I have a NAD C320 (20+ years old now) that won't power up. The symtoms appear a little different to other threads here although I've tried to follow advice from those but to no avail.
Symptoms:
Switch on the power on front panel. Red LED illuminates few a few seconds and then rather than switch to green I hear a relay click and red light goes out.
Looking at the amp one of the big 15000uF caps was bulging so I replaced both (C417 & C418). At the same time replaced the two adjcent ones (C423 & C424). Also, based on comments in other threads replaced C420, C421 & C422.
Testing the power rails with power supply board disconnected I'm seeing +46V & -46V as well as +37V & -37V so I'm fairly confident the power supply is working and the problem is with the protection circuit, although not sure as the symptoms quite line-up (I think the red light should go on after a brief green rather than off competely).
Any tips on how to diagnose this? I have the service manual but don't really know what to measure given the symptoms above.
Symptoms:
Switch on the power on front panel. Red LED illuminates few a few seconds and then rather than switch to green I hear a relay click and red light goes out.
Looking at the amp one of the big 15000uF caps was bulging so I replaced both (C417 & C418). At the same time replaced the two adjcent ones (C423 & C424). Also, based on comments in other threads replaced C420, C421 & C422.
Testing the power rails with power supply board disconnected I'm seeing +46V & -46V as well as +37V & -37V so I'm fairly confident the power supply is working and the problem is with the protection circuit, although not sure as the symptoms quite line-up (I think the red light should go on after a brief green rather than off competely).
Any tips on how to diagnose this? I have the service manual but don't really know what to measure given the symptoms above.
Measure the DC offset of each channel before the speaker relay. Each should be close to zero volts.
I'm seeing -0.14V and -0.20V shortly after power on and these seem to gradually rise and after a few minutes seem to have stabilised at -0.25 and -0.31. Power off and these slowly drop back to zero. This is repeatable.
Measured with a trusty old Fluke 73 multimeter so it should be reasonably accurate.
One other thing I just noticed: after switching the power switch off the red power LED illuminates once more before fading out as presumably a capacitor discharges.
Measured with a trusty old Fluke 73 multimeter so it should be reasonably accurate.
One other thing I just noticed: after switching the power switch off the red power LED illuminates once more before fading out as presumably a capacitor discharges.
Those results are a bit unexpected tbh, I was expecting one channel to have a large offset.
Given that both channels are similar I would do two things. First measure the +18 and -18v stabilised rails circled here and make sure they are constant. They should not drift much at all. If that seems OK then I would short out the cap C422 marked here and see if the relay operates normally. Do not connect any speakers at this point.
Given that both channels are similar I would do two things. First measure the +18 and -18v stabilised rails circled here and make sure they are constant. They should not drift much at all. If that seems OK then I would short out the cap C422 marked here and see if the relay operates normally. Do not connect any speakers at this point.
No sign of 18V at either, during or after the 2s period where the red LED is lit.
Q41: Emitter=0V, collector & base = 0.28V
Similarly
Q42: Emitter = 0V, collector & base = -0.25V
(hope I'm reading the pins in the right order)
I didn't bother try shorting C422 given these readings.
It's not obvious (to me) looking at the circuit diagram where the +/-18V should be coming from as that's helpfully labelled at the edge of the page with the voltages but I don't see an obvious source at another diagram edge, but it sounds like that's perhaps next step?
Remote diagnostics of a circuit board via a novice is an amazing skill. I admire your patience.
Q41: Emitter=0V, collector & base = 0.28V
Similarly
Q42: Emitter = 0V, collector & base = -0.25V
(hope I'm reading the pins in the right order)
I didn't bother try shorting C422 given these readings.
It's not obvious (to me) looking at the circuit diagram where the +/-18V should be coming from as that's helpfully labelled at the edge of the page with the voltages but I don't see an obvious source at another diagram edge, but it sounds like that's perhaps next step?
Remote diagnostics of a circuit board via a novice is an amazing skill. I admire your patience.
Good progress!
The source of the +/- 18V regulators is on page 12 of the service manual.
Sorry, inexperience showing here. I see +/-18V on the right-hand side, so is it the case that the source is on the left, i.e. the +/- 37V that come in via connector CZ46/CB47?
If so, what I'm measuring on this connector doesn't make much sense. What I see as 1,2,3,4,5 on the diagram, labelled -37V,+37V,I-V,AC DET,TRIG, doesn't directly correlate with the PCB which has somewhat different labels, although AC Detect is a good clue I suspect.
On the main PCB there are the following labels with voltages measured coming from the PSU board with connector both unplugged and plugged-in to main board respectively:
ACD must be AC detect = pin4, so presumbaly VB2 should be -37V and VC2 should be +37V and that's clearly not the case, with readings looking quite a lot like what we were seeing on Q41 and Q42 (makes sense!)
This is contrary to what I said earlier based on what I thought I saw last night, perhaps too hasty in thinking I was seeing what was expected (just looked at various voltages of pin pairs unplugged and could see combinations that looked like rougly +/-18V - apologies if I've led us down the wrong path - at the time I had no idea what the labels meant and was happy to see what looked reasonable/expected).
If so, what I'm measuring on this connector doesn't make much sense. What I see as 1,2,3,4,5 on the diagram, labelled -37V,+37V,I-V,AC DET,TRIG, doesn't directly correlate with the PCB which has somewhat different labels, although AC Detect is a good clue I suspect.
On the main PCB there are the following labels with voltages measured coming from the PSU board with connector both unplugged and plugged-in to main board respectively:
| Label | Unplugged | Plugged-in |
| LHD | 0V | 0V |
| ACD | +19.2V | +2V |
| LHS | +38.8V | +0.19V |
| VC2 | +0.33V | +0.3V |
| VB2 | -0.25V | -0.25V |
ACD must be AC detect = pin4, so presumbaly VB2 should be -37V and VC2 should be +37V and that's clearly not the case, with readings looking quite a lot like what we were seeing on Q41 and Q42 (makes sense!)
This is contrary to what I said earlier based on what I thought I saw last night, perhaps too hasty in thinking I was seeing what was expected (just looked at various voltages of pin pairs unplugged and could see combinations that looked like rougly +/-18V - apologies if I've led us down the wrong path - at the time I had no idea what the labels meant and was happy to see what looked reasonable/expected).
The C320 turns on with the 18vdc regulators in standby so that the preamp modules are not active and there are no noises from the speakers. The 18vdc is enabled by pressing a channel button. After that the green LED of the selected channel implies the 18vdc regulators are active.
I repaired my C320 as described in the "hot heatsinks" thread.
- https://www.diyaudio.com/community/threads/hot-heatsinks-in-vintage-amps-such-as-nad-c320.391553/
I hope fubar3's post helps resolve the problem. Do you have the remote, does the amp respond to the keypad?
The NAD schematics seem to be a mess. I'm concerned that there might be a supply problem in front of the 18V regulator system:
I agree the CZ46/CB47 connector pair appears to be the path from rectifiers to the 18V regulators. The mismatched labels are silk-screened labels on the PCB? And inconsistent with the schematic? I suspect the labels for CB47 pins 1 and 2 are probably a drafting error and should have been +/- 37V rather than +/- 17V; 17v is less than the 18V regulator outputs, so doesn't make sense anyway.
I recommend confirming +/- 37V is present at CZ46, passes through CB47 and makes its way to Q41 and Q42. This would be a good precursor test for getting the 18V regulators to activate.
The NAD schematics seem to be a mess. I'm concerned that there might be a supply problem in front of the 18V regulator system:
I agree the CZ46/CB47 connector pair appears to be the path from rectifiers to the 18V regulators. The mismatched labels are silk-screened labels on the PCB? And inconsistent with the schematic? I suspect the labels for CB47 pins 1 and 2 are probably a drafting error and should have been +/- 37V rather than +/- 17V; 17v is less than the 18V regulator outputs, so doesn't make sense anyway.
I recommend confirming +/- 37V is present at CZ46, passes through CB47 and makes its way to Q41 and Q42. This would be a good precursor test for getting the 18V regulators to activate.
Yes. (I haven't had a chance to read all through the linked thread 🙂
If any of these have failed then then check the four diode bridge for any shorts.
Thanks everyone.
The fuses are certainly OK and before the resistors (R447 & R449) I see 60VAC, so the transformer must be OK too, however when we get to the diodes I'm seeing just about nothing even when there is no load. Measuing the resitance of R447 & R449 I'm seeing mega ohms, presumably from paths elsewhere on the board, so they are clearly shot as they should be 2.2 ohm each. This is somewhat surprising as I'd expect to be seeing some visible change to the resistors as I imagine they must have had too much current through them, presumably as a result of a problem (short) further on.
These two resistors are marked with <!> on the diagram indicating that they should only be replaced with "parts recommended by the manufacturer", hard to see what could be special about a 1W 5% 2.2ohm resistor though. Could there be?
I guess before I replace these it would be useful to try to identify what caused them to fail. I've already replaced C423 and C424 although there appeared to be nothing wrong with them (if I measure the old ones now the meter sees mega ohms and I believe that electrolytic cap catastophic failure should result in a low resistance or even a short).
Measuring voltage drop (feature of the Fluke) of the 4 diodes shows 0.56V so pretty sure they are OK.
Small & frequent orders of components are going to cost me loads of postage and take lots of time so I'd rather pre-emptively order more components that might need replacing rather than a few at a time.
The fuses are certainly OK and before the resistors (R447 & R449) I see 60VAC, so the transformer must be OK too, however when we get to the diodes I'm seeing just about nothing even when there is no load. Measuing the resitance of R447 & R449 I'm seeing mega ohms, presumably from paths elsewhere on the board, so they are clearly shot as they should be 2.2 ohm each. This is somewhat surprising as I'd expect to be seeing some visible change to the resistors as I imagine they must have had too much current through them, presumably as a result of a problem (short) further on.
These two resistors are marked with <!> on the diagram indicating that they should only be replaced with "parts recommended by the manufacturer", hard to see what could be special about a 1W 5% 2.2ohm resistor though. Could there be?
I guess before I replace these it would be useful to try to identify what caused them to fail. I've already replaced C423 and C424 although there appeared to be nothing wrong with them (if I measure the old ones now the meter sees mega ohms and I believe that electrolytic cap catastophic failure should result in a low resistance or even a short).
Measuring voltage drop (feature of the Fluke) of the 4 diodes shows 0.56V so pretty sure they are OK.
Small & frequent orders of components are going to cost me loads of postage and take lots of time so I'd rather pre-emptively order more components that might need replacing rather than a few at a time.
I don't want to hijack the thread. But I also have a NAD-C320. In my NAD the problem is with the DC offset in the left channel only. The DC offset is around -1.7V. But the problem is that this DC offset disperses after some time, 30s or so after power-up. Do you have any suggestions in that case?
I wouldn't think there's anything exotic about the resistors, but flame resistant and/or fusible features spring to mind.
I'm attaching a Digikey search link because I like their search engine. Not sure if that helps in GB.
https://www.digikey.com/en/products...lsqTAFcO2EPh0sVACwC2mQyHO8WMAHRx851AA97ToYMFA
I assume the the offset is beyond the adjustment range of VR21?
Perhaps it will be an easy problem---maybe it's leakage through C216//217. Measure voltage across R253 and if it's near -1.7V, that's highly suspicious for leakage in the caps, most likely C216. (Voltage across C216 should be near 0V, indicating low offset error in the input stage.)
Good luck!
P.S. Any voltage across R249 is even more telling, but it will be much smaller than voltage across R253, by a factor of about 19.
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They may be 'safety resistors' which means when they fail they do so quietly and cleanly with no smoke (smoke frightens the punters 😀).
If nothing obvious reads short then you have to replace them. For test purposes only you can use standard 2.2 ohm 1 or 2 watt types but I can't endorse not fitting the correct type as a final repair.
You can also (as a test) run the amp up with a DBT (dim bulb tester) and link those resistors out. Any short should light the bulb. Ultimately if you can't find anything wrong then you have to power it up and work on it measuring to see where the issue is. Something (the fault) may smoke but that is all part and parcel of faultfinding.
It's going to get confusing with two of you in one thread 🙂
Check the voltage across the two diodes in each power amp that generate the -/+0.65 volts used for offset correction. If those voltages are not constant then check the 18v rails as in this thread and also the resistors feeding those diodes.
If its getting more complex then I can split your posts and replies off to e new thread.
Quick update. The 2R2 resistors arrived and once installed the amp appears to be operating normally. So far I've only powered it up and now the power LED switches to green, relay clicks and the various input selectors appear to work. Measuing at Q41 & Q42 shows +/- 18V, so that sounds very promising. I'm yet to attach speakers and an input source though.
If it's working then I'm wondering what to do about those two resistors. The ones removed are virtually open circuit (reading MΩs) with no signs of burning, which sounds quite odd and perhaps indicates they have a special safe mode of failure. A search of the part numbers listed in the service manual reveals nothing. As you said, you can't endorse using regular resistors, but what's the worst that can happen?
If it's working then I'm wondering what to do about those two resistors. The ones removed are virtually open circuit (reading MΩs) with no signs of burning, which sounds quite odd and perhaps indicates they have a special safe mode of failure. A search of the part numbers listed in the service manual reveals nothing. As you said, you can't endorse using regular resistors, but what's the worst that can happen?