Thanks, yeah that's exactly what I'm going to do. I hadn't unplugged connectors yet because someone mentioned that the protection circuit would either shut everything off or not work with things disconnected. But, unplugging it will be. The 64v and 68v rails are measuring +- 6 to 19v so it's *something*. I suspect the 34V PSU as it's readings were all over the place. Will unplug it and see if the other two become normal.
Do the C160 modules use SMD electrolytic caps? I have a C325BEE with riser-board/modules and it utilizes SMD caps. In my amp several of these failed and required replacement.
Also, reading about other C370 repairs online....the protection module, it is suggested to move the zener diode onto the backside/foil-side of the module. It generates a lot of heat that causes issues with the components nearby.
Also, reading about other C370 repairs online....the protection module, it is suggested to move the zener diode onto the backside/foil-side of the module. It generates a lot of heat that causes issues with the components nearby.
All through hole caps in the C160s and they've been replaced. The protection module went bad (the standard cap failure) a couple of years ago so I did all the required changes (including moving the diode to the back side).
Thanks.
Someone recorded a problem with the protection circuit see this link Repairing a NAD C370 Amplifier – The Slow Start Protection Module Problem
Thanks, it's not a protection circuit issue. I actually got it to power up with the 68V and 34V rails disconnected so that's my direction of investigation.
The main problems you have after servicing an amplifier is the preliminary test setting of the output standing current and the dc offset both close as possible to zero with some means of limiting current such as the light bulb in series with the transformer primary winding.
You have achieved that.
If it is not possible to adjust the standing current up then turn the trimmer in the direction for minimum.
The dc supply voltages you are getting are probably too low to bias the output stage and the protection module is probably working below the stable VCC working range of 25 to 60 volts of the Upc1237, and this is why your module acting abnormally.
I am not a fan of using light bulbs in series with transformers. I use wire-wound resistors in series with the supply rails - possible if these are fuse protected as one can clip these to ends of the fuse holders - and measure the voltage drops across these to deduce current levels.
You have achieved that.
If it is not possible to adjust the standing current up then turn the trimmer in the direction for minimum.
The dc supply voltages you are getting are probably too low to bias the output stage and the protection module is probably working below the stable VCC working range of 25 to 60 volts of the Upc1237, and this is why your module acting abnormally.
I am not a fan of using light bulbs in series with transformers. I use wire-wound resistors in series with the supply rails - possible if these are fuse protected as one can clip these to ends of the fuse holders - and measure the voltage drops across these to deduce current levels.
The bulb's been ok so far. Haven't gotten to the offset and bias yet - just trying to get stable power.
64v rails are now stable although kind of low at ~50v - is that common on these amps (I didn't think to measure before servicing)?
The positive 68v rail is bang on at 68v, I assume the negative is the same but haven't had a chance to put a meter on it yet.
Haven't measured the 34v rails yet.
I now have all secondaries plugged into the PSUs and the amp lights up and stays in operational (green power light) mode without the protection circuit kicking in. So the fault is either one of the output boards or the volume board.
BTW, thanks to everyone who's replied so far. I'm a super noob and have learned a ton doing this especially with the poignant questions and suggestions.
Have you examined the ISC circuit and checked/adjusted the voltage at TP9 (GND point) + TP10 ??
Spec is -100mV to -50mV.
Not sure if this circuit would have an issue if the protection module would be triggered. But this ISC works with supplying the higher voltage under a higher load demand from the power amp stage.
Spec is -100mV to -50mV.
Not sure if this circuit would have an issue if the protection module would be triggered. But this ISC works with supplying the higher voltage under a higher load demand from the power amp stage.
Have not gotten there yet. Still hunting for whatever fault is dragging down the PSUs.
You need to get rid of the light bulb to get any further with this. When this illuminates the filament resistance increases and this reduces the ac voltage and current levels in the transformer winding. The same thing happens in the secondary winding.
Transistors need a base-to-emitter voltage (vbe) of 0.6 to conduct. This is a bottom line in manufacturers datasheets.
It should be clear from the level of reduction in supply rail voltage your amplifier these would have impact on all transistor vbe levels. If you need proof measure one or two - do the base and emitter voltages separately with respect to earth and subtract the differences.
Transistors need a base-to-emitter voltage (vbe) of 0.6 to conduct. This is a bottom line in manufacturers datasheets.
It should be clear from the level of reduction in supply rail voltage your amplifier these would have impact on all transistor vbe levels. If you need proof measure one or two - do the base and emitter voltages separately with respect to earth and subtract the differences.
"You need to get rid of the light bulb"
+1, the bulb should be used at first power on after parts replacement to verify no shorts, then get rid of it. The bulb will drag down all voltage rails. Set bias back to zero/minimum before power up without bulb
+1, the bulb should be used at first power on after parts replacement to verify no shorts, then get rid of it. The bulb will drag down all voltage rails. Set bias back to zero/minimum before power up without bulb
Ok, done. Fires up without issue now and rail voltages are correct. It's weird though because I tried it without the bulb before but it still shut itself off. It's possible I didn't have all the offset and bias pots turned all the way down though...I don't remember.
I'd go through all the adjustments (dc offset, bias and isc) as soon as possible then monitor the amp for an extend burn-in.
Good pickup on the DBT usage by @mjona and @mbz.
I got away from DBT after I purchased an auto-transformer with AC current monitoring.
I'm sure there are good arguments for using a DBT but in addition to the autotrans + meter I use 3 DMMs to monitor voltages.
As for why the shutdown W/O the DBT on a previous test; there still may be a flaky device somewhere in the power supply structure. Supplies to check are the 5V and 12V outputs.
Good pickup on the DBT usage by @mjona and @mbz.
I got away from DBT after I purchased an auto-transformer with AC current monitoring.
I'm sure there are good arguments for using a DBT but in addition to the autotrans + meter I use 3 DMMs to monitor voltages.
As for why the shutdown W/O the DBT on a previous test; there still may be a flaky device somewhere in the power supply structure. Supplies to check are the 5V and 12V outputs.
Thanks guys lol. Glad it fired up but still uncomfortable with the fact that it didn't before. Is it possible that upon initial fire-up all caps being 100% empty forced the circuit to misbehave and it needed a few "on-protect-on-protect" cycles before stabilizing? Perhaps this is why it didn't before? I didn't give it much time - as soon as at it went into protect the first time, I shut it off for fear of the magic smoke.
Anyway, offsets and biases next...hopefully things go well.
Anyway, offsets and biases next...hopefully things go well.
A bit off topic but, if I wanted to add a safety earth/ground to the amp, do I need a cap or thermistor between chassis and ground? If so, what cap value? And it should be Y rated correct? Any benefit to adding a rectifier bridge as a ground lifter?
I expect whichever State you are living in would have their electrical wiring regulations online.
Any amp I work on I always verify that the hot side is always switched. And if the neutral prong of the AC plug is not flared then I make it flared by using pliers+snips.
If you are going to add a 3-wire AC line-in plug/wire then connect the ground wire to the chassis (safety ground)....again verifying the hot side is switched.
Safety caps; here's a website that I refer to for explaining X (across line) and Y (line to ground):
ABC's of Safety (Interference Suppression) Capacitors for Tube Radios
I'm not sure about the thermistor usage nor rectifier-bridge/ground lifter (seems like a bad idea) for line safety.
I'm not sure my state cares how I wire up my amp lol. Doubt they have an audio amplifier section of regulations...or best practices in regards to noise etc.
Thanks. The rectifier bridge ground lifter comes from this article on ground loops (start on page 23): http://hifisonix.com/wordpress/wp-content/uploads/2019/02/Ground-Loops.pdf