Hi.
Finally fixed my mate's NAD C320. This had the common problem that it took ages to come out of protection (power LED stays red rather than turning green). I foiund the circuit diagram and read various similar problems here, although I replaced a few components I believe the fix was to replace C421. This connects to pin 4 of IC44 (a uPC1237, protection IC), and pin 4 is used for the AC mains detection. The design is to feed pin 4 from the mains transformer via a half wave rectifier and a resistor, the IC detects when the mains is switched off and goes into protection. Same IC is used in loads of amps, seems to be a common chip, so other amps likely to issues with the capacitor on pin 4.
Initially I though this capacitor was OK, as pin 4 originally measured about 1.5V on my DC multimeter. However as suggested on other threads I removed C421 (4.7uF 50V) and it only measured 1uF. When replaced with a new cap pin 4 measured 1.9V. However I think previously there must have been a lot of ripple on pin 4 hence the IC saw the voltage drop below threshold (0.9V) and so thought the AC was off and triggered the protection.
I also think there is some interaction on the IC between pin 4 and pin 7 (switch on delay). Pin 7 was stuck at around 200mV rather than the required 2V, so I thought originally the problem was with the associated capacitor for pin 7 (C420).
I also replaced C420 and C422 but they measured only 30% below spec and were probably OK.
Much earlier I had replaced IC44 and D412 but I don't think they were faulty.
Lesson is to think whether a DC voltage that seems OK with a multimeter could have a lot of ripple and actually fall below protection IC threshold, in which case a scope would be needed to check.
I saw on Youtube someone repaired a C320 by replacing all the caps, I think this is overkill (but would probably have saved me a lot of time!)
Regards,
John
Finally fixed my mate's NAD C320. This had the common problem that it took ages to come out of protection (power LED stays red rather than turning green). I foiund the circuit diagram and read various similar problems here, although I replaced a few components I believe the fix was to replace C421. This connects to pin 4 of IC44 (a uPC1237, protection IC), and pin 4 is used for the AC mains detection. The design is to feed pin 4 from the mains transformer via a half wave rectifier and a resistor, the IC detects when the mains is switched off and goes into protection. Same IC is used in loads of amps, seems to be a common chip, so other amps likely to issues with the capacitor on pin 4.
Initially I though this capacitor was OK, as pin 4 originally measured about 1.5V on my DC multimeter. However as suggested on other threads I removed C421 (4.7uF 50V) and it only measured 1uF. When replaced with a new cap pin 4 measured 1.9V. However I think previously there must have been a lot of ripple on pin 4 hence the IC saw the voltage drop below threshold (0.9V) and so thought the AC was off and triggered the protection.
I also think there is some interaction on the IC between pin 4 and pin 7 (switch on delay). Pin 7 was stuck at around 200mV rather than the required 2V, so I thought originally the problem was with the associated capacitor for pin 7 (C420).
I also replaced C420 and C422 but they measured only 30% below spec and were probably OK.
Much earlier I had replaced IC44 and D412 but I don't think they were faulty.
Lesson is to think whether a DC voltage that seems OK with a multimeter could have a lot of ripple and actually fall below protection IC threshold, in which case a scope would be needed to check.
I saw on Youtube someone repaired a C320 by replacing all the caps, I think this is overkill (but would probably have saved me a lot of time!)
Regards,
John
NAD C320 fixed too
Thanks a lot for the tip about C421 on the protection circuit (pin 4 of IC44). I was stuck on this problem because my measurement of pin 7 of IC44 was, like you, very different of what I expected, according to the service manual.
By the way, this amp belongs to a friend. He has fallen down. And the first breakdown I found was a broken track in the +/- 18v circuit. Moreover, I'm surprised a fall can lead to a big change in a capacitor value, reason why I was not searching in that direction ! 2 breakdown to repear, but it's done thank to your experience ! Thanks again !
Thanks a lot for the tip about C421 on the protection circuit (pin 4 of IC44). I was stuck on this problem because my measurement of pin 7 of IC44 was, like you, very different of what I expected, according to the service manual.
By the way, this amp belongs to a friend. He has fallen down. And the first breakdown I found was a broken track in the +/- 18v circuit. Moreover, I'm surprised a fall can lead to a big change in a capacitor value, reason why I was not searching in that direction ! 2 breakdown to repear, but it's done thank to your experience ! Thanks again !
Thanks 1980sjohn. I've just been working on a NAD C320 which wouldn't come out of protection. Unfortunately I shorted one of the driver transistors while testing voltages but it was a blessing in disguise - when installing the replacement I noticed all but two of the drivers had heat-damaged traces, and using the schematic worked out somebody had installed one backwards! So some trace repair and everything seemed fine, but it still wouldn't come out of protection. Found your post, tested C421 which had only dropped to 3.5uF, but had high ESR and 19% Voltage loss (those Mega Testers are really useful), replaced it and the amp now comes out of protection. Thanks for the great information!
Thanks John!
Just wanted to post a big thank you to 1980sjohn. I'm not a big gearhead when it comes to electronics, but I'm handy enough with a soldering iron. I bought a C320BEE a few years ago which took 2 solid minutes to go green whenever it had been unplugged or powered off at the switch. Replaced C421 per the suggestion above, and voila - ready to go in 3 seconds or so.
BTW, I think this is more important with this particular model, because it draws 10W in standby. If you're not using it a ton, that seems pretty wasteful to me. Now I can turn the switch off when I'm not using it and not have to wait for it to "warm up."
If anyone is interested, my old C421 measured less than half of rated value when I took it out.
Just wanted to post a big thank you to 1980sjohn. I'm not a big gearhead when it comes to electronics, but I'm handy enough with a soldering iron. I bought a C320BEE a few years ago which took 2 solid minutes to go green whenever it had been unplugged or powered off at the switch. Replaced C421 per the suggestion above, and voila - ready to go in 3 seconds or so.
BTW, I think this is more important with this particular model, because it draws 10W in standby. If you're not using it a ton, that seems pretty wasteful to me. Now I can turn the switch off when I'm not using it and not have to wait for it to "warm up."
If anyone is interested, my old C421 measured less than half of rated value when I took it out.
This is why I like protection circuits without loads of capacitors.
For DC detect I use a pair of resistors into a 8 pin PIC microcontroller I/o pin.
Cant get much simpler than that.
The software is pretty simple too, just look for a same condition on pin for 250ms and if its there turn off relay.
For DC detect I use a pair of resistors into a 8 pin PIC microcontroller I/o pin.
Cant get much simpler than that.
The software is pretty simple too, just look for a same condition on pin for 250ms and if its there turn off relay.
How does that handle -ve DC offsets without destroying the PIC ? Surely you need some rectification at least
What I do is use two port pins.
One pin looks at +ve dc offsets and the other looks at -ve dc offsets.
I just use two 10k resistors making a potential divider into each pin.
The PIC will just put any excess current into the power supply.
It relies on a PIC input pin changing state at 1.5 volts.
Working fix - thanks
Thanks https://www.diyaudio.com/forums/members/1980sjohn.html
The fix for c421 solved my similar issue - slow time to enable speaker outputs.
Had thought it was a bias issue due to strange readings on speaker output terminals, multimeter showed a large dc offset dropping over 10 minutes or so, then the speaker relay would trigger.
However replacing only c421 resolves this & speaker relay now triggers within a second of power up. It’s a relatively easy fix, recommend printing out the pcb layout from the service manual, then invert to track down the correct solder points when working upside down.
Thanks again!
Thanks https://www.diyaudio.com/forums/members/1980sjohn.html
The fix for c421 solved my similar issue - slow time to enable speaker outputs.Had thought it was a bias issue due to strange readings on speaker output terminals, multimeter showed a large dc offset dropping over 10 minutes or so, then the speaker relay would trigger.
However replacing only c421 resolves this & speaker relay now triggers within a second of power up. It’s a relatively easy fix, recommend printing out the pcb layout from the service manual, then invert to track down the correct solder points when working upside down.
Thanks again!
Thanks https://www.diyaudio.com/forums/members/1980sjohn.html I replaced the C421 in my C320BEE. Didn't solve the issue. When I turn the unit on, the power light goes from red to yellow as it should. But as soon as I press an input, the protection circuit trips with a loud click somewhere near where the power supply is. The circuit protection trips even when the unit has no input (no tuner, etc connected). Any thoughts?
Hi
The symptom your seeing is most likely due to the large power supply electrolytic capacitors drying out.
They are mounted on the board just behind the power transformer. The NAD C320 has JH electrolytics installed and they go high resistance and low value.
Many for example in the power supply and protection circuit are close to components that dissipate heat hence the rate of failure is accelerated. I personally would recap the Amp, but as a starting point replace the two large electrolytics and the smaller ones in the protection circuit.
The symptom your seeing is most likely due to the large power supply electrolytic capacitors drying out.
They are mounted on the board just behind the power transformer. The NAD C320 has JH electrolytics installed and they go high resistance and low value.
Many for example in the power supply and protection circuit are close to components that dissipate heat hence the rate of failure is accelerated. I personally would recap the Amp, but as a starting point replace the two large electrolytics and the smaller ones in the protection circuit.
I completely agree with Audio Service. I've had to replace the main filter capacitors (15,000 @ 50v) on all the units I've had in the C 320/BEE. Also, remove the glue underneath all the large caps on that board. It's a poor design having caps near the hot regulators and rectifiers.
Also, download the service bulleting from Hifi Engine and check to see if the diodes on the primary PSU board and near the volume potentiometer have been replaced with link wires.
I would replace all capacitors too and trimmer pots as they go bad often and can be difficult to reset the bias/DC offset
Also, download the service bulleting from Hifi Engine and check to see if the diodes on the primary PSU board and near the volume potentiometer have been replaced with link wires.
I would replace all capacitors too and trimmer pots as they go bad often and can be difficult to reset the bias/DC offset
Hi. I am having the same problem with NAD320BEE slow start.
Is the right replacement product for C421 for example this one? RS PRO 4.7μF Electrolytic Capacitor 50V dc, Through Hole | RS Components
(spec: RS PRO 4.7μF Electrolytic Capacitor 50V dc, Through Hole. RS Stock No.:
711-1413)
Thankyou for all help!
Is the right replacement product for C421 for example this one? RS PRO 4.7μF Electrolytic Capacitor 50V dc, Through Hole | RS Components
(spec: RS PRO 4.7μF Electrolytic Capacitor 50V dc, Through Hole. RS Stock No.:
711-1413)
Thankyou for all help!
I've just been through this protection circuit problem with a 320BEE. It turned out to be C420 (the turn-on delay capacitor connected to pin 7 of the uPC1237). It was effectively shorting to ground, holding pin 7 low.
The effect of the STANDBY mode and +18V supply through R438/R439 (thermistor) to drive Q412 to short the supply at pin 8 is interesting. It seems that the +18V supply only comes live after coming out of STANDBY, so there is a short on pin 8 and C420 will not charge while in STANDBY. You need to take the amplifier out of standby (to turn on the supply to pin 8) and then check the voltages on pins 8 and 7 to see if there is a problem on pin 7. With C420 replaced (100uF), there still seems to be a reasonably long turn-on delay (maybe ~10 seconds?) before the speaker relay is activated, so I might fit a slightly smaller value for C420... maybe 33uF or 47uF.
The way C420 and C421 are used in this circuit should not need low ESR parts, just healthy ones. The maximum voltage allowed on pins 7 and 8 is 8 volts and the maximum voltage allowed on pin 4 is 10 volts (relative to pin 5: ground). C420 and C421 can be 10V, 16V or 25V parts, if that makes it easier to find a replacement.
The effect of the STANDBY mode and +18V supply through R438/R439 (thermistor) to drive Q412 to short the supply at pin 8 is interesting. It seems that the +18V supply only comes live after coming out of STANDBY, so there is a short on pin 8 and C420 will not charge while in STANDBY. You need to take the amplifier out of standby (to turn on the supply to pin 8) and then check the voltages on pins 8 and 7 to see if there is a problem on pin 7. With C420 replaced (100uF), there still seems to be a reasonably long turn-on delay (maybe ~10 seconds?) before the speaker relay is activated, so I might fit a slightly smaller value for C420... maybe 33uF or 47uF.
The way C420 and C421 are used in this circuit should not need low ESR parts, just healthy ones. The maximum voltage allowed on pins 7 and 8 is 8 volts and the maximum voltage allowed on pin 4 is 10 volts (relative to pin 5: ground). C420 and C421 can be 10V, 16V or 25V parts, if that makes it easier to find a replacement.
This info is golden as it just helped my reveive my C320.
First clue was that shorting out the output relay didn't give a solution, so it had to be another muting part in the amp (that otherwize measured OK, offset and bias wise).
Replacing both C420 and C421 solved it immediatedly.
Both caps measured ok on my cheap multimeter on the uF setting, and I am not sure which and what was wrong with them.
grt, Thijs
First clue was that shorting out the output relay didn't give a solution, so it had to be another muting part in the amp (that otherwize measured OK, offset and bias wise).
Replacing both C420 and C421 solved it immediatedly.
Both caps measured ok on my cheap multimeter on the uF setting, and I am not sure which and what was wrong with them.
grt, Thijs
