I bought a NAD C356BEE amplifier a few days ago.
Symptoms as described by seller: "No sound, but will power up. A friend did some repair but it still doesn't work."
🙂
When I had a look at the amp, the +/- 55V rails were OK but the -26V and -18V rails were not- just a few tenths of a volt.
I had read this thread:
NAD Amp Repair (Newbie) which had a link to the service manual for the amp. Thanks!
http://www.bluefoxnet.com/files/c_356bee.pdf
I looked at the supply circuit and measured some voltages and ordered some parts for wwhat I assumed was the regulator circuit for the -26 and -18 rails.
See pic below.
Symptoms as described by seller: "No sound, but will power up. A friend did some repair but it still doesn't work."
🙂
When I had a look at the amp, the +/- 55V rails were OK but the -26V and -18V rails were not- just a few tenths of a volt.
I had read this thread:
NAD Amp Repair (Newbie) which had a link to the service manual for the amp. Thanks!
http://www.bluefoxnet.com/files/c_356bee.pdf
I looked at the supply circuit and measured some voltages and ordered some parts for wwhat I assumed was the regulator circuit for the -26 and -18 rails.
See pic below.
Attachments
At first, I was getting too high voltages on the -26v rail (in the 40s).
The PCB traces in this amp are really delicate and once I had a good look with a magnifier I realized that the pad was missing from one leg of Q44.
Adding a jumper got all the voltages correct and the amp seems to be working...for now.
It still isn't obvious what caused the failure in Q44/Q42 (?). I replaced the 337 for the 18V supply even though I thought it was probably OK.
I replaced C46 as it pretty much fell out of the board when I was unsoldering the adjacent transistor leads.
Do these regulator components 'just fail' without something else being the reason?
Anyway, not much play value when the first guess seems to fix the problem.
(Time will tell.....).
The PCB traces in this amp are really delicate and once I had a good look with a magnifier I realized that the pad was missing from one leg of Q44.
Adding a jumper got all the voltages correct and the amp seems to be working...for now.
It still isn't obvious what caused the failure in Q44/Q42 (?). I replaced the 337 for the 18V supply even though I thought it was probably OK.
I replaced C46 as it pretty much fell out of the board when I was unsoldering the adjacent transistor leads.
Do these regulator components 'just fail' without something else being the reason?
Anyway, not much play value when the first guess seems to fix the problem.
(Time will tell.....).
Flushed with success 🙂 I thought I should check the 'alignment' following the procedures in the manual.
Please see attached picture of the schematic in that area.
I'm not sure what the instructions mean. Am I supposed to measure the voltage between TP1 and TP2, or measure the voltage between ground and each of the TPs separately?
One combination (TP1 and VR22, measuring voltage to ground) adjusts easily but the other (TP2 and VR23, measuring between TP2 and ground) doesn't seem to be adjustable- it's just sitting at 3 mV.
I'd appreciate any help on the alignment issue.
Thanks.
Please see attached picture of the schematic in that area.
I'm not sure what the instructions mean. Am I supposed to measure the voltage between TP1 and TP2, or measure the voltage between ground and each of the TPs separately?
One combination (TP1 and VR22, measuring voltage to ground) adjusts easily but the other (TP2 and VR23, measuring between TP2 and ground) doesn't seem to be adjustable- it's just sitting at 3 mV.
I'd appreciate any help on the alignment issue.
Thanks.
Attachments
Good jod! It is a good example how to repair an amp, measuring the voltages to found out the fault component and a replacement to get it fix.
The amp idling current setting like this :
Setting the VRs and measuring the voltage between TP1 and TP2 to the requirement, no need to consider the polarity of the probe, a +6mv or -6mv reading is ok.
There is 2 amp channels, so that you adjust VR22 and VR23 and measuring the voltage between TP1 and TP2 and then adjust VR12 and VR13 and measuring the voltage between TP3 and TP4.
If you need adiust the idling current, try adjust VR13 and VR23 only, do not adjust VR12 and VR22 first, if the first VR(13/23) can not set the value then adjust the second VR(12/22)
Patrick-
Thanks for the very clear explanation about the idling current, and how the VRs work.
I'll do that idle adjustment (again) tomorrow.
Will the protection circuits in this amp protect my speakers?
Thanks for the very clear explanation about the idling current, and how the VRs work.
I'll do that idle adjustment (again) tomorrow.
Will the protection circuits in this amp protect my speakers?
Excellent!
Next project is a step down in quality, I think...Cambridge Audio.
Trying to occupy my time at home during virus restrictions........
Next project is a step down in quality, I think...Cambridge Audio.
Trying to occupy my time at home during virus restrictions........
Sounds like cold soldered joints due to time and maybe corrosion.
A pad missing might mean a previous repair gone wrong.
Frail pcb's are a real pain to work with.
I worked on an old Maplin amplifier where the pcb's looked almost home made with no solder resist. I could hardly remove a component without tracks flying off. Ended up with soldering wires on for missing tracks but worked in the end. Just didnt look good.
A pad missing might mean a previous repair gone wrong.
Frail pcb's are a real pain to work with.
I worked on an old Maplin amplifier where the pcb's looked almost home made with no solder resist. I could hardly remove a component without tracks flying off. Ended up with soldering wires on for missing tracks but worked in the end. Just didnt look good.
Umm-mm 🙁.. This is on point, but I'd suggest adding one little thing.
If you hit the peg on VRx3 (Fine adjustment), center it before moving on to VRx2 (the Course adjustment). Then you only have to get 'in the neighborhood' on the Course, and the Fine will still have enough control range to polish it.
Cheers
If you hit the peg on VRx3 (Fine adjustment), center it before moving on to VRx2 (the Course adjustment). Then you only have to get 'in the neighborhood' on the Course, and the Fine will still have enough control range to polish it.
Cheers
You explained the VRs function very clear, people may confuse how to set these two VRs.
Since for a repair tuning, I think he tune the fine adjustment will be ok.
Cheers
The amp adjustments were already pretty close to specs, so it wasn't tough to tune that idle adjustment to 6mV.
Thanks for the hints!
Except for the somewhat delicate PCB (especially in the repair area where the board had gotten hot in the past), that NAD is a pretty solidly built unit.
Quite a contrast to what I saw when I took the lid off the Cambridge P500 earlier today...but that's another story!
🙂
Thanks for the hints!
Except for the somewhat delicate PCB (especially in the repair area where the board had gotten hot in the past), that NAD is a pretty solidly built unit.
Quite a contrast to what I saw when I took the lid off the Cambridge P500 earlier today...but that's another story!
🙂
Hi, i just finished reading all this, i have the same NAD both Zener Diodes burned a hole in the board, i replaced the diod's and all the caps in the area and upgraded the diodes to 5 watt and the 51ohm 1watt to a 51ohm 5watt sand block resister, its working and all the voltage are correct, the system has been running for about 2 hours, the top of the system on the right side is rather hot, the amp side is cool to the touch. is this normal ?