NAD 304 blowing fuses in preamp supply

[geemonster]

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!

[colwaiting]

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

[jaycee]

quick guess but look for zener follower regulators where it's possible that either the transistor has failed, the zener has failed or both.

[anatech]

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.

-Chris

[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.

-Sean