JVC RX-807V receiver. One channel distorted when SEA is enab

[David Farber]

David Farber wrote:

makolber@yahoo.com wrote:
On Tuesday, March 21, 2017 at 9:34:11 PM UTC-4, David Farber wrote:
David Farber wrote:
makolber@yahoo.com wrote:
As far as I can tell, the components in question are not custom.
IC 503 is a standard dual op-amp whose DC output voltages start
creeping up until it reaches Vc or 12 volts and then the positive
half of the signal is clipped.


there are either schematic documentation errors or design errors
re IC 503

The + inputs to IC503 don't appear to have ANY DC connection, all
connections are via caps. The DC voltage on the + inputs is
therefore undefined. This is is either a design error or a
documentation error. If DC voltage is being set by the the cap
leakage, then that is your problem. Use a 100K connected to a
variable DC supply and see if you can't move the DC voltage to the
correct value. If you can, then you should be able to figure out
the rest.
m

I was wondering the same thing! I will check and report back to
you. Thanks for your reply.

Your suggestion of adding some dc voltage to the + inputs led me to
this webpage:
http://www.radio-electronics.com/info/circuits/opamp_non_inverting/op_amp_non-inverting.php

There is a section there called: *AC coupling the non-inverting
op-amp circuit.Using that guide, I added 100k resistors from the +
inputs to ground and now the DC doesn't go above 20mV. I also
noticed that whoever worked on that pc board before replaced many of
the caps. Most of the replacements involved changing the 4.7? values
to 10? values. I will replace those with the original values.

Thanks to everyone for their suggestions.
--

David Farber
Los Osos, CA

wow a design error then.

glad i could help



Mark

When I receive the replacement capacitors, which will be the original
ones according to the schematic, I will remove the 100k resistors and
see if the circuit still works. You have to believe that somehow the
system worked for many years until something changed.

Thanks for your reply.

I received the new capacitors and installed them. Then I removed the 100k
resistors to see what would happen. The voltage started rising again. I
reinstalled the resistors and it's been running great for two days now.

Thanks again.
--
David Farber
Los Osos, CA

 

One those 100Ks might have been open. When you unsoldered them you "fixed" them. This will not be permanent.

That was a problem sometimes, I learned to just use circuit values. Unless it is tube stuff you you cannot even check them in circuit, and you heat the leads to take them out of circuit and many of the "heal up" all the sudden, which can lead you down the garden path.

Been there done that. I would change them, what are we taking here ten cents ? That vs customer satisfaction is easy to figure out. Any circuit with even those slightly ambiguous results, change the high value resistors.

 

[David Farber]

jurb6006@gmail.com wrote:

One those 100Ks might have been open. When you unsoldered them you
"fixed" them. This will not be permanent.

That was a problem sometimes, I learned to just use circuit values.
Unless it is tube stuff you you cannot even check them in circuit,
and you heat the leads to take them out of circuit and many of the
"heal up" all the sudden, which can lead you down the garden path.

Been there done that. I would change them, what are we taking here
ten cents ? That vs customer satisfaction is easy to figure out. Any
circuit with even those slightly ambiguous results, change the high
value resistors.

I think you misunderstood what happened. I inserted those 100k resistors
myself as a modification to see if the DC voltages would stabilize. They
were not part of the original circuit. After I installed them, operation
returned to normal. When I changed the capacitors on that pc board to match
the values in the schematic, I removed the resistors just to see if the
circuit would work "as designed." (I put that in quotes because we never
really know if the schematic matches what comes off the production line.)
Without those resistors, the problem recurred. Therefore I reinstalled the
resistors and it's all good now.

I hope that made sense.
--
David Farber
Los Osos, CA

 

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-807vtn.pdf/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

 

[David Farber]

mikeberlak@gmail.com 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-807vtn.pdf/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

 

On Monday, May 22, 2017 at 7:40:15 PM UTC-4, David Farber wrote:

mikeberlak@gmail.com 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-807vtn.pdf/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

 

[David Farber]

makolber@yahoo.com wrote:

On Monday, May 22, 2017 at 7:40:15 PM UTC-4, David Farber wrote:
mikeberlak@gmail.com 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-807vtn.pdf/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

I agree with your conclusion about the design error but how did this
receiver ever work properly as initially designed? I imagine there would
have been a lot of returns!

Thanks for your reply.
--
David Farber
Los Osos, CA

 

"but how did this
receiver ever work properly as initially designed"

Having dealt with leakage on PC boards it might be that internal leakage in the IC pulled it to ground but with very high resistance.

Over years and exposure to humidity it picks up other leakage from traces on the PC board and throws off the bias.

Either that or there were high value resistors internal to the IC that have gone open over the years. the higher the value the more likely they are to open up. True of regular resistors, why not ones internal to an IC. I have actually had this on a TV once. Had to add a 1 meg to ground to fix it, or replace the IC. The 1 meg did just fine.

 

Hello as i observed the board i suspect the ic501 and ic 502 do you think thats the problem i baught those is from ebay

 

I think the ic 501 is bad

 

On Wednesday, May 2, 2018 at 1:17:49 PM UTC-4, jeffrey....@gmail.com wrote:
> I think the ic 501 is bad

make the modification i described in the previous post.
mark

 

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

Hi David. I know this thread is a bit old. How do you solder the 100k resistor to noninverting pins #3 and #5? Do you happen to have a picture of the actual solder of the resistor? Also on the resistor, what watt is the resistor? Thanks!

 

[John-Del]

On Monday, February 25, 2019 at 6:29:48 PM UTC-5, cpt...@gmail.com wrote:

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

Hi David. I know this thread is a bit old. How do you solder the 100k resistor to noninverting pins #3 and #5? Do you happen to have a picture of the actual solder of the resistor? Also on the resistor, what watt is the resistor? Thanks!

The wattage is unimportant. If you're using leaded resistors, use 1/8 watt and go from pins 3 and 5 to the nearest ground (like pin 4). What I would do is use a couple of smd resistors and tack them from pins 3 and 5 to pin 4 ground.

Did anyone try actually replacing those op-amps?