Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Solid State Talk all about solid state amplification.

NAD 319 Repair
NAD 319 Repair
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 6th December 2018, 05:30 PM   #1
Passmore is offline Passmore
diyAudio Member
 
Join Date: Dec 2018
Default NAD 319 Repair

I need help with an old NAD319 amp . I have attached an excerpt of the schematic which is referenced heavily in this post. Can attach the full service manual if needed - i wasn't sure if that was allowed.

So - R202, one of the fusible resistors was blown. After checking the board over and testing for any obvious shorts/failed components, I decided to temporarily replace it with a 10ohm non-fusing resistor, so that i could try to find what was heating up and drawing too much current through it. The culprit appeared to be C211, which was heating up and swelling up gradually once the unit was powered. The cap tested fine out of circuit.

Anyways, I replaced it with a new cap, and unfortunately this cap does exactly the same thing! I have checked all the diodes and little ceramic caps around c211 and all seem fine. Also checked the ceramic caps around the bridge rectifier and no issues there. I've also checked the 6 4700uf caps on the positive side of C211.

I believe I have tested every possible component which could solely cause a lot of current to pass through C211. The curious thing is - if the problem is elsewhere in the circuit, I would expect either C210 or C213 to be heating up too, which they are not.

I have scanned the board many times and can't see any solder bridges or anything else which might cause this to happen.

Important notes...
1. The test point +67v over to the far left of the picture below is measuring well over +100v (i say well over because it keeps rising and i'm only leaving the amp on for a few seconds at a time).
2. The test point -67v over to the far left of the picture is measuring about -16v. This indicates to me that whatever is drawing too much power away from that line and through C211 is not a total short.
3. If i test the 4 pins of the bridge rectifier, I get perfect voltage readings, meaning the problem definitely starts after that.
4. The mains fuse was not blown and isn't blowing, none of the other fusing resistors are blowing other than R202.

Click the image to open in full size.

Last edited by Passmore; 6th December 2018 at 05:54 PM. Reason: Forgot to attach image
  Reply With Quote
Old 6th December 2018, 05:54 PM   #2
Ylli is offline Ylli  United States
diyAudio Member
 
Join Date: Oct 2018
Location: Upper midwest, US
Almost has to be excess ripple current through C211. Have you checked C207? Regardless of what your meter says, I would go ahead and replace D203, D205, and D207, and probably C207.
  Reply With Quote
Old 6th December 2018, 05:56 PM   #3
rayma is offline rayma  United States
diyAudio Member
 
Join Date: Apr 2011
Quote:
Originally Posted by Passmore View Post
The test point +67v over to the far left of the picture below is measuring well over +100v
Check/replace C206 and D202.
  Reply With Quote
Old 6th December 2018, 06:07 PM   #4
Passmore is offline Passmore
diyAudio Member
 
Join Date: Dec 2018
Quote:
Originally Posted by Ylli View Post
Almost has to be excess ripple current through C211. Have you checked C207? Regardless of what your meter says, I would go ahead and replace D203, D205, and D207, and probably C207.
All of those test fine out of circuit, but I'm happy to replace them to confirm.

Quote:
Originally Posted by rayma View Post
Check/replace C206 and D202.
Both of those test fine out of circuit - replace anyways?
  Reply With Quote
Old 6th December 2018, 06:13 PM   #5
rayma is offline rayma  United States
diyAudio Member
 
Join Date: Apr 2011
Quote:
Originally Posted by Passmore View Post
Both of those test fine out of circuit - replace anyways?
Yes, how are you testing these parts?

I would replace C206 207 210 211 212 213 and related diodes.
Check R201, 202, 203, 204.

Last edited by rayma; 6th December 2018 at 06:30 PM.
  Reply With Quote
Old 6th December 2018, 06:16 PM   #6
Passmore is offline Passmore
diyAudio Member
 
Join Date: Dec 2018
Quote:
Originally Posted by rayma View Post
Yes, how are you testing these parts?
With a Fluke 117 Multimeter and an Atlas ESR70 for the caps.
  Reply With Quote
Old 6th December 2018, 06:43 PM   #7
rayma is offline rayma  United States
diyAudio Member
 
Join Date: Apr 2011
Quote:
Originally Posted by Passmore View Post
With a Fluke 117 Multimeter and an Atlas ESR70 for the caps.
I wonder whether the Atlas can detect shorted capacitors.
  Reply With Quote
Old 6th December 2018, 07:38 PM   #8
Ylli is offline Ylli  United States
diyAudio Member
 
Join Date: Oct 2018
Location: Upper midwest, US
"1. The test point +67v over to the far left of the picture below is measuring well over +100v"

Where do you have the negative voltmeter lead connected? To get 100 volts on the 67 volt buss would require;
1. CT of transformer open - open black wire AC0, use your ohmmeter to confirm transformer center tap is intact and properly connect to the system ground (0V, SPOR, SPOL, OL, OR)
2. Failure in the +67 volt regulator

Note that the -67 volt line is actually regulated via a pair of zener diodes (D208, D209) and the +67 volt line is regulated as a mirror of the - 67 v.

For test purposes, lift one end of R203 and R204. Then measure the voltages across C210 and C211.
  Reply With Quote
Old 6th December 2018, 08:19 PM   #9
Ketje is offline Ketje  Belgium
diyAudio Member
 
Ketje's Avatar
 
Join Date: Dec 2012
Location: Flanders
It's normal for the +67 to go up when the -62 fails.The minus supply provide the reference for the positive side.
Mona
  Reply With Quote
Old 6th December 2018, 08:28 PM   #10
Mooly is offline Mooly  United Kingdom
diyAudio Moderator
 
Mooly's Avatar
 
Join Date: Sep 2007
NAD 319 Repair
These voltage multiplier circuits are often trouble (speaking generally).

D207 would be favourite but I would absolutely change them all plus the caps as has been mentioned. Replace all the diodes with 1N4007 (1000v PIV) rather than 1N4003 (which is only 200v PIV)
  Reply With Quote

Reply


NAD 319 RepairHide 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

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
What did you last repair?? Dude111 The Lounge 761 Today 06:49 AM
QED A 240 CD Can Anyone Pls Help With Repair Rol G Solid State 38 8th March 2018 08:30 AM
Please help amp repair! HerrNuki Car Audio 11 13th May 2014 07:24 PM
Car Amp repair cnitty76 Car Audio 6 13th April 2011 03:46 AM
Need some help on amp repair pityocamptes Car Audio 2 2nd October 2007 08:59 PM


New To Site? Need Help?

All times are GMT. The time now is 06:16 PM.


Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2020 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 14.29%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2020 DragonByte Technologies Ltd.
Copyright ©1999-2020 diyAudio
Wiki