Hi! I’m new here. I don’t usually ask about my problems on forums because I usually find the answer I need from older posts.
I can’t find anything that could help me resolve my problem.
I bought a Nad 312 amplifier from a guy thinking it was at least powering up. Sadly, it doesn’t. There is not even a glimpse of sound or light.
I’ve already had experience repairing old amplifier when they gave some sign of life but the sound wasn’t great. Changing caps and putting contact cleaner spray.
As I said, there is no green light at the front and no apparent buzz or sound coming out of the transistors.
It must come from the beginning of the circuit on mains side, but I’m wondering if you had any idea of where to start looking.
Thanks for your help. Wish you a good day!
PS: here are some pictures of the amp circuit
I can’t find anything that could help me resolve my problem.
I bought a Nad 312 amplifier from a guy thinking it was at least powering up. Sadly, it doesn’t. There is not even a glimpse of sound or light.
I’ve already had experience repairing old amplifier when they gave some sign of life but the sound wasn’t great. Changing caps and putting contact cleaner spray.
As I said, there is no green light at the front and no apparent buzz or sound coming out of the transistors.
It must come from the beginning of the circuit on mains side, but I’m wondering if you had any idea of where to start looking.
Thanks for your help. Wish you a good day!
PS: here are some pictures of the amp circuit
Check if there is DC resistance across live and neutral on the mains plug when the power switch is on. It should read quite low, perhaps under 10 ohms depending on the transformer size. You measure that with the plug OUT of the wall socket of course. If there is not and it reads open circuit then suspect a thermal fuse within the transformer (non replaceable usually) or a simple continuity problem like a fuse or switch open.
(You should be able to check basic rail voltages without any service information)
That's a good point. Make sure you understand any safety implications.
Hello, thanks a lot for your replies. I have a multimeter, an oscilloscope and a soldering iron. I’m well aware of the dangers of mains voltage and I take extreme precautions when measuring it.
This is what I found so far:
They are coming directly from the transformer and seem rather low, considering that it needs to make +/- 33V DC on the blue/green side as seen on the data sheet.
I measured what was coming to the led pins, and to my surprise I got 5V one time, and now nothing.
I redid the solder joints in the cables coming from the transformer but it did not change anything.
Also, here are the voltages I got from the mains board with the switch on it.
No relay clicking sound and no sound coming out of the speakers.
I tried measuring what was coming out of the full bridge rectifier. It should output negative voltage on one side and positive voltage on the other but both output are at 0.85V AC (with the chassis as a reference to ground).
Maybe the transformer is dead but if so, my chances of repairing this amp are over considering the unusual type of transformer that it is made of.
Here are the resistance measurement. I have an open loop between the neutral and the line and between white and blk which would coincide with your approach of the thermal fuse having blown up :/
Does that mean that this amp is unfixable ? Thanks again for your help.
This is what I found so far:
- 240V on the L/N wires coming from the outlet
- Blue/Blue/Green wires from the transformer are at 1.5V AC (or less, not stable)
- Grey/Grey/Black wires also from the transformer are at 1.3V AC (or less, not stable)
They are coming directly from the transformer and seem rather low, considering that it needs to make +/- 33V DC on the blue/green side as seen on the data sheet.
I measured what was coming to the led pins, and to my surprise I got 5V one time, and now nothing.
I redid the solder joints in the cables coming from the transformer but it did not change anything.
Also, here are the voltages I got from the mains board with the switch on it.
No relay clicking sound and no sound coming out of the speakers.
I tried measuring what was coming out of the full bridge rectifier. It should output negative voltage on one side and positive voltage on the other but both output are at 0.85V AC (with the chassis as a reference to ground).
Maybe the transformer is dead but if so, my chances of repairing this amp are over considering the unusual type of transformer that it is made of.
Here are the resistance measurement. I have an open loop between the neutral and the line and between white and blk which would coincide with your approach of the thermal fuse having blown up :/
Does that mean that this amp is unfixable ? Thanks again for your help.
Your diagram shows the thermal fuse between P521 and P527and you have written OL. Of OL means 'open' then its failed. It should read as a short between those points.
If the fuse is embedded within the windings then you can not replace it. They often are unfortunately. You may also find that there is a 'genuine' fault on the secondary side that has caused the failure in the first place.
So yes, if it has failed and is not replaceable then the amp is not fixable on safety grounds unfortunately.
If the fuse is embedded within the windings then you can not replace it. They often are unfortunately. You may also find that there is a 'genuine' fault on the secondary side that has caused the failure in the first place.
So yes, if it has failed and is not replaceable then the amp is not fixable on safety grounds unfortunately.
Hi again,
Your answer made me sad :/
I made some research on the side. This guy has the same problem as I have : https://www.diyaudio.com/community/threads/nad-304-not-powering-up.320027/
They suggest putting a 1A slow blow fuse between the white and black wire to bypass the thermal fuse inside the transformer. Is it a « good » idea in my case ? What should I verify on the secondary side to ensure that nothing has failed ?
Here are my measurements coming from the secondary side.
Is there any website where I could buy a used transformer for my amp ? (I live in France).
Your answer made me sad :/
I made some research on the side. This guy has the same problem as I have : https://www.diyaudio.com/community/threads/nad-304-not-powering-up.320027/
They suggest putting a 1A slow blow fuse between the white and black wire to bypass the thermal fuse inside the transformer. Is it a « good » idea in my case ? What should I verify on the secondary side to ensure that nothing has failed ?
Here are my measurements coming from the secondary side.
Is there any website where I could buy a used transformer for my amp ? (I live in France).
I'm late to the party. Your post #6 seems to show continuity between all windings but but open circuit between Line and Neutral. Can't be as simple as non-conducting power switch! I'm notoriously blind. What am I overlooking?
Edit: Now I see it. Open winding between P527 and P521.
If Neutral were moved to P527, what would be the desired line voltage for proper operation?
Edit: Now I see it. Open winding between P527 and P521.
If Neutral were moved to P527, what would be the desired line voltage for proper operation?
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Firstly and most importantly, I can't recommend defeating a safety feature such as a thermal fuse. Its there for a reason, to prevent damage and what could be a possible fire risk if there were a further issue and the unit was left unattended.
For curiosity you can use a DBT (dim bulb tester) across the thermal fuse and see if the amp works or not. If not you have the opportunity to fault find in the normal way and fix it but as stated, I can not recommend defeating the manufacturers safety features as a permanent thing.
That's going to defeat the protection though as it bypasses the embedded fuse.
I wouldn't focus on the transformer unless (and until) you have verified that the amp is working and stable, which looks like it will require taking the amp board out and powering it with a lab supply.
If the transformer has failed, it's done so for a reason and so investing in a new transfomer is a bit pointless unless you know that the amp is working
If the transformer has failed, it's done so for a reason and so investing in a new transfomer is a bit pointless unless you know that the amp is working
Hi. Recently had to repair two different amplifiers , different brands , but same problem - failed thermal fuse inside transformer. If a fuse is wired to external pins , and measures as opens , its failed. In my case transformers were overheated, as owner used all four speakers (A+B pair) at same time , and they were 4 ohms... With one transforner fuse was not accessible, transformer was welded , so i've had to cut welding point and disassemble it , to replace fuse. After that owner welded it again( he had that possibility). Transforner works. Amplfier too. In another amplfier fuse in transforner was replaced before and was easy accessible from side, so it was easy replacement , just need to desolder pcb board with wires. In both cases we keep safety as before , not just bypass the fuse with wire , as many does.
Very good advice to check with dim bulb tester , but i would recommend first to measure all output transistors, if they are not shorted ( no 0,0 display on dmm in ohms or diode range). If amplfier would show some signs of life, voltage appears on big filter capacitors ,no dc voltage on speaker terminals, you may try increase lamp power and test with sound input and speakers connected , at small volume.
Very good advice to check with dim bulb tester , but i would recommend first to measure all output transistors, if they are not shorted ( no 0,0 display on dmm in ohms or diode range). If amplfier would show some signs of life, voltage appears on big filter capacitors ,no dc voltage on speaker terminals, you may try increase lamp power and test with sound input and speakers connected , at small volume.
Hello guys,
I have to thank you all for your great advices that helped me narrow down the problem. I was afraid I’d break the transformer by opening it to change the thermal fuse so I decided not to do it.
I was also unsure of removing the safety measures that were there in the first place.
I sold it to another person who might have some spares in hand to repair it.
It’s not the end of my amplifier adventure cause I have another Denon PMA-320A to repair.
Thanks for everything and good luck everyone !
I have to thank you all for your great advices that helped me narrow down the problem. I was afraid I’d break the transformer by opening it to change the thermal fuse so I decided not to do it.
I was also unsure of removing the safety measures that were there in the first place.
I sold it to another person who might have some spares in hand to repair it.
It’s not the end of my amplifier adventure cause I have another Denon PMA-320A to repair.
Thanks for everything and good luck everyone !