Blown fuses on NAD 7240PE

[d-ray657]

I won a NAD 7240PE on ebay for a price that reflected its being listed as having no sound output. I opened it up tonight and found all four fuses blown. I have to wait until tomorrow to get replacements, but my question is: does the fact that all four fuses are blown indicate a likelihood that there are greater problems than blown fuses? Is it common for other fuses to follow in short order when one blows? Is it likely that I got lucky and got a good receiver for cheap with an easy fix? All collective wisdom is appreciated.

The fuses and the fuse holders were pretty badly oxidized, but nothing else inside looked fried.

Regards,

D-Ray

 

[chadnliz]

I saw a guy blow a fuse once, I crossed the street really quick!
I have blown a fuse before but only 1 at a time so your likely just gonna have to hope for the best and get advice from there if that didnt do the trick. Good luck!

 

[Troutsqueezr]

I'll give you a 50% chance!

 

[d-ray657]

Nix on the quick fix. I put four new fuses in and the four new fuses were immediately blown. I did hear a speaker "thump" right as the fuses blew (cheap sacrificial American Acoustics, so no real speaker risk). The only wires in close proximity to the fuses went to the switch that selects speaker impedance. It was set on 8 ohms, which matches the connected speakers. Besides finding a service manual and a schematic, and suggestions on the first thing to look at? Thanks.

Regards,

D-Ray

 

[asoundhound]

check for any indication of burning or loose solder joints on the power suppy board.

 

[BrassTeacher]

You have blown output transistors. That may not be all even. First, visually inspect resistors R461, R463, R462, and R464. If any of the four appear "toasted", or discolored from heat, don't even bother putting the two 5A and the two 4A fuses back in, skip to paragraph 2. If the afore-mentioned resistors look ok, remove all four output transistors, fit new fuses and start it back up. If the fuses blow again, don't even bother putting any more back in, skip to paragraph 2. Even if the fuses hold, look for signs of heat and/or smoke from any of the driver transistors. If such is noticeable, pull the fuses and don't put them back, skip to paragraph 2. If everything appears normal, you can gamble on putting new output transistors in, and there is a good chance everything will be ok. However, if you want to be sure, and you have a DMM, and experience working on a powered up amplifier with the cover off (in other words, if you do harm to yourself, you are proceeding of your own free will, and you have been warned that this is dangerous if you do not know what you are doing), check the voltages on the pads where the output transistors should be. You should see either + or - 40V or so on the collector, zero volts on the emitter (and I mean ZERO), and just a few millivolts on the base. If there is abnormally high voltage on the base, say more than 100mV, skip to paragraph 2. If there is around 30mV on the base, it should be safe to fit new output transistors, do not proceed to paragraph 2.

Paragraph 2: The amp will cost more to repair than it's worth.:tears: Luckily, the preamp and tuner will still function well with the main amp fuses missing, and the unit can be used as such.:yes: It will be a good preamp (basically a 1240), a good tuner, and a decent phono stage.

Oh, by the way, I just did service on a 7240PE yesterday, so this is all fresh in my mind.

 

[magnabox]

You have blown output transistors. That may not be all even. First, visually inspect resistors R461, R463, R462, and R464. If any of the four appear "toasted", or discolored from heat, don't even bother putting the two 5A and the two 4A fuses back in, skip to paragraph 2. If the afore-mentioned resistors look ok, remove all four output transistors, fit new fuses and start it back up. If the fuses blow again, don't even bother putting any more back in, skip to paragraph 2. Even if the fuses hold, look for signs of heat and/or smoke from any of the driver transistors. If such is noticeable, pull the fuses and don't put them back, skip to paragraph 2. If everything appears normal, you can gamble on putting new output transistors in, and there is a good chance everything will be ok. However, if you want to be sure, and you have a DMM, and experience working on a powered up amplifier with the cover off (in other words, if you do harm to yourself, you are proceeding of your own free will, and you have been warned that this is dangerous if you do not know what you are doing), check the voltages on the pads where the output transistors should be. You should see either + or - 40V or so on the collector, zero volts on the emitter (and I mean ZERO), and just a few millivolts on the base. If there is abnormally high voltage on the base, say more than 100mV, skip to paragraph 2. If there is around 30mV on the base, it should be safe to fit new output transistors, do not proceed to paragraph 2.

Paragraph 2: The amp will cost more to repair than it's worth.:tears: Luckily, the preamp and tuner will still function well with the main amp fuses missing, and the unit can be used as such.:yes: It will be a good preamp (basically a 1240), a good tuner, and a decent phono stage.

Oh, by the way, I just did service on a 7240PE yesterday, so this is all fresh in my mind.

This is excellent information. I am quoting it so I can try this fix myself. I usually leave fixes up to professionals but I may have to do this myself. I have a NAD 7240PE that seems to have a good front end but a bad amplifier. Could I use the pre-amp on this unit and pair it up w/ 2 Crown D150A II (Right and Left)?

 

[E.Man]

Not sure exactly on your 'Crown' but I'm making a wild guess that you *could*.

Have used 3240PE (Intergrated Amp version of 7240PE) preamp stage ahead of left and right amp stages without drama.