Couple weeks ago I noticed that when the amp is on and silent a while longer, it starts hum and pop the speaker. So I opened it and read information about it and looked at the schematic diagram. Then I decided to check and set the quiescent current. Left channel was fine all around but I noticed there was not any quiescent current at right channel. I replaced badly worn bias trimmerpot.
Then I re-heated soldering points at the circuit board and after that the bias circuit was energized again. Now I noticed the amp is in protection mode, led is red constantly. I measured and confirmed the bias circuit was functioning normally. Some more staring at schematics.
Then I measured amp stage output to see if the problem is with faulty protection circuit or the stage itself. There was about 25 volts DC at the output so protection circuit is working as it should. I even took the output wire off the circuit board and then of course the protection circuit noticed it and green led was lighting. So something at the right channel is causing supply voltage to leak to the output. I measured all transistors and found no faulty ones.
Then I re-heated soldering points at the circuit board and after that the bias circuit was energized again. Now I noticed the amp is in protection mode, led is red constantly. I measured and confirmed the bias circuit was functioning normally. Some more staring at schematics.
Then I measured amp stage output to see if the problem is with faulty protection circuit or the stage itself. There was about 25 volts DC at the output so protection circuit is working as it should. I even took the output wire off the circuit board and then of course the protection circuit noticed it and green led was lighting. So something at the right channel is causing supply voltage to leak to the output. I measured all transistors and found no faulty ones.
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Ok, managed to advance. I changed all metal film resistors. Then I connected output to the protection circuit normally and powered the amp. The protection led turned from red to green almost immediately and i measured the output DC voltage to be at zero level.
I am still testing further but I think I managed to fix problems. So bias setting circuit works normally and output dc problem is no more. Protection circuit has not engaged within this 3 hours of power-on time
What I learned from this is: replace resistors, re-heat soldering at pcb, treat pcb carefully and final measurement should be done with protection circuit connected. The two aforementioned problems could be related to same defective parts. I will test more and will tell what the conclusion is.
I am still testing further but I think I managed to fix problems. So bias setting circuit works normally and output dc problem is no more. Protection circuit has not engaged within this 3 hours of power-on time
What I learned from this is: replace resistors, re-heat soldering at pcb, treat pcb carefully and final measurement should be done with protection circuit connected. The two aforementioned problems could be related to same defective parts. I will test more and will tell what the conclusion is.
When you try to fix problems by the shotgun method of replacing every part, you learn nothing because neither the problem or the symptom are identified. This problem could still recur even though you replaced most components. Looking at the service manual, notice there are a number of fusible resistors around the power output stages and flameproof power resistors in the power supplies, as shown by triangular symbols and in the parts list. These are an important part of the design and were popular safety features in the period of manufacture. Unfortunately, those fusible resistors caused problems by making troubleshooting a difficult and uncertain process for DIY repairs if their failure goes unnoticed. Even experienced technicians have problems but at least they will know where to look for and quickly locate the parts and check for local faults that could cause the fusible resistor to blow - intentionally, btw.
At the end of your repair, are you sure that the resistors you replaced are all suitable for the protection specified in the NAD 214 design? I suspect not, but I've also seen similar electronics to power amplifiers survive for years longer than the original products, with all the fusibles simply replaced by standard film resistors too. I have also had problems with them, so use the free manual and study the locations and known issues first before blasting away with a shotgun and fitting so many unnecessary replacements. Just keep in mind that the fusibles were for protection and safety - I'm no expert in their use at the design level but be cautious in using the amplifier now - don't leave it powered up without someone being around to monitor it.
Having said that, re-flowing suspect solder joints as you found, is certainly a good idea. The wave soldering employed in assembly at that time caused massive numbers of latent faults and scrapped products because the problems could not be easily found or fixed.
I'll bet it's good to have it working, though 🙂
At the end of your repair, are you sure that the resistors you replaced are all suitable for the protection specified in the NAD 214 design? I suspect not, but I've also seen similar electronics to power amplifiers survive for years longer than the original products, with all the fusibles simply replaced by standard film resistors too. I have also had problems with them, so use the free manual and study the locations and known issues first before blasting away with a shotgun and fitting so many unnecessary replacements. Just keep in mind that the fusibles were for protection and safety - I'm no expert in their use at the design level but be cautious in using the amplifier now - don't leave it powered up without someone being around to monitor it.
Having said that, re-flowing suspect solder joints as you found, is certainly a good idea. The wave soldering employed in assembly at that time caused massive numbers of latent faults and scrapped products because the problems could not be easily found or fixed.
I'll bet it's good to have it working, though 🙂
I also had dc offset problems with my 214. At the end I found that it was the small ribbon cables connecting the power supply board to the output boards. The pins on the plugs corrodes and causes intermittent connections.
Ian, I am sure about the safety not being compromised. I have replaced flameproof resistors with flameproof new ones where needed and looked for schematics to avoid compromising design with substandard replacements. Btw, I have not replaced any of the fusible resistors...
I can understand your point but I have not created a bomb.
I can understand your point but I have not created a bomb.
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No problem, though as you had replaced so many components but made no mention of your experience, I could only assume you were new to electronic design and repair. Sorry if I misjudged you.
I found one thing also. Right channel signal input cable was almost cut and when it was slightly moved the contact made amp to go protection mode and output to swing wildly with dc. This without doubt contributed to the strange dc behaviour. I replaced the cable with better and now this is a rock solid amplifier. I have extensively tested amp and everything works flawlessly. Knowledge gained also. So all is good.
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