NAD 304 blowing fuses in preamp supply - 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
Thread Tools Search this Thread
Old 24th March 2005, 03:57 PM   #1
diyAudio Member
Join Date: Mar 2005
Location: Austin, TX
Send a message via Yahoo to geemonster
Question NAD 304 blowing fuses in preamp supply

I've got an NAD 304 integrated amp that is having some issues. The two 250mA fuses feeding what I believe is the preamp power supply are blowing instantly obviously indicating that there is some kind of short in that part of the amplifier. I was wondering if anyone had come across a similar problem with a 304 or 314 and where I should look next. I'm fortunate enough to have a service manual/schematic (thanks guys!) but this is my first repair job so I could use some guidance as to what to look for/at. Any help is greatly appreciated.

The usual suspects with this model R333,R334, R917, R918 all appear to be fine. I've checked the main amplifier supply and the voltages all look good so I'm assuming the transformer is fine and that it is not a main amplifier problem. Not really sure what AC voltage should be present at the F901 and F902 supply fuses since the schematic doesn't say but they are toast instantly. Is there a particular component in the low current supply that when it goes bad creates a short? In just poking around I didn't find any resistors that seemed out of wack and most of the diodes I probed seemed good but that was only in the power supply section and the problem could obviously be in the pre-amp portion itself.

Thanks again for any help!
  Reply With Quote
Old 24th March 2005, 05:17 PM   #2
diyAudio Member
colwaiting's Avatar
Join Date: Dec 2002
Location: boston Ma.
Default Nad 304

Your supply will be regulated.
Nad has used transistor regulation, not fixed regulators, in the past
In mine they ran hot, could be a failure point. These guys will have small Alu heatsiks. Look for burning. They are in pairs for +/- supply..
In my 1300 Pre the voltage was 65 CT ac, so high! of course a lot of hum was eliminated in RC circuits to get the +/- dcv to usable levels-
(+/- 22 for the discrte circuits) . if there is enough trouble on the board, a repair will be too troublesome. Another supply could be added, If your schematic has the voltages needed for the pre-amp stages
  Reply With Quote
Old 25th March 2005, 12:25 AM   #3
jaycee is offline jaycee  United Kingdom
diyAudio Member
Join Date: Apr 2004
Location: Norwich, UK
quick guess but look for zener follower regulators where it's possible that either the transistor has failed, the zener has failed or both.
  Reply With Quote
Old 25th March 2005, 01:47 AM   #4
anatech is offline anatech  Canada
diyAudio Moderator
anatech's Avatar
Join Date: Jun 2004
Location: Georgetown, On
Yup, when the pass transistor shorts the emitter - base junction normally breaks down reverse (shorting) and kills the zener. I've also seen the small electrolytic caps short.

  Reply With Quote
Old 25th March 2005, 05:16 PM   #5
diyAudio Member
Join Date: Mar 2005
Location: Austin, TX
Send a message via Yahoo to geemonster
I think there are definitely some bad regulator transistors in that power supply although they don't show any physical signs of burnout around the heatsinks. I'm measuring some low resistances across the terminals on a couple of them in both directions.

I'll also look at any zener's in that circuit as well but thus far they all seemed fine.

Thanks for the tips.

  Reply With Quote


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
Keep Blowing Fuses Whiting Tubes / Valves 9 11th September 2008 09:44 PM
Blowing fuses balance Tubes / Valves 2 5th March 2005 02:16 AM
blowing fuses homegrown45167 Car Audio 2 22nd October 2004 12:16 PM
Power Supply blowing fuses cm961 Parts 7 21st January 2003 04:25 AM
Why does my PSU KEEP blowing fuses?! Rarkov Parts 29 17th October 2002 03:35 AM

New To Site? Need Help?

All times are GMT. The time now is 01:01 AM.

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