JVC 838-major problems

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teamstress

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Aug 24, 2009
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chicopee, ma, usa
Here's the problems: right channel cuts in and out and sounds very garbled,static-y also both the bass and treble controls do nothing. :sad: So I'm thinking bad sliders right? I think the function switch isn't working too well either. I've tried the deoxit route a bunch of times, it hasn't helped. I've looked at the sliders and I'm not up to disassembling them(not brave enough).
So I'm looking for suggestions on what to do. I'm amazed that it's actually working at all. The metal was very corroded and there seemed to be beach sand inside! I'm listen to it as I type this.
I'll probably take it to my local repair guy and see if he can help it.
 

Superduper

Member (SA)
Yep. Sounds like it needs new sliders. Bad news is -- NLA from JVC. No suggestions except to disassemble the sliders and rebuild them. Problem is that inside the sliders are little feelers. Those sliders are made by ALPS corp and were very poor quality. The feelers that bridge 2 contacts usually break off -- sometimes they are still in there and you might be able to somehow glue them back if they aren't mangled too bad by the back/forth operation. Oftentimes, they are missing entirely and it's virtually impossible to fabricate those feelers. The easiest solution then is to buy new ALPs sliders (if you can find them) and sacrifice them for parts. You can not use the NOS Alps sliders because these JVC's use a very HTF tonal curve that I doubt you'll locate. That's why it's not a simple swap and you'll need to find a sacrificial lamb. The tonal curve is built into the resistor board which is what you'll need to keep.

The problem you are having is very common with these JVC's, although I suspect a lot of folks that have this problem don't even know it. :w00t: :lol:
 

Superduper

Member (SA)
I had one. Ended up having to rebuild the balance, bass, treble and both mic level sliders -- all bad. Thank goodness the vol slider was OK. Also, had to fix the power switch, and rebuild the broken power switch linkage. Then had to replace the cassette belts, then had to replace the FM demodulator module. Still ended up with 2 of the SW bands silent but decided not to spend any more time on it since I don't listen to SW, ever. Then eventually sold it.
 

2steppa

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May 11, 2009
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These are a real pig - my two vol sliders on my M70 failed and I managed to repair one of them successfully but the other has an actual physical problem with the centre pin not actually connecting to the carbon track, seemingly for no apparent reason. I can only assume there's some kind of tiny fracture in the actual layer somewhere. :-/
 

Superduper

Member (SA)
2steppa said:
These are a real pig - my two vol sliders on my M70 failed and I managed to repair one of them successfully but the other has an actual physical problem with the centre pin not actually connecting to the carbon track, seemingly for no apparent reason. I can only assume there's some kind of tiny fracture in the actual layer somewhere. :-/
Interesting 2-stepper. I've fixed just exactly that problem for a few members, and on my own boxes as well. I don't want to do another 838, but the M70 -- that's too important a boombox to ignore.
 

2steppa

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May 11, 2009
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Interesting SD.... the wiper contact issue was fairly straightforward, but this other problem seems really odd. The centre pin comes up into what looks like an SMT resistor or some kind of tiny block. One (good) slider shows that the centre pin must show conductivity to this 'block' whereas the other shows open circuit. This seems to explain why theres no variation in resistance even though the other parts of the assembly are fine. Can't see how or if this can be fixed hence it's (still) sitting at my workplace in bits. :-/
 

Superduper

Member (SA)
There is no SMT or any other specially integrated component in the slider assembly. It's a simple straightforward slider variable resistor with a center tap, and the center pin merely is a midpoint tie of the carbon resistor. Are you getting connectivity at both ends of the carbon resistor? If you are but the center tap shows no connectivity with either end of the resistor, then the center tap has an open connection. If the resistor is not getting connectivity from end to end, then it's possible the carbon track either has a fracture or is worn through in the middle breaking connectivity with the center tap and the the two ends of the resistor. Also verify that the broken connection is not the pin itself. The pin should show connectivity with the center tie block. The "fix" all depends upon exactly which scenario you are experiencing.

My best guess is that the center "tie block" has fractured it's connection with the two resistors on each end of the center terminal. Verify this with a VOM using the tips of the probe along the resistor strip (carefully to prevent scoring it) to find the actual fracture. Probing the resistor strip between ANY 2 places of the strip using the 2m ohm scale of your meter should yield connectivity. Once you've found the break, use a trace pen or other trace repair compound to repair it. The only thing is -- you can expect the "repaired" slider to wear much faster than before since the brass wipers are going to wear the repair much faster than the original resistor material which is actual quite hard and durable compound. My suggestion: keep the volume at a nice comfortable level and don't goof with it too much once repaired. Those metal case tabs are easily stressed and the tabs WILL break off after 2 or so disassemblies so you can't repair the thing ad infinitum.

--------------------->EDIT:
 

2steppa

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May 11, 2009
628
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Ahh, OK, I wasn't sure if that block was an actual component or just a fix/tie point as you have clarified. Thanks.

I'll have to examine the whole assembly again in more detail when I have more time. I can't remember exactly which points I measured the resisitance at but they were the same two pins that showed a variation when the wiper was moved on the other (good) volume slider. This one showed no change whatsoever. The track *looks* good at first glance but that is no guarantee of it's integrity obviously. I will persevere as I'd like this box to at least work even if it's not in the best cosmetic shape.
 

Superduper

Member (SA)
In every case that I've seen where there is a break in continuity, there was no visible or obvious fracture. Mostly, I've found the break to occur where the pin rivet contacts the resistor carbon track. I've fixed it with silver trace pen or silver epoxy. ONLY fix the break. If you spread too much, you could change the resistance value of the slider and the left/right side might not play even anymore.

If you are measuring the slider assembled, then make sure you take the measurements from the metal strip to either end of the resistor strip. If you are measuring from end to end of the resistor strip, then no matter where the slider is, resistance will not change.
 

JVC Floyd

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May 6, 2009
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Maryland USA.
I HAVE A PARTS BOX fixer upper not great looking but mostly works might need the controls sprayed aside /cassette i think plays too fast but works and the door has a busted hinge ,i'll have to have a few days to fully check it out but i will report with pics as soon as i get the time if you are interested let me know oh and the tuner works.
 

decks&cassettes

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Mar 2, 2017
179
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north of toronto ontario
try electrical contact cleaner on a cotton swab worked for my bass and treble sliders seems they were just super dirty after i cleaned them i moved them up and down a bunch waited a while to make sure the cleaner had evaporated completely then gave it a go and it seemed to work good
i just followed a local repair guys instructions its worth a try
 
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