M-70 channel operation guide or help requested.

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majestic

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Jul 12, 2009
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HEY to all you M-70 owners. While in stereo mode do both of your speakers move equally or does one channel move more than the other with Bass...I've noticed that my right speaker does not produce the bass that the left channel does. Only when I switch it to Mono do they respond the same to bass.....I notice that actually it appears that in Stereo mode the left channel is producing mid to low and the right channel is producing mid to high somewhat......It still sounds great but would one suspension wear out before the other????/

ANY thoughts :hmmm:
 

Superduper

Member (SA)
One thing about JVC's that I hate, at least on the models with sliders is that they use Alps sliders that are just flat out crappy. They fall apart internally. I had a b**tch of a job on an RC-838 where 4 of the 5 sliders (not counting the volume) were bad, and by bad, I mean fell apart inside.

So had to search high, searched low and finally found someone with some NOS Alps sliders, same type except different tonal curve and value. I had to remove the old sliders, carefully disassembled them, took the broken slide and replaced them keeping the old carbon resistor base. Well, what bugged me was that on the new ones, one was already broken (same way) brand new out the box. This tells me it failed not due to wear but POOR design. I reinforced mine with superglue and finally fixed it but it was not fun.

Not sayin that your M70 has the same problem but I would not be suprised at all. Mine wouldn't respond to cleaning and I finally decided to take the sliders apart after all indications were that the problem (dead channel) was in the tonal control circuits and lead to the balance control. After fixing that, I checked the non-responsive tonal controls just in case and sure enough discovered that the same thing was occuring in those as well.

The problem I'm talking about is that the "feelers" that slide and contacts the resistor breaks off internally. No contact between feelers and resistors mean no change in tone. In the balance control, no feelers mean one dead channel (or both, if both are broken).
 

majestic

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Jul 12, 2009
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Thanks for the reply's guys....... I would have observed the speaker movement when new as a base for comparison but I have never owned one of these so I was not sure if this was normal. I know they move together when in MONO but differ in Stereo. I was not sure if the stereo signal was redirected during playback like when you use Matrix on some speaker models. This box is still loud and bumps to my pleasure so I will probably enjoy it as long as it lives........

Thanks Super for techno side of things. I hope to have the open door later If I have any questions.......

Rock on............................... :thumbsup:
 

Superduper

Member (SA)
I do get rather long winded in my responses but with respect to mono, stereo, and wide modes:

Stereo has 2 distinctly seperate signal paths from input to the power amp.

Mono either takes the signal from one side only and feeds it into both amps or takes both channels and mixes them (same phase) together feeding the combined signal to the amplifier.

Wide mode usually has 2 separate signals (like in stereo) but mixes the signals either just before the power amps or into a seperate input of the amp module that is designed specifically for wide/spatial effect. The way the spatial effect is introduced is accomplished by playing true stereo AND feeding / mixing the signal from the other stereo channel (criss-cross) slightly out of phase giving a slight echo effect.

As to how the tonal controls figure into this -- these controls are dual channel controls meaning that a single slide actually simultaneously controls both channels together but seperately. (Did that make sense?) What I was trying to convey in my previous post was that if one side of the dual slide control broke, then one side would continue to adjust the tone while the other channel (broken) would have no effect since there is no continuity.
 

majestic

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Jul 12, 2009
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:hmmm: ........So potentially I have a slider giving me crap........Well if it is I will have to suck it up since,

A. I do not have another slider control and it appears they are not easy to obtain and

B. I most likely would not be able to rebuild one without losing the whole thing...

At least it plays.... :thumbsup:
 

2steppa

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May 11, 2009
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Got the same problem on my M70, first the right now the left slider (vol) has ceased to operate :-/

Got a strip down planned soon to investigate that and other faults... :hmmm:
 

Superduper

Member (SA)
2steppa said:
Got the same problem on my M70, first the right now the left slider (vol) has ceased to operate :-/

Got a strip down planned soon to investigate that and other faults... :hmmm:
Oh NO! I hope you get it fixed 2steppa. I know that these parts are obsolete. Let me just say I tried, really tried hard to find a solution to my slider issues. Finally got a hold of JVC and they confirmed that those parts are NLA. That was when I started thinking about opening up the controls and rebuilding those instead.

Might be time to start searching for a parts box to salvage parts from. Good news is that M70's are plenty in pretty junky, non/partially - functional condition. Bad news is that even crappy ones will fetch a pretty penny.
 

2steppa

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May 11, 2009
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I hope I can get it sorted too, SuperDuper.....

I'll have a crack at 'fixing' the sliders but I must say I have strong doubts as to my chances of success, as you say these sliders are likely suffering from more than just normal wear, probably the wiper has broken up rather than the usual worn/dirty resistive track... :sad:

Having heard it working once in both channels and been blown away by the sound, my resolve is pretty strong to fix it up and get it singing again. Then I can worry about the tape wow & flutter issue and the cosmetics.

Worst case scenario I'll junk it and write it off as a bad job and then seek out a better example of this legendery machine.
 

analogboi

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Jul 16, 2009
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You can try headphones (or external speakers) if you think one of the M70's speakers is bad...

But, I tend to agree that the potentiometer may be funk'd up in some way. :annoyed:

I also notice I have a "favorite spot" for the bass and treble sliders that does not move when I change styles of music. You might want to experiment with fixed resistors to create the same potentiometer position. You could then leave the slider intact "just for looks."

Don't rule out a cracked solder joint either. Remelt the solder where the slider pot attaches to the PCB.

:beer2:
 

majestic

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Jul 12, 2009
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Thanks all........And everyone that has added the concerns on the boxes they own. It helps me realize that it is more widespread than mine.....I'm the type of person that if there are other options I'll use them. If there were a way to use rotaries then I'd use them when the time came......The box sounds to good to abandon. Even if I had to make a custom plate to do it.....I'm just sayin......


If there is anyone else with issues please add them...

Thanks
 

majestic

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Jul 12, 2009
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Hey Super....I think your right on the slider. I disassembled the Box tonight to replaced a missing handle screw and of coarse I had to remove the top plate......Well After removing and reinstalling the slider knob I played some music and noticed that both speakers moved equally to the beat......Most likely when I pried off the knobs it may have moved the feelers enough to make better contact.........Life is good.....Real Good............ :yes: :cool:
 

majestic

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Jul 12, 2009
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2steppa said:
Got the same problem on my M70, first the right now the left slider (vol) has ceased to operate :-/

Got a strip down planned soon to investigate that and other faults... :hmmm:

Well you know Steppa, when I first plugged in my box I knew one channel did not play as I purcahsed that way.....It was the right channel. The left was pure static....I thought right away it was the volume slider. I spent one night ohming my wires and checking the circuits and connectors....

As you read I found when I plugged in my headphones I had both channels loud and smooth. I then focused between the main connector from the channel PCB and the headphone jack where I discovered the broken solder points....

If you plug in your headphones and you hear music fine out of both channels then you know your close.....Does not hurt to try and you do not have to take apart your box.....
 

Superduper

Member (SA)
A lot of people don't know this but in many boomboxes, the speaker cables go through the headphone jacks. A switch inside the jack disconnects the speakers and pipes the sound to the headphones. If used extensively, it is possible that the spring inside the headphone jacks lose their tension and there is no continuity to the speakers.

Also, I've seen cases where stuff (coffee, solvent, who knows) got into the headphone jack area and rusted or corroded it out. From the outside, everything looks cool because plastic doesn't rust. But the jack breaks the connection to the speakers due to the corrossion. In cases like these, working a cleaner in there MIGHT restore continuity. However, the jack really should be changed.
 

2steppa

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May 11, 2009
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I'll try the headphone jack suggestion, although there's definite crackle and moments of sound cutting in and out when I move the sliders, but it never stays on and seems to be worse every time I go to it.
Plus, the other side of the sliders gives a healthy hum over each each channel when you touch it, suggesting that the jack is in the correct configuration without headphones inserted.
 

Superduper

Member (SA)
2Steppa. No no, I don't think your situation is related to the headphones. The headphone thing was just a response to Majestics experience with his.

Sounds to me like your slider has crapped out. If that is the case, STOP playing with the sliders and verify continuity using a DMM. The reason is that on my slider, the feelers actually broke off and was loose inside the control. If you can recover those feelers, then you can attempt to superglue it back. Otherwise, continually working the control could cause the loose copper feelers to grind off the fragile carbon layer inside the resistor making it even harder, if not impossible, to repair. They sell carbon restoration coatings but you may never be able to reproduce the electrical characteristics of the original resistor which may affect your ability to achieve equal volume on both sides. Unfortunately, I was out of luck and had to search out some similar replacements to disassemble for parts because on several, the feelers were completely gone -- nowhere to be found. If yours just went out and you never had your unit upside down, the feelers are probably still there.
 

2steppa

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May 11, 2009
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Aaahhhh, OK Supe.....

I haven't really worked the sliders that hard (honest!) but I agree it's best to just leave them until the teardown and then fingers crossed there may be at least a slender chance of effecting a repair.

Here's hoping :-/ ;-)
 

Eric

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May 7, 2009
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Just to throw this out there and give you all some more to think about.....I have found the tape radio line in selector swtich to be the culprit in a few of these M70's. It can produce alot of strange issues including low volume no volume crackling volume and other strange behavior. It is best to desolder the switch, take it completely apart and clean the contacts. This too has very small sliders that make contact with each of the connections and over time and they just seem to gum up. deoxit does help but I have found the best way is to take them apart and clean them. It can be tricky putting them back together......just some more to think about. :huh:
 
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