Hi Everyone,
I recently got a old NAD C370 that someone asked me to repair.
I'm not a electronics expert but i know something about electronics and repair lots of older electronics.
The Amp was in protection mode, so i downloaded the service manual to see how it is built.
So when i got it in and opened it up i immediately saw a whole bunch of cooked electrolytic capacitors, so i took
out my capacitor box and started replacing 90% of all caps for good ones with low ESR and preferably 105c ones (mostly Nichicon and Rubicon i still had),
cause the ones that where in there where 85c and a whole lot of them where in the vicinity of either a large resistor or heatsink,
and even the ones that wheren't in a hot spot where cooked as well. Some 100uf caps where only 100nf with a ESR of 25,
which is obviously due to the Amp being a bad design when it comes to component placement and choice of components,
the large power supply caps where already starting to bulge as well, so i ordered new ones and put them in,
also for the 1000uf and 2200uf ones on the main board, all Nichicon ones.
Also recapped the power amp modules, left power amp had a faulty bav21 diode (determinded that after systematic power up)
Also changed the resistor and zener in the protection board to the backside, since the resistor there cooked pretty much all the caps.
I know some people say the zener does thatbut the zener is on top of the print radiating it's heat to only a single cap, and the rest goes up.
the large resistor however was the main culprit in cooking the lower 2 caps.
Than i powered up the amp without the power amp modules attached, and got my -64 and +64 +- volts at both sides, not 68.
Figured that was good enough to try it out.
So i first connected the left Power amp, no sound, took it out, found a faulty diode, replaced it, left side started working perfectly.
Than i connected the right side, it had sound but i think only half the power it should have, no distortion however, clean sound, but just a lot softer than left.
But after like 2 minutes (playings softly ofcourse, not want to overstress the circuit), heard some noise and crackling and the sound dropped to very low volume, still not distorted however.
Switched it off, took out the right power amp, and checked every single diode by taking one leg out and measure, took out every single semiconductor and tested them all, they where all good,
even the power transistors, everything measured fine (with transistor tester, all had nominal hFE for their type), small film caps where still all in spec, all resistors are in spec.
Worked the 2x 22K resistors that have all come loose to the side of the board extending the legs with thicker copper wire turned around the legs and soldered them to 1cm scraped
tracks on the pcb so they will never come loose again and radiate their heat into open space instead of cooking another cap.
Actualy did that wherever i could, get the heat away from the caps as much as possible to prevent future needlesly early damage to the caps.
Anyway, all measured fine, reflowed all solder joints on the right power amp to make sure it wasn't a bad connection and put it back in.
When i started it up, it had like 5 second a normal volume and sound and after that again a hissy noise some crackle and again a very low undistorted volume.
When i shut it down, the volume comes back like for about 400 milliseconds, if i than power it right up, volume is still low (also on speakers B which never had been connected,
so those relais are still good for sure), if i wait for like 30 minutes and power it up, i again have a few seconds normal operation, and than it goes soft again.
If i let it play at like 40% volume (which is very soft) the power transistors get warmer like they will normall do at 40 percent, not really hot though, but just 45 degrees celcius or so i think.
Maybe after 5 minutes.
So i took the Amp out again and replaced the 3x 2sd669a's and the 3x 2sb649a's to make sure it wasn't the drivers that where faulting, but i don't have the smaller transistors lying around, the 2sa1015, 2sa872, 2sa1775 and the 2sc1815 laying around, i do have the TL431 voltage regulators, but i suspect thy're good, i don't know for sure cause it's design doesn't easily let me acces the spots where i can measure voltage, maybe i'll solder a wire to those spots to be able to check it when it's on.
I did order the small semi's so i can batch replace them all, since they all measured fine i thought mayb one goes into a fault when it gets it's voltage, so best to just swap them all out for new ones to be sure, they're not very expensive so..
But to make a long story short, has anyone ever had this on this model and have an idea what the fault could be ??
I recently got a old NAD C370 that someone asked me to repair.
I'm not a electronics expert but i know something about electronics and repair lots of older electronics.
The Amp was in protection mode, so i downloaded the service manual to see how it is built.
So when i got it in and opened it up i immediately saw a whole bunch of cooked electrolytic capacitors, so i took
out my capacitor box and started replacing 90% of all caps for good ones with low ESR and preferably 105c ones (mostly Nichicon and Rubicon i still had),
cause the ones that where in there where 85c and a whole lot of them where in the vicinity of either a large resistor or heatsink,
and even the ones that wheren't in a hot spot where cooked as well. Some 100uf caps where only 100nf with a ESR of 25,
which is obviously due to the Amp being a bad design when it comes to component placement and choice of components,
the large power supply caps where already starting to bulge as well, so i ordered new ones and put them in,
also for the 1000uf and 2200uf ones on the main board, all Nichicon ones.
Also recapped the power amp modules, left power amp had a faulty bav21 diode (determinded that after systematic power up)
Also changed the resistor and zener in the protection board to the backside, since the resistor there cooked pretty much all the caps.
I know some people say the zener does thatbut the zener is on top of the print radiating it's heat to only a single cap, and the rest goes up.
the large resistor however was the main culprit in cooking the lower 2 caps.
Than i powered up the amp without the power amp modules attached, and got my -64 and +64 +- volts at both sides, not 68.
Figured that was good enough to try it out.
So i first connected the left Power amp, no sound, took it out, found a faulty diode, replaced it, left side started working perfectly.
Than i connected the right side, it had sound but i think only half the power it should have, no distortion however, clean sound, but just a lot softer than left.
But after like 2 minutes (playings softly ofcourse, not want to overstress the circuit), heard some noise and crackling and the sound dropped to very low volume, still not distorted however.
Switched it off, took out the right power amp, and checked every single diode by taking one leg out and measure, took out every single semiconductor and tested them all, they where all good,
even the power transistors, everything measured fine (with transistor tester, all had nominal hFE for their type), small film caps where still all in spec, all resistors are in spec.
Worked the 2x 22K resistors that have all come loose to the side of the board extending the legs with thicker copper wire turned around the legs and soldered them to 1cm scraped
tracks on the pcb so they will never come loose again and radiate their heat into open space instead of cooking another cap.
Actualy did that wherever i could, get the heat away from the caps as much as possible to prevent future needlesly early damage to the caps.
Anyway, all measured fine, reflowed all solder joints on the right power amp to make sure it wasn't a bad connection and put it back in.
When i started it up, it had like 5 second a normal volume and sound and after that again a hissy noise some crackle and again a very low undistorted volume.
When i shut it down, the volume comes back like for about 400 milliseconds, if i than power it right up, volume is still low (also on speakers B which never had been connected,
so those relais are still good for sure), if i wait for like 30 minutes and power it up, i again have a few seconds normal operation, and than it goes soft again.
If i let it play at like 40% volume (which is very soft) the power transistors get warmer like they will normall do at 40 percent, not really hot though, but just 45 degrees celcius or so i think.
Maybe after 5 minutes.
So i took the Amp out again and replaced the 3x 2sd669a's and the 3x 2sb649a's to make sure it wasn't the drivers that where faulting, but i don't have the smaller transistors lying around, the 2sa1015, 2sa872, 2sa1775 and the 2sc1815 laying around, i do have the TL431 voltage regulators, but i suspect thy're good, i don't know for sure cause it's design doesn't easily let me acces the spots where i can measure voltage, maybe i'll solder a wire to those spots to be able to check it when it's on.
I did order the small semi's so i can batch replace them all, since they all measured fine i thought mayb one goes into a fault when it gets it's voltage, so best to just swap them all out for new ones to be sure, they're not very expensive so..
But to make a long story short, has anyone ever had this on this model and have an idea what the fault could be ??
