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NAD 7225PE Restore Fail!

notdigital

notdigital

AK Subscriber
Subscriber
I finally got around to digging into a 7225 receiver that had blown a few years back after doing gentle office duty. What I found was that the power transistors for the left channel were blown together with a couple of the smaller transistors in that channel. I replaced all the blown components with devices I had on hand. While I don't have a distinct memory of where I got them, what I do remember is that the smaller devices (2SB649A & 2SD669A) are true OEM Hitachi components. I suspect that the power transistors were purchased at auction with uncertain pedigree.

Here's the problem: It works but sounds like crap!! Gone is the resonant bass that I recall and is obviously present in another NAD piece I have going at another location. I know the "NAD sound" since I've always had one running somewhere. Unfortunately, this one now sounds like music from a tin can!

Before I cracked it open I powered it up to remind me what was wrong. The right channel was fine but, as I've mentioned, the left was kaput. After replacing the cooked parts I performed the amp adjustments per the service manual and the voltage measurements were well within spec. Still, bass is extremely anemic to non-existent. The LOUDNESS button helps but I shouldn't have to go there. What gives?

If the consensus is that the (possibly fake) power transistors are at fault, then can someone recommend a reliable source for bona fide devices? I think DigiKey carries what I need but the options have me a little thrown. Guidance in this regard would be appreciated!!!
 
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Shot in the dark here, but have you verified that you have the speakers hooked up in phase? Anemic bass does result from being out of phase. Also, if you haven't changed anything on the right channel, it's bass should be fine, but would be cancelled by the left speaker if it was out of phase. You can cross check things by using the headphone jack; that isn't dependent on your speaker wiring.
 
Hmmmmm...don't think so but I'll check tonight re phase. I'm pretty careful about connections but I might have goofed. I'll check that and the headphone test as well. Thanks for the suggestion!
 
Maybe check the small signal interstage capacitors around the transistors you replaced. Maybe one of them fried their base or gate to v+ causing the capacitor to blow itself somewhat open or leaky. I've seen it happen before. If you have an oscilloscope you could inject a low frequency square wave to the input and trace where the signal is rolling off by comparing to the other good stereo channel.
 
Yeah, agree somewhat on the caps, NADs I've seen had very tired/didn't test well caps.
If the unit has PRE-OUT/MAIN-IN connections then remove the jumpers and use an RCA cable but cross them, ie, Left PreOut to Right Main-IN.
Likewise Right Preout to Left Main IN. If the cr@p sound stays on the left then power amp issue
 
darklife said:
Maybe check the small signal interstage capacitors around the transistors you replaced. Maybe one of them fried their base or gate to v+ causing the capacitor to blow itself somewhat open or leaky. I've seen it happen before. If you have an oscilloscope you could inject a low frequency square wave to the input and trace where the signal is rolling off by comparing to the other good stereo channel.
Click to expand...
+1
If you have the test equipment input a 400 hz square wave and probe through it with an oscilloscope. Easiest way to pinpoint these kind of troubles.
 
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