Distortion in R channel: JVC JR-S100

HAZE33

New Member
Hey guys, I hope this is the right section for this... I'm having issues with an older (I was told '80s) JVC stereo receiver.

It sounded great for a while, and then the right channel sounded like the speaker was blown. However, the speakers sound fine on another receiver, and I've tried other wires too.

It's basically a weak, fuzzy, distorted sound. It doesn't get any better or worse throughout the volume range. And only the right channel.

So I sprayed contact cleaner in the volume sliders, eq sliders, speaker selector switch, etc. No change. Speaker protect fuses are not blown, no obvious signs of failed components on the board.

I was told there are speaker protect relays that sometimes fail, but I don't see any relays in this receiver. I'm hoping someone here will have some experience with this receiver, or just some general troubleshooting tips. I still haven't been able to find a free service manual for it. Guess I should probably just buy it...

Thanks in advance
 
I'd look near where the speakers hook up, look for bad soldering on that board by having the unit on with the cover off (don't touch anything), tap lightly using the back plastic handle of a screwdriver. Start near the output terminals with , just lightly. If no change work over where the output transistors are, (the part of the board where the heatsink is near). My first instinct when i read this was an open emitter resistor (usually four or eight white ceramic components near the heatsink and output transistors, it could be one of the transistors blown, but that usually takes out the line fuse.

If you're afraid to open the amp, just lightly drop the amp (like a quarter of an inch) when it's on and if it starts working than you could rule out any component failures, it would be a bad solder connection, or bad connection.


Is it possible that you connected 4 ohm speakers to an 8 ohm amp, even the best of us can make this mistake once or twice. That would stress the amp and blow the weakest channel first. Also make sure it has the right fuse in it and not some foil:no:. I work on amps for a living and have seen this alot, the fuse is there for a reason.

Hope you get it working.:thmbsp:

Keith
 
Thanks for the suggestions Keith!

I've traced the speaker wires inside the chassis and made sure there are no breaks or bad solder joints. Furthermore, the fuzzy sound comes out the right channel on both sets of speaker outputs.

I've got 8 ohm speakers connected to it, although it is 4 ohm stable.

How would you suggest I test the emitter resistors? Should they all have similar voltage readings on either side? See if one is drastically different and replace if necessary?

I'll take some pictures of the board and post them if that would help with the diagnosis.

Thanks again
 
This JVC is from 1979. If you open the machine and watch the transformer, you will see two daughter boards. One board has the main fuses. Another board is next to it, it is mounted vertically.
This vertical board has the components that manage the speaker relay. On this board you will also find the speaker relay. You can open it and try to clean it, but i'd suggest spend a few bucks and buy a new one.
 
Hopefully he resolved the problem. He hasn't posted in over 5 years.

Hope so too ;-). But these messages can be found through search engines, so even if he dumped his machine we can still try to answer the question for another person with the same problem.
 
You should first use Deoxit (or something like that) to clean all the switches and potentiometers (volume etc). Inject the fluid into the controls and wiggle them vigorously. Potentiometers will either be open or have a small gap you can inject the fluid into. Let it dry for some hours and try again. Also try if the problem exists on all inputs and all speaker outputs. But often this type of problem indicates an issue in the hardware, likely a sudden death of one of the small driver transistors. Either that or a smaller capacitor has failed. Or both in a cascade Sometimes solder joints can be a problem but usually that would either cause a full audio drop out or a dropout of specific inputs. If the sound is good on headphones the problem is in the end stage amplifier.
 
Okay, So I sprayed all 4 sliders with contact cleaner, I also did the "Drop fix" mentioned earlier. Then I flexed the whole unit by lifting each corner about an inch one at a time. that seems to have fixed the issue, hopefully it will remain stable. I do have another issue though. The Treble has to be maxed out to sound any good. Are there any common reasons for the treble to sound low?

BTW thanks for the help! The sliders would crackle a lot before I sprayed them.
 
Drop fixing will only cause more damage to the fragile bonds of the soldering. Perhaps it has fixed the issue for now, it will return sooner or later as that 'fix' implies that the soldering has to be retouched. The reason for the treble function's misbehaviour can be manifold. It might be your ears and your speakers, the treble circuit is a bog common one in this machine. If that is not the case, bad solder joints are again a suspect; might be the pot itself or the surrounding components. Also by spraying the potentiometer's insides (don't know what you used) it might have thrown the resistance off track, it can happen with some sprays. Wait a few days until the spray has dried and perhaps it will clear up. It's difficult to pinpoint an exact solution, these are old machines and generally they are like ticking time bombs.
 
Gotcha, man that is spot-on! The "Right channel" issue reappeared today, unfortunately dropping or flexing didn't resolve this time. I guess i must hunt down the bad joint somehow. And the issue with the treble seems the same today, but I'll wait it out. FYI I'm using MotoMaster Electrical Contact Cleaner, as it's what I have. I should probably get some deoxit, but it feels silly spending $35 on a spray that might not even fix the issue, and when the amp is only worth ~$75.

Here is a video of the issue:
https://drive.google.com/file/d/0B_wNLSjX3rW5a0w2a3Z2MFpERndSalZZZXZrdGYweEZLOGI0/view

first you can hear the clean hiss of white noise in the left channel, then there is a buzz when I increase the volume in the right channel. Maybe this will provide insight into the issue...

Thanks again for your help. I'd appreciate any more advice. otherwise, I'll attempt to locate the bad joint.
 
Hissing / buzzing all implicate a failing transistor or capacitor, but that does not have to be the case. Other than that it is neigh impossible to tell you what the exact problem is, these old machines have a tendency to create a problem in one place that actually originates from somewhere else. If you have a signal tracer, you can see if you can find the spot(s) where the signal goes off. You can DIY one of these for a few dollars using a cheap mini amplifier from China. I believe there are topics on Audiokarma to help you along (it's easy).

Another thing you can do is to desolder the equalizer pcb. It is where the sliders are on (right). You need to dismantle the front to do this. Afterwards you can desolder the sliding potentiometers and clean them; if you are handy you can open them and clean them using some isopropanol (alchohol) with a cotton swab. Be careful not to destroy the track, don't over clean it. There are some capacitors on there, low esr. You can change them to film if you want, they are low value.

To retrace solder tracks you need to have a secure hand, a high wattage solder iron with a small tip. Don't use a standard round tip. Don't use that solder iron your grandfather gave you. No honestly, a cheapo modern station by some Chinese company will be your best friend. Use a scalpel or chisel tip. Before you do any soldering, clean the pcb. Use a toothpick to gently scratch away the old solder flux. It's the brown hard stuff. Use alcohol to clean afterwards. It will probably leave a white residu, you can use a swab with detergent to clean it and then a again with some denaturated water. Make sure it is clean and dry; you can use a hairdryer but not too hot and not too long. For retracing / resoldering, add a small bit of flux to the tracks and solder joints, wet your solder iron with some solder, then solder it. You may need to use a tiny bit of extra solder. Sometimes it is easier to desolder the joint and then resolder it. Patience is your friend because if you solder too long you will damage these old pcbs really, really easy.

Afterwards, reassemble and put it on a dim bulb if you have one. If you don't fingers crossed, hands on the off button. If you see smoke or something blows, turn it off because you've done something wrong.

Good luck ;-)
 
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