Go Back   Home > Forums > Amplifiers > Solid State

Solid State Talk all about solid state amplification.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 11th March 2007, 04:30 PM   #1
cyteen is offline cyteen  England
diyAudio Member
 
cyteen's Avatar
 
Join Date: Mar 2004
Location: S.E
Default NAD C370 Diagnosis and repair help

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.
Attached Images
File Type: jpg nad_c370_circuit-small.jpg (99.5 KB, 836 views)
  Reply With Quote
Old 12th March 2007, 06:55 PM   #2
cyteen is offline cyteen  England
diyAudio Member
 
cyteen's Avatar
 
Join Date: Mar 2004
Location: S.E
Just in case the image was too small:

Click the image to open in full size.
  Reply With Quote
Old 12th March 2007, 08:35 PM   #3
diyAudio Member
 
FastEddy's Avatar
 
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.)
  Reply With Quote
Old 13th March 2007, 01:02 AM   #4
cyteen is offline cyteen  England
diyAudio Member
 
cyteen's Avatar
 
Join Date: Mar 2004
Location: S.E
Thanks very much for the reply. I'll look into those areas next.
  Reply With Quote
Old 13th March 2007, 12:22 PM   #5
cyteen is offline cyteen  England
diyAudio Member
 
cyteen's Avatar
 
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?
  Reply With Quote
Old 13th March 2007, 01:14 PM   #6
cyteen is offline cyteen  England
diyAudio Member
 
cyteen's Avatar
 
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.
  Reply With Quote
Old 13th March 2007, 03:36 PM   #7
diyAudio Member
 
FastEddy's Avatar
 
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/ )
  Reply With Quote
Old 13th March 2007, 03:45 PM   #8
jaycee is offline jaycee  United Kingdom
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.
  Reply With Quote
Old 14th March 2007, 03:43 AM   #9
KISS is offline KISS  United States
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.
  Reply With Quote
Old 14th March 2007, 09:21 AM   #10
cyteen is offline cyteen  England
diyAudio Member
 
cyteen's Avatar
 
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.
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
NAD C370 power-on problem windfall123 Solid State 13 19th September 2013 12:47 PM
Replacing CAPs in NAD C370 silversweden Solid State 9 28th December 2009 07:36 PM
power supply cap replacement: NAD C370 nick78 Solid State 7 16th July 2009 07:15 AM
Noise on R-channel NAD C370 sandeman Solid State 4 25th July 2008 03:32 PM
Electronics diagnosis and repair learning web-sites? KP11520 Equipment & Tools 20 13th June 2007 03:08 AM


New To Site? Need Help?

All times are GMT. The time now is 01:22 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2