Hi all, I recently was given my dad's old 7240PE which he said just stopped working one day. Taking a look inside I saw all of the 4a and 5a fuses had blown. For the rest of this post I will be referencing the service manual found here: https://www.audioservicemanuals.com/n/nad/nad-60-7600/nad-7240/15238-nad-7240-pe-service-manual
My first step was taking out the driver transistors (Q427,428,429,430) and putting new fuses back in. Turning it on, R495 burst in a flame of glory. It's worth noting that the fuses did not blow, but I'm still a little concerned to do any further damage. Because of this I plan on proceeding with caution... and with the help of the internet.
I've poked around with my DMM and noticed that C427's points on the board are shorted even after I desoldered it (the cap itself was fine). Any ideas where to go next?
My first step was taking out the driver transistors (Q427,428,429,430) and putting new fuses back in. Turning it on, R495 burst in a flame of glory. It's worth noting that the fuses did not blow, but I'm still a little concerned to do any further damage. Because of this I plan on proceeding with caution... and with the help of the internet.
I've poked around with my DMM and noticed that C427's points on the board are shorted even after I desoldered it (the cap itself was fine). Any ideas where to go next?
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Hi jommint,
Welcome to the forum!
I suspect this will be a challenging repair. I've attached a pdf of a service manual that seems to have better print quality than the one from your link. You should probably confirm it's appropriate for your amp's SN. It came from: https://www.manualslib.com/manual/1724963/Nad-3240.html#manual I hope other members may have repair experience with this amp; I'll be working solely from the schematic.
Note that page 5 of the attachment recommends using a light bulb current limiter for testing after repair. Do you have a scope? Perhaps current-limited bench supplies?
Good luck!
Welcome to the forum!
I suspect this will be a challenging repair. I've attached a pdf of a service manual that seems to have better print quality than the one from your link. You should probably confirm it's appropriate for your amp's SN. It came from: https://www.manualslib.com/manual/1724963/Nad-3240.html#manual I hope other members may have repair experience with this amp; I'll be working solely from the schematic.
Note that page 5 of the attachment recommends using a light bulb current limiter for testing after repair. Do you have a scope? Perhaps current-limited bench supplies?
So, were Q427,428,429,430 absent from the board during your first test? (I'm not saying this would be a bad idea.)
From the schematic, I would think you would see at least R457's 10 Ohm resistance, probably more. Would you double check? (and try exchanging meter leads for polarity check.)
Good luck!
Always you have a failed device in your workbench use a limited current supply. This way you prevent future damage. A light bulb of the filament type (no led nor fluorescent) in series to the live cord of the line. You may do it with a dedicated outlet or wire a lamp holder with a couple of cucodrile clips, thus remove the DUT's fuse and use it in place of the said fuse.
The lamp must be about 1 to 5 times the power of the DUT, not much more. Under normal conditions you should see a flash in the lamp immediatly after power on, because of capacitor charge. After it, lamp must be completely dark or a soft red in the filament.
The lamp must be about 1 to 5 times the power of the DUT, not much more. Under normal conditions you should see a flash in the lamp immediatly after power on, because of capacitor charge. After it, lamp must be completely dark or a soft red in the filament.