC junctions test OK, make sure you also check between CE (Collector and Emitter) for a short.(Many times I've seen BE & BC check OK but CE is shorted) Also, the right resistor in parallel with a junction can make it look like it is good, but actually open. Other times, a low value resistor in parallel will make it look bad. I always treat a "pass" diode check on a transistor junction as a "maybe it is ok".

You've obviously got multiple faults on this PCB. But double check C23 as well because that seems to be a path that would burn out R254. Especially since Q24, Q201, and R215 visually seem to have escaped being overheated.
 

Offline paul_g_787

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Re: NAD C325BEE Will not power.
« Reply #43 on: January 21, 2022, 09:24:56 pm »
When you are checking the transistors, I assume you are using the diode check function on your multimeter. If the BE and BC junctions test OK, make sure you also check between CE (Collector and Emitter) for a short.(Many times I've seen BE & BC check OK but CE is shorted) Also, the right resistor in parallel with a junction can make it look like it is good, but actually open. Other times, a low value resistor in parallel will make it look bad. I always treat a "pass" diode check on a transistor junction as a "maybe it is ok".
Yes I am doing BE, BC. I am getting 0.6ish volts across the junctions. The probes the other way I get OL open circuit. CE I get OL, open circuit in both directions.

You've obviously got multiple faults on this PCB. But double check C23 as well because that seems to be a path that would burn out R254. Especially since Q24, Q201, and R215 visually seem to have escaped being overheated.

Indeed! This is a tricky one for me as I have never had to look this in-depth before at a power amp. This is quite a complicated looking design to me too.

I will see if I can remove C23 and check it with my meters out of circuit.

Do you think I should replace R254? I have some through hole resistors, I could temporarily install one for now and order some SMD ones later.
« Last Edit: January 21, 2022, 10:26:53 pm by paul_g_787 »
 

Offline paul_g_787

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Re: NAD C325BEE Will not power.
« Reply #44 on: January 21, 2022, 09:42:15 pm »
C23 is measuring 9.99uF with 19.6 ohms ESR. This ESR seems a bit high to me.

C24 is 9.96uF with 9.8 ohm ESR.
« Last Edit: January 21, 2022, 10:02:08 pm by paul_g_787 »
 

Offline paul_g_787

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Re: NAD C325BEE Will not power.
« Reply #45 on: January 21, 2022, 10:49:37 pm »
OK so I replaced R254 with a new 10Ω resistor. The old one was measuring 72Ω out of circuit and is clearly labelled "100" 10Ω.

I also removed R223 which was measuring 39K in circuit, however once removed it was measuring bang on 47K!! So I have re-installed it. (I really hate SMD resistors, they are so fiddly).

Is this a clue as to the location of the fault maybe?
 

Offline Kim Christensen

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Re: NAD C325BEE Will not power.
« Reply #46 on: January 22, 2022, 04:52:41 am »
I tested Q201 and If I put the + multimeter probe on the Collector and the - probe on the Base I get a reading of 0.6V. I repeated this test on Q101 and I get the same measurement.
If I swap the probes so the + is on the Base and the - on the collector I get a reading of 0.2V This seems wrong to me... I repeated this test on Q101 and I get an infinite measurement here.

Yes, I think you should revisit this. Q201 could be bad. I can't see why the BC junction would measure 0.2V when forward biased.

Quote
Yes I am doing BE, BC. I am getting 0.6ish volts across the junctions. The probes the other way I get OL open circuit. CE I get OL, open circuit in both directions.

Yup, sounds good to me.

Check: R230 on the mainboard because it feeds the +46V to the module you took out. (R219 is darkened)
To better understand this amplifier, I've highlighted some of the signal paths. I guess they drew it in this weird manner because the input stage of the amp is on a daughter board. Maybe it'll help others too:

 

Offline paul_g_787

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Re: NAD C325BEE Will not power.
« Reply #47 on: January 22, 2022, 11:55:34 am »
Thank you for that, it is really helpful.

I think the weird layout it throwing me off.

You are right on the money, R230 is looking a bit crusty and it is measuring 18Meg. Attached is a picture.

R130 on the other channel is bang on 82 ohms.
 

Offline Per Hansson

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Re: NAD C325BEE Will not power.
« Reply #48 on: January 22, 2022, 06:32:44 pm »
C23 is measuring 9.99uF with 19.6 ohms ESR. This ESR seems a bit high to me.

C24 is 9.96uF with 9.8 ohm ESR.
Yea that is bad, they are the same type of caps so their ESR should be identical.

You are right on the money, R230 is looking a bit crusty and it is measuring 18Meg. Attached is a picture.

R130 on the other channel is bang on 82 ohms.
That is really bizarre how C211 could be ok if that nearby resistor got so hot that it overheated and failed.
I noticed those caps are black on your unit: in mine all original caps where blue and brown.
One cap had been replaced previously in my unit: C47, and it was black too, perhaps someone worked on your unit before also?
« Last Edit: January 22, 2022, 07:39:11 pm by Per Hansson »
 

Offline paul_g_787

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Re: NAD C325BEE Will not power.
« Reply #49 on: January 22, 2022, 06:53:32 pm »
Yes I thought that strange too. Maybe if the ESR has gone up due to getting hot?

The owner of the amp said they have had it from new and it has been in his home studio the whole time and had no faults before.

Then it suddenly went all distorted and protection came on.

Then he brought it to me.
« Last Edit: January 24, 2022, 05:10:46 pm by paul_g_787 »
 


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