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October 3, 2022

Repairs of The Carver Receiver 2000

Filed under: Uncategorized — admin @ 6:57 am

One of the more complex repairs that I’ve done lately was of this Carver Receiver 2000 that a climbing friend brought my way. This is a rare piece of high end audio gear, manufactured in late 1980s. As received, the unit had no left channel output, and no A speaker output. B output and headphone output was fine for right channel.

I used a simple signal generator applied to the inputs and an oscilloscope to trace through the system. The preamp output turned out to be fine for both channels. The final power output board was where the problems were. First, I found that the output of the left-dedicated channel of a NJM072DE dual JFET input buffer op amp, IC701 in the schematic, was becoming badly distorted and then shutting down completely after a few seconds of operation, just before the speaker relays were being energized -essentially a thermal intermittent. Scope trace below is of the distorted response to a low amplitude sine wave input.

That op amp is out of production but I happened to have a newer replacement, a Motorola LF353N with equivalent specs in my parts bins. After replacing the IC, the left channel output was now consistently on, but very distorted past that op amp. The culprit turned out to be yet another bad electrolytic capacitor, C309. Upon replacing that and fixing some problematic soldering at R715, the amplifier output was fine; a very clean sine wave even at high amplitude.

One final problem remained; the A speaker output. The obvious culprit was the A speaker relay, a 24V Matsushita device. Rather than buy a new one for $144US, I cracked it open and cleaned the contacts with fine sandpaper. Tests showed it to be fully working afterward.

All fixed now. A very interesting high power amplifier.

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