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

[David Farber]

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

[jurb...@gmail.com]

I think you should resolder the board. When shit like this happens in one channel and then the other independently it is almost not going to be the power supply, which is usually the first logical guess. But in this case with both channels intermittent I think it has bad connections.

They may look good, especially with lead free solder, but the reason that you experience this the way you described could be due to thermal issues.

I am so disgusted with lead free solder that I quit. Think about that. I almost refuse to work on anything with it. It is too unpredictable and also it has corrosion issues you cannot see. Resolder it with 63/37 and cook it. I mean COOK IT. But look for the connections that might be possible for the problem, do not do all the connections. In fact if you have a floating iron you can do it while it is running and find the bad ones more easily. However, do not expect simple resoldering to fix it. You have to take all that lead free shit off and clean the connections, which means the board as well as the pins or whatever, and use the 63/37 on it.

If you do not want a recall that is.

[dansabr...@yahoo.com]

Isolate the problem first. Does the distortion change with a change in volume? I would guess that there is a transistor in the front end that is starting to fail because of heat. It doesn't take much.

You have an advantage here, one channel works as expected. Take some voltage measurements and compare left to right channels. Use a scope and see where the distortion starts.

IC431 (4580) might be a suspect. Follow the audio signal in both channels.

Let us know how you make out.

Dan

[Bruce Esquibel]

jurb...@gmail.com wrote:
> I think you should resolder the board. When shit like this happens in one
> channel and then the other independently it is almost not going to be the
> power supply, which is usually the first logical guess. But in this case
> with both channels intermittent I think it has bad connections.

> They may look good, especially with lead free solder, but the reason that
> you experience this the way you described could be due to thermal issues.

Hmmm, but isn't that receiver from the late 80's, early 90's?

Not sure the "lead free solder" problems would apply.

-bruce
b...@ripco.com

[Phil Allison]

Bruce Esquibel wrote:

>
> Hmmm, but isn't that receiver from the late 80's, early 90's?
>
> Not sure the "lead free solder" problems would apply.
>

** The JVC RX807V is a thoroughly modern, home theatre, multi channel nightmare.

Pb free solder is 100% guaranteed.

But it ain't all that bad.

IME, most Pb free stuff is far from impossible to deal with.

A bit of 60/40 on top of any dodgy looking joint works wonders ..




... Phil

[David Farber]

The date on the service manual is 1992 and the solder looks very shiny and
easy to work with. I would say that it is not lead-free solder.

Thanks for your reply.

[mako...@yahoo.com]

>
> What is the function of the JFET, Q553?

It looks like an audio switch that connects the L and R channels together in some mode controlled by the Mode 2 signal, for some reason. Is there a mono mode?

I'd disconnect this first to troubleshoot the other problem, it may be related or not.

Once you have the two channels isolated from each other, it may be easier to trace the distortion problem.

Mark

[mako...@yahoo.com]

also looks like a design error to me re Q551 and Q552. It is hard to follow the way they are drawing upside down from how we normally see this circuit, but there should be a pullup resistor on Q552 like 10K from base to emmitter to ensure Q552 is cutoff when Q551 is off. There may be enough leakage when it gets warm that Q552 starts to turn on when it should not. That may be the cause of the distortion?

Mark

[David Farber]

There is no mono mode that I can see. I do not have the remote control.
Maybe it could be activated in certain surround modes? (This receiver has
Pro Logic surround sound.)

Are you saying I should just remove the JFET, Q553, to isolate left from
right and see what happens?

Thanks for your reply.

[David Farber]

Hi Dan,

I powered it on again and after about 15 minutes, both channels were
distorted. The signal going in and out of IC431 is fine. Adjusting the
volume does not affect the distortion. The top half of the wave is clipped.
I did notice that both dual op-amps, IC431 and IC503 are inline 4580's. I
swapped them to see if that made a difference but the distortion stayed the
same after warm up at IC503. Inverting input pins 2 and 6 were distorted
while noninverting pins 3 and 5 were not distorted.

Thanks for your reply.

[Trevor Wilson]

On 17/03/2017 10:20 PM, Phil Allison wrote:
> Bruce Esquibel wrote:
>
>>
>> Hmmm, but isn't that receiver from the late 80's, early 90's?
>>
>> Not sure the "lead free solder" problems would apply.
>>
>
> ** The JVC RX807V is a thoroughly modern, home theatre, multi channel nightmare.

**Nah, it's ancient history in the world of home cinema. A nightmare?
Certainly. Horrible thing. Only good for use as a door-stop. Hell, I
doubt you can find a TV, Blu Ray or DVD to connect it to anymore.
Composite only. Absolute POS. If I recall correctly, it has no removable
base-plate, making service a very lengthy and certainly non-viable
option. Why anyone would contemplate repairing one of these things is
beyond me. It was not even a high end model with decent audio output
stages.

>
> Pb free solder is 100% guaranteed.

**Nah. I doubt it. Too old.

>
> But it ain't all that bad.
>
> IME, most Pb free stuff is far from impossible to deal with.
>
> A bit of 60/40 on top of any dodgy looking joint works wonders ..

**Sure does. Luckily, proper solder is still easy enough to buy.


--
Trevor Wilson
www.rageaudio.com.au

[ohg...@gmail.com]

Have you tried preheating the assy and see if the distortion appears more quickly or even immediately? A good percentage of timed issues are also thermal ones.

[dansabr...@yahoo.com]

Actually, use freeze spray. spray one IC at a time and give it a few seconds to react. You should be able to isolate the offending IC that way. Works for transistors too.

[Bruce Esquibel]

David Farber <farberbe...@aol.com> wrote:

> I'm servicing a JVC RX-807V receiver.

You know, there was something about that model number that bugged me and
finally realized there is a similar one in the junk pile.

It's a model RX-905VTN, I'm sure it's in the same generation but not the
same series. I'm pretty sure it was one of those, if you could fix it let me
know, if not use it for parts. That was like 20 years ago.

Anyway, in case the problem with yours ends up being some weird obsolete
jvc-only IC or something else similar, feel free to ask for the boards out
of the thing. It's complete and actually looks brand new but would probably
be more to ship whole anywhere than it's worth.

I didn't see a service manual anywhere to see if it's similar but I can take
some snaps with the cover off to see if any of the circuit boards are
similar.

-bruce
b...@ripco.com

[David Farber]

Hi Bruce,

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.

The board in question is ENC-093 version C. It's a plug-in board as many of
the circuit boards are.

Thanks for your reply.

[mako...@yahoo.com]

>
> 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

[David Farber]

I was wondering the same thing! I will check and report back to you.

[David Farber]

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番 values to 10番 values. I will replace those with
the original values.

Thanks to everyone for their suggestions.

[mako...@yahoo.com]

wow a design error then.

glad i could help

:-)

Mark

[David Farber]

>> 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.

[David Farber]

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.

[jurb...@gmail.com]

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]

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.

[mikeb...@gmail.com]

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]

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.

[mako...@yahoo.com]

appears to be an actual design error
m

[David Farber]

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.

[jurb...@gmail.com]

>"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.

[jeffrey....@gmail.com]

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

[jeffrey....@gmail.com]

I think the ic 501 is bad

[mako...@yahoo.com]

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

[cpt...@gmail.com]

>
> 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]

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?