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David Farber David Farber is offline
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Default JVC RX-807V receiver. One channel distorted when SEA is enabled.

I'm servicing a JVC RX-807V receiver. After about 5 minutes of warm up, the
left channel starts to get distorted. Injecting a sine wave into the tape
input shows the top half of the wave is getting clipped. This only happens
when the SEA equalizer is enabled. Intermittently, the same problem will
happen on the other channel. When I tested it extensively last night, it was
only the left channel having the problem. I've narrowed the problem down to
the ENC-093 pc board. You can have a look at the schematic here.
https://www.elektrotanya.com/jvc_rx-.../download.html

I took some voltage measurements of IC 503 which is a dual op-amp:

Before the left channel starts clipping, pins 2 and 3 are about 2.0 and 2.4
volts respectively. Pin 1, the output, is at 3.0 volts. When the clipping
happens, pins 2 and 3 are about 6.7 volts, and pin 1 is at 12.0 volts. The
working channel has its own interesting measurements. Pins 5 and 6 are
about -4.0 volts and pin 7 is about -4.5 volts (there is no distortion with
these measurements in the right channel.) The input to both channels is
clean at IC503 even when the output is distorted. I want to replace IC 503
but before I do, I'd like to know.

What is the function of the JFET, Q553? the source(?) goes directly to pin 6
which is the inverting input of the right channel of IC503. I would think
that since this is a stereo receiver that there would be some symmetry to
the opposite inverting input, pin 2, of IC503 but instead pin 2 is wired
back to the drain of Q553 through an RC network. The gate of Q553 is -10
volts instead of +10 volts because Q551 and Q552 apparently are switched
off. I believe there is an error in the schematic that shows C516 connected
to pin 6 of IC503 rather than pin 7.

Thanks for your replies.

--

David Farber
Los Osos, CA