NAD 7250PE Repair From Driving a Shorted Voice Coil? - diyAudio
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Old 27th March 2014, 07:22 AM   #1
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Location: Upstate NY
Default NAD 7250PE Repair From Driving a Shorted Voice Coil?

Hi, I'm Nick, I'm a college student and love audio and electronics. It looks like I'm in good company here, part of why I signed up. The other part is that this is the best source I've found for information about the insides of amplifiers online.

I have read a few threads here relating to this receiver, they have gotten me this far, and provided what schematics I have. (Here, for further reference: example is in pretty great shape, except that it drove a shorted load on the right channel for a while. I'm trying to resurrect it in the hopes of using it in my home audio setup.

My autopsy found that the two rail voltage switching transistors were definitely blown. I've checked every other transistor on the right channel and compared it to the equivalent in the left channel; nothing else is shorted.

The plan of attack then became to short the two failed transistors, and power it up gently to make some measurements. I build a little gadget with a switch and light bulb socket wired in series with the mains and tried it.
No magic smoke, light dimmed right away, great. No relay click either though.

Speaker outputs: no DC across them. Great! Rails measure in at +55 and -55, left channel outputs measure within reason on the board, but the right channel has rail voltage at all collectors and emitters. No trail of destruction from a blown output stage, all four .22 ohm resistors measured out okay, so WTF is going on here?

The one idea I have left (short of tediously lifting every leg and testing every component, something I neither have the time or wallet to do) is that one or more of the little screw pots to adjust idle current and DC offset has blown. Is this reasonable, or am I just hoping for a cheap fix?

TL;DR: Amp drove shorted load on right channel. Binding posts measure 0 vdc across them, left channel okay, right channel has rail at the collector and emitter of every output transistor, but they do not test as failed (and there is no trail of destruction). Ideas?
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Old 27th March 2014, 07:37 AM   #2
ishiru is offline ishiru  Indonesia
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Location: Jakarta, ID - Nagasaki, JP
what about the DC fuse if there's available?.they might be blowned
iono wat i do
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Old 27th March 2014, 07:48 AM   #3
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Location: Upstate NY
Originally Posted by ishiru View Post
what about the DC fuse if there's available?.they might be blowned
The only two fuses are on the preamp board, they are both good.
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