Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems.

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Old March 15th 17, 10:52 PM posted to sci.electronics.repair
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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



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Old March 17th 17, 01:19 AM posted to sci.electronics.repair
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Default JVC RX-807V receiver. One channel distorted when SEA is enabled.

I think you should resolder the board. When **** 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 **** 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.
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Old March 17th 17, 03:33 AM posted to sci.electronics.repair
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Default JVC RX-807V receiver. One channel distorted when SEA is enabled.

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
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Old March 17th 17, 12:20 PM posted to sci.electronics.repair
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Default JVC RX-807V receiver. One channel distorted when SEA is enabled.

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


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Old March 17th 17, 06:32 PM posted to sci.electronics.repair
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Default JVC RX-807V receiver. One channel distorted when SEA is enabled.



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
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Old March 17th 17, 06:40 PM posted to sci.electronics.repair
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Default JVC RX-807V receiver. One channel distorted when SEA is enabled.

On Friday, March 17, 2017 at 1:32:20 PM UTC-4, wrote:

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


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

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