NAD C370 Diagnosis and repair help

cyteen · 11th March 2007, 04:30 PM

A friends C370 has recently stopped working and I'm having a look at it now. Apparently it had been running hot for a while then made a few unexpected noises and then switched into protection mode. After being off for a while it will produce sound for a short time before protect mode kicks in again.

So I've measured the resistors connected to the output transistors and these measure 0.7 ohms rather than 0.2 on the schematic. (R370, R352 R354, R349 R353 R355) and I measured the resistors which seem to get hotist (judging from the heat marks on the lid) and these measured 1k5 ohms instead of 2k2 on the schematic. (R334, R344)

The main 10,000uF capacitors look a little bulgy and there is a little staining around their base on the pcb.

I'd appreciate any advice regarding what might be happening and further tests I can do to pinpoint the root cause of the problem.

cyteen · 12th March 2007, 06:55 PM

Just in case the image was too small:

FastEddy · 12th March 2007, 08:35 PM

" ... sound for a short time before protect mode kicks in again ... I've measured the resistors connected to the output transistors and these measure 0.7 ohms rather than 0.2 on the schematic. ..."

Age and repeated over range heat will do this = power on/off over several years. Get ready to replace these. Consider a little higher wattage if there is room = 2 watt or 5 watt. "Audiophile quality" recommended, +/- 1% or better.

" ... The main 10,000uF capacitors look a little bulgy and there is a little staining around their base on the pcb. ..."

This may be serious. Consider replacing these with modern audiophile type caps.

You might also try to figure out where the excess heat is coming from, specifically. Power supply? PS caps warm? Outputs over heating? The first audio stage(s)? Left side or right side or both, equally?

Fix the heat problem first before upgrading elsewhere. Also measure the voltage across the Zeners (TL431 time 2 times two channels = http://www.fairchildsemi.com/ds/TL/TL431A.pdf ). This voltage should be equal on each channel and on left & right. One of these running outta spec could be the culpret. (These are adjustable, according to the schemo, but after adjustments to factory specs, none should be more than 5% variation.)

cyteen · 13th March 2007, 01:02 AM

Thanks very much for the reply. I'll look into those areas next.

cyteen · 13th March 2007, 12:22 PM

I'm unsure what would classify as "Audiophile quality" and the only 1% 0.2r I've found is rated at 3W

Vishay LVR-3

Is wirewound appropriate in this position?

cyteen · 13th March 2007, 01:14 PM

I've just found the service manual and it specifies "Fusible Resistor" for the 0.2r and metal oxide film for the 2k2. Again I don't know what would constitute an "audiophile grade" of these.

FastEddy · 13th March 2007, 03:36 PM

" ... Is wirewound appropriate in this position? ..."

I would not use wire wound resistors anywhere near a signal path 'cause you don't know how the inductive component of these will act or react to the other associated components = wire wound resistors are just that = a coil of resistance wire wound around a form = an inductor.

There are plenty of places for wire wound resistors, like in the current / power path of a power supply where these can actually be benifitial to the purpose = remove a little of the noise from the power path, but in the signal path, these can remove some of the audio signal as well ...

" ... I don't know what would constitute an "audiophile grade" of these. ..."

I use the term relative to the low cost, lower performance "ordinary" components. "Audiophile quality" and "Golden Ear quality" are very relative terms used in this case to distinguish between the ordinary.

In the case of capacitors, there are many ranges of quality of result. For the purposes of power supplies, as in this example (the NAD amp), one could add smaller value polystyrene caps in parallel with the bigger electrolytic caps and achieve similar results in improvement of performance approaching "audiophile quality" = better response to higher frequency noise components in the power supply ...

"Truth is one, paths are many ... " - The Dalai Lama ( http://www.dalailama.com/ )

jaycee · 13th March 2007, 03:45 PM

Ignore the "Audiophile Grade" comments. I think the aim here is to get it working. Metal Film resistors will do.

If the resistors are Fusible, you can use regular ones but only to TEST with... the idea obviously is that the fusible resistors protect from serious damage/fire by going open circuit if too much current flows.

KISS · 14th March 2007, 03:43 AM

Be carefull trying to measure 0.2 ohm resistors. The reliable ways are:

A 4-wire ohmeter

An ohmeter with a zero adjust

Measure the voltage across the resistor and the current through it
and compute the value of R.

The way fusable resistors are made, they usually fail completely open.
Metal film resistors can puddle. Carbon composition resistors usually get larger in value when they fail.

cyteen · 14th March 2007, 09:21 AM

I appreciate the information, I was a little wary of trusting the cheap DMM for the low value resistors but good to have confirmation.

12th March 2007, 08:35 PM	#3
FastEddy diyAudio Member Join Date: Jun 2006 Location: North Californie	" ... sound for a short time before protect mode kicks in again ... I've measured the resistors connected to the output transistors and these measure 0.7 ohms rather than 0.2 on the schematic. ..." Age and repeated over range heat will do this = power on/off over several years. Get ready to replace these. Consider a little higher wattage if there is room = 2 watt or 5 watt. "Audiophile quality" recommended, +/- 1% or better. " ... The main 10,000uF capacitors look a little bulgy and there is a little staining around their base on the pcb. ..." This may be serious. Consider replacing these with modern audiophile type caps. You might also try to figure out where the excess heat is coming from, specifically. Power supply? PS caps warm? Outputs over heating? The first audio stage(s)? Left side or right side or both, equally? Fix the heat problem first before upgrading elsewhere. Also measure the voltage across the Zeners (TL431 time 2 times two channels = http://www.fairchildsemi.com/ds/TL/TL431A.pdf ). This voltage should be equal on each channel and on left & right. One of these running outta spec could be the culpret. (These are adjustable, according to the schemo, but after adjustments to factory specs, none should be more than 5% variation.) __________________ Over compression is a problem with modern CD recordings

13th March 2007, 03:36 PM	#7
FastEddy diyAudio Member Join Date: Jun 2006 Location: North Californie	" ... Is wirewound appropriate in this position? ..." I would not use wire wound resistors anywhere near a signal path 'cause you don't know how the inductive component of these will act or react to the other associated components = wire wound resistors are just that = a coil of resistance wire wound around a form = an inductor. There are plenty of places for wire wound resistors, like in the current / power path of a power supply where these can actually be benifitial to the purpose = remove a little of the noise from the power path, but in the signal path, these can remove some of the audio signal as well ... " ... I don't know what would constitute an "audiophile grade" of these. ..." I use the term relative to the low cost, lower performance "ordinary" components. "Audiophile quality" and "Golden Ear quality" are very relative terms used in this case to distinguish between the ordinary. In the case of capacitors, there are many ranges of quality of result. For the purposes of power supplies, as in this example (the NAD amp), one could add smaller value polystyrene caps in parallel with the bigger electrolytic caps and achieve similar results in improvement of performance approaching "audiophile quality" = better response to higher frequency noise components in the power supply ... "Truth is one, paths are many ... " - The Dalai Lama ( http://www.dalailama.com/ ) __________________ Over compression is a problem with modern CD recordings

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12th March 2007, 06:55 PM	#2
cyteen diyAudio Member Join Date: Mar 2004 Location: S.E	Just in case the image was too small:

13th March 2007, 01:02 AM	#4
cyteen diyAudio Member Join Date: Mar 2004 Location: S.E	Thanks very much for the reply. I'll look into those areas next.

13th March 2007, 12:22 PM	#5
cyteen diyAudio Member Join Date: Mar 2004 Location: S.E	I'm unsure what would classify as "Audiophile quality" and the only 1% 0.2r I've found is rated at 3W Vishay LVR-3 Is wirewound appropriate in this position?

13th March 2007, 01:14 PM	#6
cyteen diyAudio Member Join Date: Mar 2004 Location: S.E	I've just found the service manual and it specifies "Fusible Resistor" for the 0.2r and metal oxide film for the 2k2. Again I don't know what would constitute an "audiophile grade" of these.

13th March 2007, 03:45 PM	#8
jaycee diyAudio Member Join Date: Apr 2004 Location: Norwich, UK	Ignore the "Audiophile Grade" comments. I think the aim here is to get it working. Metal Film resistors will do. If the resistors are Fusible, you can use regular ones but only to TEST with... the idea obviously is that the fusible resistors protect from serious damage/fire by going open circuit if too much current flows.

14th March 2007, 03:43 AM	#9
KISS diyAudio Member Join Date: Jan 2007	Be carefull trying to measure 0.2 ohm resistors. The reliable ways are: A 4-wire ohmeter An ohmeter with a zero adjust Measure the voltage across the resistor and the current through it and compute the value of R. The way fusable resistors are made, they usually fail completely open. Metal film resistors can puddle. Carbon composition resistors usually get larger in value when they fail.

14th March 2007, 09:21 AM	#10
cyteen diyAudio Member Join Date: Mar 2004 Location: S.E	I appreciate the information, I was a little wary of trusting the cheap DMM for the low value resistors but good to have confirmation.