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JVC M70


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#1 BoomboxLover48

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Posted 03 December 2010 - 11:40 PM

I have a JVC M70 that got one channel is very weak and the other not strong enough either. When I increase the volume on the channel that is weak it got no base and teh volume slightly icreases to half of its way and then remains at the same level. Is this an issue with components in the the amplifier stage or is a problem with the pre amplifier stage?

I cleaned the sliding volume controller pots stereo/mono switches with no luck. Is this a common problem with JVC M70? I also like to change the belts. I looks very tricky to open and access the belt area. If I can pull out the whole chassis things would have been much easier. I removed most of the screws holding the black plastic chassis and all corners are free but the centre are is stuck. I don't know which screw I missed to pull out the whole chassis leaving the speakers attached to the front section.

I like to get a service manual for M70. Let me know from where I can obtain one.



Please advice

#2 Superduper

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Posted 03 December 2010 - 11:53 PM

I immediately suspect that the tonal control board is shot (the sliders are the bane of this model) and needs rebuilding. The sliders themselves are no longer available, from any source, so they need to be removed, disassembled, and painstainkenly restored. If you think the belts are hard to do, then I suspect rebuilding the tonal board is probably not a diy'er type job for you.

#3 BoomboxLover48

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Posted 04 December 2010 - 12:16 AM

Hi Super,

Thanks for your replly. So the slide control needs to be restored right? Do you have any suggestions on how to do it?

The belt replacement is pretty easy and I can do it without much issues. I am least worried about it.

Thanks again!

#4 baddboybill

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Posted 04 December 2010 - 12:22 AM

:hi: Here is a service manual from the archives ;-) Hope it will be more helpful but if i can make a suggestion and start small... Like Superduper says rebuilding the controls will be hell.. You may be better off trying to find a donor box or parts box with working parts :-D Good luck
http://www.mediafire...5w2ol8iuuzmv8uu

#5 BoomboxLover48

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Posted 04 December 2010 - 12:38 AM

Hi baddboybill,

Thanks a million for the manual!

I will take your advice! I will look for getting the sliding control (VC) for the M70. I was wondering how I would get the resistance strip with the right value to fit in that tiny slot, and restore it. Hope I will find one soon!

Thanks again!

#6 Superduper

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Posted 04 December 2010 - 12:51 AM

Can't really give you an enumerated procedure. You either know how to do it or not. But basically, this is the board we are talking about (if that is what's bad in your case). Basically, remove the board, remove the sliders, disassemble the sliders, rebuild the broken pieces. Check the resistor board for proper continuity. Repair any broken traces on the board. Replace the capacitors on the board. Then reassemble.

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Oh, and the resistor strip? There is NO replacement. If it's broken and no longer salvageable, then you'll need to find a parts box and hope it's rebuildable. That is one of THE items on the board that needs to be salvaged and repaired during the rebuild. It has a unique tonal response curve that is not standard so even if you found a replacement that will physically fit, it won't perform the same. The original parts were discontinued long ago. So if it's bad then you'll need to try to repair it. Also, the parts that's almost always broken are the feelers. The original Alps design was terrible, and they break off pretty easily -- even on new ones that have never been installed..

#7 BoomboxLover48

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Posted 04 December 2010 - 02:03 AM

Hi Superduper,

I am so grateful for all the detailed help you guys give here. Wow!
:-D

Initially I thought the problem could be due to aging parts in the amplifier section. I am so glad to know it is the sliders causing the issue.

I have a Panasonic RX5350 that also got the same issue. The left is not as strong as the right channel for bass but not that noticeable at higher levels and reamains almost the same at 1/2 the volume. That one is a rotary type of pot.

I am pretty good at working with delicate parts. This will be a very interesting project to work on. Now with the help of having the service manual I can carefully remove the board and take the sliders out, open it up and rebuild broken pieces.

:-)

Hats off to you all!

God Bless!

#8 baddboybill

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Posted 04 December 2010 - 02:09 AM

I just noticed your from chi town :thumbsup: so where abouts :huh: I am around 30 miles south of Joliet :cool:

#9 Superduper

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Posted 04 December 2010 - 02:13 AM

Well, let's not get ahead of ourselves. Lots of things can go wrong and I'm just going by the symptoms you provided. Again, it could be any number of things but bad sliders are very common on that model. Bad amps are not a weak link on the M70. In fact, I have heard of very few failures in that dept and the M70's amp is robust indeed, especially if the speakers are kept at 8 ohms and not tinkered with and/or replaced.

The setup of the RX-5350 is entirely different so do not presume you have the same issue. I have not heard that to be a problem with the RX-5350 but I certainly won't exclude that as a possibility. Although the volume control is as you say a rotary control, the tonal controls however are indeed sliders in this model as well. You may not recognize it as such but the 5-band equalizer is basically a 5-band tone control compared to 2-band tone control on those with bass/treble. You will note that the RX-5350 does not have bass/treble controls and all tonal tailoring is done by those 5 equalizers.

#10 2steppa

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Posted 04 December 2010 - 03:27 AM

My M70 had (has) the same issue, I was able to restore one slider but success with the other has eluded me so far!
It wasn't too bad a job, just worked methodically and with care and patience. (And surprisingly little of the F and C words!) :-P

#11 BoomboxLover48

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Posted 04 December 2010 - 08:54 AM

I just noticed your from chi town :thumbsup: so where abouts :huh: I am around 30 miles south of Joliet :cool:



Hi Bill,

I live very close to O'Hare area.

#12 BoomboxLover48

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Posted 04 December 2010 - 10:30 AM

My M70 had (has) the same issue, I was able to restore one slider but success with the other has eluded me so far!
It wasn't too bad a job, just worked methodically and with care and patience. (And surprisingly little of the F and C words!) :-P


There is some bass on the right side, but the volume is 1/2 way powerful and no full power. The left channel slowly increase volume till half way with no bass and after we reach 1/2 the way, there is no effect at all if you slide it to full. The treble is excellent on both channels. Was this the same issue with your M70 too? :huh:
Just wanted to make a little more clear with the problem I have with the M70. :sad:

#13 BoomboxLover48

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Posted 08 December 2010 - 04:55 PM

Hi,

I was wondering if anyone has a JVC M70 to sell to me for parts. I love to fix the problem with my M70.

I could see JVC Floyd helped one guy with a VC potentiometer problem with Sharp 777.

Please help! :-)

Thanks

#14 BoomboxLover48

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Posted 15 December 2010 - 10:59 PM

Well, let's not get ahead of ourselves. Lots of things can go wrong and I'm just going by the symptoms you provided. Again, it could be any number of things but bad sliders are very common on that model. Bad amps are not a weak link on the M70. In fact, I have heard of very few failures in that dept and the M70's amp is robust indeed, especially if the speakers are kept at 8 ohms and not tinkered with and/or replaced.

The setup of the RX-5350 is entirely different so do not presume you have the same issue. I have not heard that to be a problem with the RX-5350 but I certainly won't exclude that as a possibility. Although the volume control is as you say a rotary control, the tonal controls however are indeed sliders in this model as well. You may not recognize it as such but the 5-band equalizer is basically a 5-band tone control compared to 2-band tone control on those with bass/treble. You will note that the RX-5350 does not have bass/treble controls and all tonal tailoring is done by those 5 equalizers.



Hi Super,
You are so right! This evening I took out both the sliding pots and when I opened up to check the pot with the 1/2 volume issue, I could see that the carbon resistance strip had a hairline crack right above the 1/2 volume (center tap). When I checked the resistance from the center tap to one of the end connectors on the carbon strip I was getting a resistance value of ~24 K ohms. When I checked it from the center tap to the other end, it was found open. I slowly glided the DMM probe “very gently” (taking care not to scrape off the carbon) over the carbon strip I found the place where it was cracked; beyond that point there was no resistance reading at all.

If I use conductive paint at the crack will it join it? Just wondering how I can fix that crack on the carbon resistance strip.

Now the other sliding VC resistor looks good to me. It gave me a 50K ohms value when I checked. I was wondering why that channel volume is not maxing out. I would say it is around 2/3rd of the M70 channel volume. I didn't open this slider control yet.

Your suggestions are very much appreciated.

Thanks again

#15 Superduper

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Posted 16 December 2010 - 12:01 AM

When they start to go, it's like an avalanche. Just FYI, the volume, bass and treble all affect the sound. You already took one apart. You need to take them all out, clean them one at a time, then test them individually. A failure in any one of those pots will have an affect on the tonal quality and sound output level. Also, I never saw a resistor board with a break in the carbon. Always at the connecting strip near the ends. There are compounds to bridge the break. There are flexible carbon based ones, designed more for repairing rubber buttons like those in car remotes. Then there are acrylic based silver compounds that are more durable. But none of them is going to be very durable with the feelers rubbing them all the time.

#16 BoomboxLover48

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Posted 16 December 2010 - 11:57 AM

When they start to go, it's like an avalanche. Just FYI, the volume, bass and treble all affect the sound. You already took one apart. You need to take them all out, clean them one at a time, then test them individually. A failure in any one of those pots will have an affect on the tonal quality and sound output level. Also, I never saw a resistor board with a break in the carbon. Always at the connecting strip near the ends. There are compounds to bridge the break. There are flexible carbon based ones, designed more for repairing rubber buttons like those in car remotes. Then there are acrylic based silver compounds that are more durable. But none of them is going to be very durable with the feelers rubbing them all the time.



Hi Superduper,

Thanks again for your valuable suggestions and help! You are the MVP here! :thumbsup:

As a desperate attempt I will try using the silver conductive paint to fill the hairline crack. I don't know if it will even get inside the ~0.2mm cleavage. The particle size of the silver dust looks much bigger. I hate to make a "V" cut to accomodate more paint there. I cannot think of a way to electrochemically deposit copper or silver there at the crack and level it off without making a bump. This is a real challenge! The feeler tips looks not that agressive and they still have the curved and smooth stylus with good spring action. :hmmm:

#17 Superduper

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Posted 16 December 2010 - 01:42 PM

The silver paint, especially the ones with pen tip makes pretty wide swaths (approx 1/16" at least) so use sparingly. If too wide, you will diminish the resistive value of the board. Then it will sound weird as the volume increase normally, then all of a sudden have a lull in the response curve until it starts up again.

There are 2 competing products out there that I know of. One manufactured by Caig (the makers of deoxit) and one by MG chemicals. It's my experience that the MG chemicals one is a better product (costs twice as much) as the acrylic finish seems more durable.

#18 BoomboxLover48

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Posted 16 December 2010 - 10:49 PM

The silver paint, especially the ones with pen tip makes pretty wide swaths (approx 1/16" at least) so use sparingly. If too wide, you will diminish the resistive value of the board. Then it will sound weird as the volume increase normally, then all of a sudden have a lull in the response curve until it starts up again.

There are 2 competing products out there that I know of. One manufactured by Caig (the makers of deoxit) and one by MG chemicals. It's my experience that the MG chemicals one is a better product (costs twice as much) as the acrylic finish seems more durable.




Hi Superduper,
The conductive silver paint I have was bought from FRYS for $22.50 (2 ounce). They put code number on it and I couldn't peel and look what brand it was. I can get that information from thier website soon.

http://www.frys.com/... ... IN_RSLT_PG

Actually the metal wafer strip coming close to the carbon was cut off. So I put a little bit of the silver paint just to touch the carbon strip end from the wafer strip area. That did the job. I am getting 49.1 Kohms on the one was getting volume to 1/2(left channel). The other one (right channel) that was not that powerful was reading 63 Kohms before I opened it. This might be the reason for the right channel not getting the full volume, right? One side from the center tap was reading higher. So I could see that somehow there is some extra resistance buld up somewhere. I put a little bit of the conductive paint at the end part where the wafer contact film meets the carbon disc. Now the total resistance reads 46.5 Kohms, which is sligtly off by 3.5 Kohms. I might have made a little overlap over the carbon strip. I will wait till tomorrow evening to fully cure the paint. I am just giving additional time for full cure.

Do you think the resistance difference is within the tolerence limits? Will this pot resistance difference cause any other issues? Please advice. :hmmm:

#19 Superduper

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Posted 16 December 2010 - 11:09 PM

It's a 50k ohm pot, so I think it's OK. There might be a slight difference in volume between left/right but I don't think it's going to be noticable at all, certainly not when both speakers are working at the same time. I trust you cleaned off the entire board with soap, water, then follow up with isopropyl before the treatment? The break at the ends between the rivet and the carbon strip is where I normally see breaks.

As for overlapping -- yep. If you overlap too much, then it can certainly affect the resistance. I think the conductive silver is rated at something like .2 ohms/mm. The carbon would be much higher.

#20 BoomboxLover48

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Posted 17 December 2010 - 07:11 AM

It's a 50k ohm pot, so I think it's OK. There might be a slight difference in volume between left/right but I don't think it's going to be noticable at all, certainly not when both speakers are working at the same time. I trust you cleaned off the entire board with soap, water, then follow up with isopropyl before the treatment? The break at the ends between the rivet and the carbon strip is where I normally see breaks.

As for overlapping -- yep. If you overlap too much, then it can certainly affect the resistance. I think the conductive silver is rated at something like .2 ohms/mm. The carbon would be much higher.



Hi Superduper,

Wow! Thanks again for your valuable advice and help! Greatly appreciated! I used a cotton swab and cleaned it with isopropyl alcohol. What kind of treatment I should give to the contacts? Deoxit 5?

I was wondering where I can get the belt of M70. I knew a guy before who sells it and he is out of business now.

#21 baddboybill

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Posted 17 December 2010 - 08:07 AM

It's a 50k ohm pot, so I think it's OK. There might be a slight difference in volume between left/right but I don't think it's going to be noticable at all, certainly not when both speakers are working at the same time. I trust you cleaned off the entire board with soap, water, then follow up with isopropyl before the treatment? The break at the ends between the rivet and the carbon strip is where I normally see breaks.

As for overlapping -- yep. If you overlap too much, then it can certainly affect the resistance. I think the conductive silver is rated at something like .2 ohms/mm. The carbon would be much higher.



Hi Superduper,

Wow! Thanks again for your valuable advice and help! Greatly appreciated! I used a cotton swab and cleaned it with isopropyl alcohol. What kind of treatment I should give to the contacts? Deoxit 5?

I was wondering where I can get the belt of M70. I knew a guy before who sells it and he is out of business now.

best belt supplier central... :-D
http://www.studiosoundelectronics.com/
frm 9.6
scy 7.0 :-D

#22 BoomboxLover48

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Posted 17 December 2010 - 11:00 AM

It's a 50k ohm pot, so I think it's OK. There might be a slight difference in volume between left/right but I don't think it's going to be noticable at all, certainly not when both speakers are working at the same time. I trust you cleaned off the entire board with soap, water, then follow up with isopropyl before the treatment? The break at the ends between the rivet and the carbon strip is where I normally see breaks.

As for overlapping -- yep. If you overlap too much, then it can certainly affect the resistance. I think the conductive silver is rated at something like .2 ohms/mm. The carbon would be much higher.



Hi Superduper,

Wow! Thanks again for your valuable advice and help! Greatly appreciated! I used a cotton swab and cleaned it with isopropyl alcohol. What kind of treatment I should give to the contacts? Deoxit 5?

I was wondering where I can get the belt of M70. I knew a guy before who sells it and he is out of business now.

best belt supplier central... :-D
http://www.studiosoundelectronics.com/
frm 9.6
scy 7.0 :-D



Hi baddboybill,

You guys are awesome! :thumbsup: God Bless You!

#23 BoomboxLover48

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Posted 17 December 2010 - 11:52 PM

It's a 50k ohm pot, so I think it's OK. There might be a slight difference in volume between left/right but I don't think it's going to be noticable at all, certainly not when both speakers are working at the same time. I trust you cleaned off the entire board with soap, water, then follow up with isopropyl before the treatment? The break at the ends between the rivet and the carbon strip is where I normally see breaks.

As for overlapping -- yep. If you overlap too much, then it can certainly affect the resistance. I think the conductive silver is rated at something like .2 ohms/mm. The carbon would be much higher.




Hi Superduper,

Wow! Thanks again for your valuable advice and help! Greatly appreciated! I used a cotton swab and cleaned it with isopropyl alcohol. What kind of treatment I should give to the contacts? Deoxit 5?

I was wondering where I can get the belt of M70. I knew a guy before who sells it and he is out of business now.




Hi Superduper and BadboyBill,

FIXED IT
:thumbsup:

Thank you all and the others who helped me here. Just put the tone control board back and the M70 ROCKS! :thumbsup: :-D :breakdance:

God Bless You all!

Without your help I would have never figured out a way to fix it. My initial guess was a weak amp.

Wow! It is 11:50PM and i am dancing with joy! I now got a beautiful M70 that you guys helped me to fix it.

A million thanks to you all! I will post pictures later. :breakdance:

#24 Superduper

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Posted 17 December 2010 - 11:59 PM

A million thanks to you all! I will post pictures later. :breakdance:


A million bucks would be better. check ur PM for my Paypal address. :lol: :yes: ;-)

#25 BoomboxLover48

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Posted 18 December 2010 - 01:26 AM

A million thanks to you all! I will post pictures later. :breakdance:


A million bucks would be better. check ur PM for my Paypal address. :lol: :yes: ;-)



:breakdance: :thumbsup: :surf: :lol:

You guys are awesome!

I am still awake listening to the M70. I do have other boomboxes like Panasonic RX7200, RX5350, RX5700, RX7000, Pioneers, JVC, Akai and so on...but this M70 is very unique! The radio reception and tone quality is amazing!

#26 baddboybill

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Posted 18 December 2010 - 01:37 PM

It's a 50k ohm pot, so I think it's OK. There might be a slight difference in volume between left/right but I don't think it's going to be noticable at all, certainly not when both speakers are working at the same time. I trust you cleaned off the entire board with soap, water, then follow up with isopropyl before the treatment? The break at the ends between the rivet and the carbon strip is where I normally see breaks.

As for overlapping -- yep. If you overlap too much, then it can certainly affect the resistance. I think the conductive silver is rated at something like .2 ohms/mm. The carbon would be much higher.



Hi Superduper,

Wow! Thanks again for your valuable advice and help! Greatly appreciated! I used a cotton swab and cleaned it with isopropyl alcohol. What kind of treatment I should give to the contacts? Deoxit 5?

I was wondering where I can get the belt of M70. I knew a guy before who sells it and he is out of business now.




Hi Superduper and BadboyBill,

FIXED IT
:thumbsup:

Thank you all and the others who helped me here. Just put the tone control board back and the M70 ROCKS! :thumbsup: :-D :breakdance:

God Bless You all!

Without your help I would have never fugured out a way to fix it. My initial guess was a weak amp.

Wow! It is 11:50PM and i am dancing with joy! I now got a beautiful M70 that you guys helped me to fix it.

A million thanks to you all! I will post pictures later. :breakdance:

No problem :thumbsup: Glad to hear its 100% :-D :w00t: :w00t:

#27 Alf

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Posted 20 December 2010 - 09:59 AM

I have extra alps sliders available for parts if needed. :) I'm in the middle of a move but I'll see if I can dig em up. :)

#28 Superduper

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Posted 20 December 2010 - 01:11 PM

Were there the sliders that were dead out of the box, and needed immediate rebuilding Alf?

#29 Alf

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Posted 20 December 2010 - 01:48 PM

I have the set of alps I purchased off of ebay that time. If I remember correctly, we were unable to determine if they had the same tonal curve of the oem M70 sliders, but the rest of the parts are good. I.e. the sticks with the feelers, etc.

#30 Superduper

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Posted 20 December 2010 - 02:06 PM

OK. I thought I recalled you installed one but the problems persisted. Maybe I recalled wrong.