I've got a C300 here which was working well until someone accidentally turned up the volume up past halfway, which killed the Mosfets on the right channel, blown 2 fuses, and took out a rectifier. These Mosfets are like delicate little flowers lol I've fixed a few of these previously.
I've replaced the faulty rectifier and fuses with the same type.
I have stable and correct voltages on the output of the regulators.
The voltages at the Mosfets are:
Left channel (Mosfets measure good)
Q307
G, 2.3V D, 30.4V S, 0V
Q308
G, -27.7V D, 0V S, -30.1V
Right channel (blows Mosfets)
Q407
G, -26.8V D, 0V S, -30.1V
Q406
G, -2.3V D, 30.5V S, 0V
I have tested all the transistors and diodes in the right channel, pulled one leg from resistors and they all appear good. I even pulled the vertical line/driver board and all transistors and diodes measure good. I replaced the 22uF cap as it measured very high ESR. The emitter resistor tests well.
Fuses don't blow anymore yet R/C Mosfets blow on startup with no load.
The service manual is here NAD C300 Stereo Integrated Amplifier Manual | HiFi Engine
What am I missing? Much appreciate any guidance in advance
I've replaced the faulty rectifier and fuses with the same type.
I have stable and correct voltages on the output of the regulators.
The voltages at the Mosfets are:
Left channel (Mosfets measure good)
Q307
G, 2.3V D, 30.4V S, 0V
Q308
G, -27.7V D, 0V S, -30.1V
Right channel (blows Mosfets)
Q407
G, -26.8V D, 0V S, -30.1V
Q406
G, -2.3V D, 30.5V S, 0V
I have tested all the transistors and diodes in the right channel, pulled one leg from resistors and they all appear good. I even pulled the vertical line/driver board and all transistors and diodes measure good. I replaced the 22uF cap as it measured very high ESR. The emitter resistor tests well.
Fuses don't blow anymore yet R/C Mosfets blow on startup with no load.
The service manual is here NAD C300 Stereo Integrated Amplifier Manual | HiFi Engine
What am I missing? Much appreciate any guidance in advance
Mosfets are "delicate little flowers " -- not the ones I use .
The ones in your amplifier are rated at 55 Amp/60Volts but this is reduced ( derated ) at 100C
What the "problem " is that they are not large die mosfets and if they are already hot and the heatsink doesn't allow much overload they "burn up " (blow ).
If the mosfets blow on start up ( no signal ) then you could have a HF oscillation feeding a large signal into them as they need a gate voltage to conduct but I don't see a gate/source zener diode to help with protection.
The ones in your amplifier are rated at 55 Amp/60Volts but this is reduced ( derated ) at 100C
What the "problem " is that they are not large die mosfets and if they are already hot and the heatsink doesn't allow much overload they "burn up " (blow ).
If the mosfets blow on start up ( no signal ) then you could have a HF oscillation feeding a large signal into them as they need a gate voltage to conduct but I don't see a gate/source zener diode to help with protection.
Thanks for the reply Duncan
I agree, there are some awesome Mosfets and these are not a good example of Mosfets in general.
I do actually think this is one of NAD's better-sounding amps from this era for low-level listening. Bargain low-cost amp. Maybe my Pass Labs DAC and Philips CD Player gets the best out of it though.
You're right, no Zener to help there. I may add some when I fix it. At least they have Zeners bypassing the regulators though. I'm just going 'round in circles with it and must be something simple I'm not finding. All solder joints are good. Had a load of new caps about a year ago which now measure well
I agree, there are some awesome Mosfets and these are not a good example of Mosfets in general.
I do actually think this is one of NAD's better-sounding amps from this era for low-level listening. Bargain low-cost amp. Maybe my Pass Labs DAC and Philips CD Player gets the best out of it though.
You're right, no Zener to help there. I may add some when I fix it. At least they have Zeners bypassing the regulators though. I'm just going 'round in circles with it and must be something simple I'm not finding. All solder joints are good. Had a load of new caps about a year ago which now measure well
Have you any gear to check for AC oscillation ?
If you don't have an oscilloscope then you need a very wide-band AC voltmeter ( big frequency response ) the usual cheap digital meters are useless FR is very low.
If you don't have an oscilloscope then you need a very wide-band AC voltmeter ( big frequency response ) the usual cheap digital meters are useless FR is very low.
My oscilloscope is miles away in Warrington with other test equipment in storage. All I have here at home is a cheap DMM and an old Beckman Ind. T120B. I'll have to check its FR specs.
Do you mean checking the AC on the transformer? I think I have the bits needed to whip up a variable PS if need be
Do you mean checking the AC on the transformer? I think I have the bits needed to whip up a variable PS if need be
Out of interest are the replacement MOSFETs new original STP55NE06L
They can be purchased from here.
STMicroelectronics STP55NE06L MOSFET | PRICE, STOCK, DATASHEET, | silicon-ark
They can be purchased from here.
STMicroelectronics STP55NE06L MOSFET | PRICE, STOCK, DATASHEET, | silicon-ark
Could be one of many problems.
1/ Oscillation frying the mosfets.
2/ Dodgy bias pot wiper breaking contact and over biasing the outputs.
3/ Power supply mains spike rejection problem. Spikes on mains are pushing mosfets into breakdown and so self destruction. Use 100nf's across smoothing caps.
4/ A component that is intermittent and affecting mosfet bias.
5/ A simple carefu lvisual inspection should show up bad solder joints.
6/ Flaky edge connector or wire to pcb.
1/ Oscillation frying the mosfets.
2/ Dodgy bias pot wiper breaking contact and over biasing the outputs.
3/ Power supply mains spike rejection problem. Spikes on mains are pushing mosfets into breakdown and so self destruction. Use 100nf's across smoothing caps.
4/ A component that is intermittent and affecting mosfet bias.
5/ A simple carefu lvisual inspection should show up bad solder joints.
6/ Flaky edge connector or wire to pcb.
I'm assuming from your post that we are talking about an intermittent problem with the right channel and not a situation where the mosfets blow every time the amp is powered up?
Have had a few of these amps in with cracked solder joints on some of the heavier components on the main board. They are not always obvious until you give the board a wobble.
The faulty 22uF cap you found might be one of many - they are all 20 years old so may be past their best. If that's the case then it's possible that the amp is going into oscillation at switch on.
It wouldn't necessarily mean that the faulty capacitors are limited to the right channel. I had an Audiolab amp in that was blowing outputs on one side and the problem was the main smoothing capacitors - I guess things become unpredictable when your amp has turned into a transmitter.
Have had a few of these amps in with cracked solder joints on some of the heavier components on the main board. They are not always obvious until you give the board a wobble.
The faulty 22uF cap you found might be one of many - they are all 20 years old so may be past their best. If that's the case then it's possible that the amp is going into oscillation at switch on.
It wouldn't necessarily mean that the faulty capacitors are limited to the right channel. I had an Audiolab amp in that was blowing outputs on one side and the problem was the main smoothing capacitors - I guess things become unpredictable when your amp has turned into a transmitter.
I don't mean the power supply Stewart unless its of the large heavily stablised type .
All the HF oscillations I have come across are down to circuit faults in the actual power amplifier or if you are designing one -wrong values of capacitors /resistors /compensation capacitors etc.
As you have replaced components I would (slightly ) up the value of the small compensation capacitors , don't worry about FR etc we are trying to locate a fault not design an audio amplifier, things can be reversed when you find a fault .
All the HF oscillations I have come across are down to circuit faults in the actual power amplifier or if you are designing one -wrong values of capacitors /resistors /compensation capacitors etc.
As you have replaced components I would (slightly ) up the value of the small compensation capacitors , don't worry about FR etc we are trying to locate a fault not design an audio amplifier, things can be reversed when you find a fault .
Thank you Audio Service. IIRC I bought 10 pieces a few years ago but not from my usual supplier, RS, as they didn't have them in stock. It May of been a seller on eBay which could be an issue with the consistency. They were branded ST. I've not used sellers on eBay now for years. I'll order them from your link and check them out.
Thanks, Nigel. I usually replace the pots and relays of NAD's this age as I've had issues with them previously. I thought they could be an issue with the pots yet forgot to order some multi-turn ones. I'll get onto that. The solder joints were all good under a microscope. I reflowed most of them that were thin to no difference. Just a power resistor that runs warm the traces have lifted slightly yet I get continuity to components before and after and the resistor measures well. I'll put some film caps across the smoothing caps, interesting that is.
Thanks for your input too Martin. I had about 25 caps replaced about a year ago and reflowed a lot of bad solder joints back then, these NAD's are suspect to bad solder joints, I agree. I've had no issues since the recap and it sounded better after the recap. The smoothing caps were replaced with the same value. The only time I've replaced just the smoothing caps that actually improved the sound a lot. Usually just get tighter bass, if anything at all. It's not intermittent, the Mosfets blow as soon as I power on Dim bulb goes bright. With the R/C Mosfets removed the dim bulb flashes on turn-on then goes out, relay clicks, no fuses blow now. I've sent two pairs of Mosfets on a suicide run so far. Mosfets don't touch the heatsinks either.
Thanks again Duncan, you have definitely pointed out an issue I've never looked into before and I'll certainly find it interesting to learn more now.
I'm well impressed with everyone's input here, really appreciate it. I have a load of new A1015 transistors yet don't want to just throw new parts in if the existing parts test good and then not learn why the fault happens
Thanks, Nigel. I usually replace the pots and relays of NAD's this age as I've had issues with them previously. I thought they could be an issue with the pots yet forgot to order some multi-turn ones. I'll get onto that. The solder joints were all good under a microscope. I reflowed most of them that were thin to no difference. Just a power resistor that runs warm the traces have lifted slightly yet I get continuity to components before and after and the resistor measures well. I'll put some film caps across the smoothing caps, interesting that is.
Thanks for your input too Martin. I had about 25 caps replaced about a year ago and reflowed a lot of bad solder joints back then, these NAD's are suspect to bad solder joints, I agree. I've had no issues since the recap and it sounded better after the recap. The smoothing caps were replaced with the same value. The only time I've replaced just the smoothing caps that actually improved the sound a lot. Usually just get tighter bass, if anything at all. It's not intermittent, the Mosfets blow as soon as I power on Dim bulb goes bright. With the R/C Mosfets removed the dim bulb flashes on turn-on then goes out, relay clicks, no fuses blow now. I've sent two pairs of Mosfets on a suicide run so far. Mosfets don't touch the heatsinks either.
Thanks again Duncan, you have definitely pointed out an issue I've never looked into before and I'll certainly find it interesting to learn more now.
I'm well impressed with everyone's input here, really appreciate it. I have a load of new A1015 transistors yet don't want to just throw new parts in if the existing parts test good and then not learn why the fault happens
Hi all
I replaced the Mosfets again and some 0.1uF mylar caps on the smoothing caps.
I noticed the previous chips were labelled P55NF, not P55NE. Probably my fault.
So I took the chance and installed them. I powered the amp up with a DBT and this time the bulb didn't stay lit.
I tested the bias'. Normal on one channel (170mA) and only 6.7mA on the channel that was faulty. DC offset was 20mA or less on both channels. So I put in a signal and hooked up some scrap speakers and still very low output, only just audioable at about 2 o clock.
Done my head in. I buttoned it back up and finally accepted I need to get my scope, which is at the back of my storage room behind about 50 amps etc
I replaced the Mosfets again and some 0.1uF mylar caps on the smoothing caps.
I noticed the previous chips were labelled P55NF, not P55NE. Probably my fault.
So I took the chance and installed them. I powered the amp up with a DBT and this time the bulb didn't stay lit.
I tested the bias'. Normal on one channel (170mA) and only 6.7mA on the channel that was faulty. DC offset was 20mA or less on both channels. So I put in a signal and hooked up some scrap speakers and still very low output, only just audioable at about 2 o clock.
Done my head in. I buttoned it back up and finally accepted I need to get my scope, which is at the back of my storage room behind about 50 amps etc