NAD C320 fixed (another protection circuit problem) - diyAudio
Go Back   Home > Forums > Amplifiers > Solid State
Home Forums Rules Articles diyAudio Store Gallery Wiki Blogs Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

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 5th April 2014, 06:49 PM   #1
diyAudio Member
 
Join Date: Mar 2013
Default NAD C320 fixed (another protection circuit problem)

Hi.

Finally fixed my mate's NAD C320. This had the common problem that it took ages to come out of protection (power LED stays red rather than turning green). I foiund the circuit diagram and read various similar problems here, although I replaced a few components I believe the fix was to replace C421. This connects to pin 4 of IC44 (a uPC1237, protection IC), and pin 4 is used for the AC mains detection. The design is to feed pin 4 from the mains transformer via a half wave rectifier and a resistor, the IC detects when the mains is switched off and goes into protection. Same IC is used in loads of amps, seems to be a common chip, so other amps likely to issues with the capacitor on pin 4.

Initially I though this capacitor was OK, as pin 4 originally measured about 1.5V on my DC multimeter. However as suggested on other threads I removed C421 (4.7uF 50V) and it only measured 1uF. When replaced with a new cap pin 4 measured 1.9V. However I think previously there must have been a lot of ripple on pin 4 hence the IC saw the voltage drop below threshold (0.9V) and so thought the AC was off and triggered the protection.

I also think there is some interaction on the IC between pin 4 and pin 7 (switch on delay). Pin 7 was stuck at around 200mV rather than the required 2V, so I thought originally the problem was with the associated capacitor for pin 7 (C420).

I also replaced C420 and C422 but they measured only 30% below spec and were probably OK.

Much earlier I had replaced IC44 and D412 but I don't think they were faulty.

Lesson is to think whether a DC voltage that seems OK with a multimeter could have a lot of ripple and actually fall below protection IC threshold, in which case a scope would be needed to check.

I saw on Youtube someone repaired a C320 by replacing all the caps, I think this is overkill (but would probably have saved me a lot of time!)

Regards,
John
  Reply With Quote
Old 5th April 2014, 09:26 PM   #2
sbrads is offline sbrads  United Kingdom
diyAudio Member
 
Join Date: Jun 2005
Location: Kent, UK
Sounds exactly like a fault I had with a Harman Kardon sub amp. A mains detection circuit cap was only 10% low in capacitance causing shutdown.
  Reply With Quote
Old 8th May 2014, 01:04 AM   #3
diyAudio Member
 
Join Date: May 2014
Default NAD C320 fixed too

Thanks a lot for the tip about C421 on the protection circuit (pin 4 of IC44). I was stuck on this problem because my measurement of pin 7 of IC44 was, like you, very different of what I expected, according to the service manual.
Click the image to open in full size.

By the way, this amp belongs to a friend. He has fallen down. And the first breakdown I found was a broken track in the +/- 18v circuit. Moreover, I'm surprised a fall can lead to a big change in a capacitor value, reason why I was not searching in that direction ! 2 breakdown to repear, but it's done thank to your experience ! Thanks again !
  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
Overactive NAD 3155 Protection Circuit dman535 Solid State 8 3rd May 2017 12:05 PM
NAD c350 protection module problem hostarn Solid State 17 19th January 2015 05:58 PM
NAD C320 Can the protection be adjusted Rossyb Solid State 0 3rd September 2011 07:49 PM
nad c320 reddish75 Solid State 3 16th September 2008 02:05 PM
NAD C320 Amp Protection delay help please Kajman Solid State 2 15th December 2007 03:24 PM


New To Site? Need Help?

All times are GMT. The time now is 03:39 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.
Copyright 1999-2017 diyAudio

Content Relevant URLs by vBSEO 3.3.2
Wiki