Need advice from an NAD amp guru . . .

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Picked up a semi-working NAD 3225PE the other day (less than $10), the preamp section is just fine, but the poweramp section was dead when I got it. There were a couple of obviously burned components, 2 resistors and a small mylar cap, that I replaced before putting new fuses in and powering it up. Had an older (expendable) set of speakers plugged into it for the first test, and when switched on, the amp was obviously in oscillation due to the pretty one-note song it was singing :)
The bias trimpots were near the failed resistors (physically), so I replaced them with a spare set of multi-turns, and to be safe, set them to double the resistance of the originals so I could turn the bias up very slowly. On power-up, the fuses held, but one of the resistors I replaced got HOT real quick (I had covered them in electrical tape as an "early warning system" just in case). Switched it off quickly, but now I'm stuck as to what to check next. The resistor is R454, 180 ohm, .25w. It goes across the bases of the output transistors, or across the emitters of the last gain stage, depending on your point of view. Do I have a dead output transistor, a dead gain-stage transistor, or a dead driver transistor, or something else? None of the transistors I mentioned got hot to the touch at all, but the outputs and the driver transistors all share a pretty substantial heat sink. Anyone have any pointers on what to look for, and how to check it before I let ALL of the smoke out of it? I'm stuck.
I've replaced (upgraded), and rebiased the output transistors in very similar NAD 3020 before, but that unit was working when I started modifying it. My current knowledge level of electronics can't take me much past that.
TIA,
BT

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Topic - Need advice from an NAD amp guru . . . - BrassTeacher 22:05:30 11/20/05 (2)

Re: Need advice from an NAD amp guru . . . - clifff 01:25:15 11/21/05 (1)

Re: Need advice from an NAD amp guru . . . - mikee55 02:55:38 11/21/05 (0)

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