Hi folks,
I've been AWOL for a while (life gets in the way sometimes😉).
I need some help with a fault on my NAD C320. The amp is failing to come on line. When I press the front panel on button the LED stays amber for 30-45 seconds then the protection relay kicks in and the red LED comes on.
Further investigation revealed that the R channel in the power amp circuit was getting warm pretty quickly on power up compared to the L channel. I checked out the voltage for the idling current settings between TP 3 & 4 and it was 11mV (should be 3-5mV). I confirmed the L channel between TP 1 & 2 and it was bang on 3mV.
Adjustment of VR12 had zero effect on the idling current on the R channel. There is no sign of physical damage or overheating on any any of the adjacent components.
Any ideas?
I've been AWOL for a while (life gets in the way sometimes😉).
I need some help with a fault on my NAD C320. The amp is failing to come on line. When I press the front panel on button the LED stays amber for 30-45 seconds then the protection relay kicks in and the red LED comes on.
Further investigation revealed that the R channel in the power amp circuit was getting warm pretty quickly on power up compared to the L channel. I checked out the voltage for the idling current settings between TP 3 & 4 and it was 11mV (should be 3-5mV). I confirmed the L channel between TP 1 & 2 and it was bang on 3mV.
Adjustment of VR12 had zero effect on the idling current on the R channel. There is no sign of physical damage or overheating on any any of the adjacent components.
Any ideas?
Attachments
Have you checked the DC offset on the bad channel?
Voltage on L11 should be zero. Also check with a scope on L11 (the end that connects to the output stage) that there is no unwanted oscillation.
Voltage on L11 should be zero. Also check with a scope on L11 (the end that connects to the output stage) that there is no unwanted oscillation.
Hi,
Thanks for the reply.
Just checked DC offset at L11 and when I power on it jumps up to around 14.5v and then slowly starts to drop back down again when I power off.
I'll not bother scoping the output as 14.5v is definitely not right 😕
Thanks for the reply.
Just checked DC offset at L11 and when I power on it jumps up to around 14.5v and then slowly starts to drop back down again when I power off.
I'll not bother scoping the output as 14.5v is definitely not right 😕
I think what Mooly was getting at is whether there is a short circuit in the output components OR HF oscillation is occurring so using an oscilloscope in this case is helpful to eliminate that type of fault .
This isn't unusual sometimes compensation capacitors go faulty causing a massive oscillation--seen it many times .
This isn't unusual sometimes compensation capacitors go faulty causing a massive oscillation--seen it many times .
That's right, the offset is tripping the relay but 14.5v isn't what you would see with a shorted output stage.
Hf oscillation can be very asymmetric and so cause a DVM to show an apparent voltage. It does really need that scope check.
If R151 is the slightest bit warm then that shows oscillation is present. Worth making sure it is correct value as well.
Hf oscillation can be very asymmetric and so cause a DVM to show an apparent voltage. It does really need that scope check.
If R151 is the slightest bit warm then that shows oscillation is present. Worth making sure it is correct value as well.
Ah, okay.😱
I've put the amp away now, but when I get a moment over the weekend I'll whip the cover off again and scope the output.
I've put the amp away now, but when I get a moment over the weekend I'll whip the cover off again and scope the output.
sounds like a plan.Couldn't wait... I had to know😉
So, scope on the output..... nothing. R151 not even tepid. But what was curious is that (and probably coincidence) the amp sprang to life after the scope had been connected for a couple of minutes; green light on.
Whilst it was on I managed to set the idling current on the offending channel, so at least I know that is good now. Left it on for a good 10 minutes and then turned it off again. I've had a look for any obvious dry joints or bulging capacitors and can't see anything, so I'm at a loss now as to what it could be; possibly a degraded cap that improves when warm???
😕
So, scope on the output..... nothing. R151 not even tepid. But what was curious is that (and probably coincidence) the amp sprang to life after the scope had been connected for a couple of minutes; green light on.
Whilst it was on I managed to set the idling current on the offending channel, so at least I know that is good now. Left it on for a good 10 minutes and then turned it off again. I've had a look for any obvious dry joints or bulging capacitors and can't see anything, so I'm at a loss now as to what it could be; possibly a degraded cap that improves when warm???
😕
Well at least you don't have a short in the output otherwise it would never have sprung into life .
I still wouldn't eliminate a HF oscillation as the oscilloscope loading would put a small value capacitance across the output.
If you are sure there isn't a HR soldered joint then you could be right about a faulty capacitor.
I still wouldn't eliminate a HF oscillation as the oscilloscope loading would put a small value capacitance across the output.
If you are sure there isn't a HR soldered joint then you could be right about a faulty capacitor.
I've done a bit more digging.
I left the amp to cool and then tried it again with the scope connected; it failed to turn on. I left it powered for 10 minutes and tried to switch on and after about 10 seconds it came on (green LED) then went back to red. I tried again and it came on and stayed on.
I'm leaning towards cap's, possibly first stage amplifier., but not discounting the protection circuit. I'm open to the posibiliy there is more than one fault.
I left the amp to cool and then tried it again with the scope connected; it failed to turn on. I left it powered for 10 minutes and tried to switch on and after about 10 seconds it came on (green LED) then went back to red. I tried again and it came on and stayed on.
I'm leaning towards cap's, possibly first stage amplifier., but not discounting the protection circuit. I'm open to the posibiliy there is more than one fault.
I would also look at Q110 and Q111 as these will run warm/hot. Check them for dries but before you do try and see if you can get a voltage reading in the faulty state between L11 and R123 which is base of the two pre drivers.
Connect the black meter lead to L11 for the measurement as that will be the reference point. If you see a high negative voltage on R123 then that points to a problem around Q110
Connect the black meter lead to L11 for the measurement as that will be the reference point. If you see a high negative voltage on R123 then that points to a problem around Q110
Worst thing in NAD amplifiers, there are some fuseble resistors. It may be fused and there is no sign on it. And not easy to know a resistor fused. Be carefull for this.
Just a point to note with this particular amp;
It's been in storage for 4 years (in the house, so warm and dry) and not powered on until last Thursday. Before it was put away it was used pretty much every day for around 5 years and never skipped a beat.
It's been in storage for 4 years (in the house, so warm and dry) and not powered on until last Thursday. Before it was put away it was used pretty much every day for around 5 years and never skipped a beat.
From your second sentence logically it should be a capacitor /HR soldered joint fault but when capacitors go they sometimes take other components.
Before all, check voltages, main unit supply (+- 37? I'm not sure), +18v and -18v and +-5.5 supplies. All must be + and - check and must be nearly equal.
Based on the four years in storage I would also suspect a bad electrolytic cap(s). I have seen several new, unused products that have been in storage for years, fail on first use for electrolytic caps. One manufacturer even gave me a "cap re-forming" procedure to bring their unit back to life. And the data sheet for an electrolytic cap usually has a "shelf life" spec which indicates that capacitance, ESR and leakage all degrade with time. Anyway, before you try to find offending cap you could just get the unit powered up, as you have, and leave it on for a few days. This may re-form the cap enough to get it going reliably and then it will continue to re-form with use.
Will try this, but the R-channel is running hot, so I'm slightly concerned it may be too much for the amp to cope with.
Hi Mooly,
I've checked the voltages on the R-channel between L11 and directly at the base of Q111 (as I can't see R123 on the board) and I'm getting a steadily rising positive voltage (currently 19.2v).
Q110 started off at -75.8v. As the amp is warming up the voltage is steadily rising (currently -58v).
L-channel in comparison is pretty solid: Q210 -37.7v, Q211 +39.0v measured from L21.
The module on the amp marked as CB11 is getting physically warm after about 20 second The other 3 modules (CB's 21,32,33) are still cold do the touch at this point.
Since it probably worked prior to storage, then something likely failed at first power up. As others have mentioned there are many fusible resistors in this unit, and it's possible one of these went out. They are identified in the parts list and on the schematic.
That rising voltage seems to show a problem around there.
It is worth checking you have 4.7 volts across both those Zeners and also that those 2k7 fusibles feeding them are OK and not going high in value. That can happen with such parts.
Also check the two 3.3 ohm and two 3300 fusibles in that chain.
It is worth checking you have 4.7 volts across both those Zeners and also that those 2k7 fusibles feeding them are OK and not going high in value. That can happen with such parts.
Also check the two 3.3 ohm and two 3300 fusibles in that chain.