On Monday, May 22, 2017 at 7:40:15 PM UTC-4, David Farber wrote:
wrote:
On Wednesday, March 15, 2017 at 5:53:01 PM UTC-4, David Farber wrote:
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


Hello David --
I just picked up a used RX707 that is doing the same exact
thing...after heating up the right channel is distorting badly, but
only when SEA is switched in. From what I can take from your
discussion above, you solved the problem by adding 100k resistors
from two of the pins of IC503 to ground? Which two pins get the
resistors? Has that fix lasted long term? I love the fact that you
can basically save a different EQ for each of your sources, something
that today's products don't allow. Thanks for the info if you can
provide it.
Mike

Hi Mike,

Yes, the problem seems to have been fixed long term. The pins in question on
IC503 where I installed the 100k resistors to ground were the noninverting
pins, #3 and #5.

Good luck.
--
David Farber
Los Osos, CA

appears to be an actual design error
m