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[FIXED] JVC M70 - Right Speaker Whispering (Unable to cutoff)


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#1 Dancorp

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Posted 15 April 2017 - 02:49 AM

Hi everyone,

 

In advance sorry for the english, i'm just a poor frenchy :)

I restore my first boombox thanks to this amazing board.

Oxeded lever buttons, power switch, source switch... most of issues have been solved.

I still got one:  i'm unable to turn off right speaker, it still whispering.

I'm also unable to fine increase the volume at start. it jump with a loud sound.

Lever have been cleared many times with a french elec. contact cleaner.

It sound like a ground problem, (when a touch the lever, i got like small impulsion in the speaker) I have already check solders without success

 

A video of the problem :

 

 

Thanks in advance for your help

 

Dan

 

EDIT : Fixed ! https://boomboxery.c...ff/#entry349597

 

 

 

 

 



#2 baddboybill

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Posted 15 April 2017 - 05:27 AM

Most likely you need the volume sliders rebuilt

#3 Fatdog

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Posted 15 April 2017 - 07:57 AM

Most likely you need the volume sliders rebuilt

I agree with Bill.  :yes:

 

Welcome aboard, Dancorp!



#4 Dancorp

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Posted 16 April 2017 - 02:46 AM

Thanks a lot for your help, I will try to disassemble it.

#5 Dancorp

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Posted 16 April 2017 - 02:48 AM

Double post

#6 baddboybill

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Posted 18 April 2017 - 10:30 AM

Thanks a lot for your help, I will try to disassemble it.


Your best bet is to message Superduper. (Norm) as he restores the slider controls. ;-) not sure if hes to busy but it dont hurt to ask

#7 Dancorp

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Posted 18 April 2017 - 01:47 PM

I think i will try to PM him... i have disassembled and cleaned it without success.



#8 Dancorp

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Posted 18 April 2017 - 03:07 PM

Before PM Superdumper, i have a question.

Is that possible to swith a volume slider with one used for the recording level ?



#9 Superduper

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Posted 18 April 2017 - 04:47 PM

Is that possible to swith a volume slider with one used for the recording level ?

 

No, not compatible.  Record level pot is 50k ohm "A" curve and no center tap.  Volume pot is 550k ohm, "B" curve with center tap.



#10 Dancorp

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Posted 19 October 2017 - 03:30 AM

After a little break, i come back here to find a solution.

I have some question about slides pot.

- Is that possible to use a 500k pot instead of a 550k one ?

- What's the Volume pot size ?

In fact, is that possible to fit this kind of pot :

https://www.aliexpre...2767612039.html

 

Thanks in advance for your help :)

Dan



#11 Superduper

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Posted 19 October 2017 - 09:37 AM

500 vs 550 should be ok. BUT..... The pins are in wrong position, the taper is different & the shaft is not the same so you won’t be able fit the original knobs.

#12 Dancorp

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Posted 19 October 2017 - 04:01 PM

i have to disassemble my M70 to get sliders sizes, pins localisation, the shaft... to check if i would be able to fit 3th party sliders on my boombox.
Of course that will be not easy to find the same shape or to fit it, but originals pots are not available anymore. this solution must be study.


#13 caution

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Posted 19 October 2017 - 04:30 PM

When I was looking for M70 sliders, I did find some that had the B curve and a center tap, but either had different dimensions or pin locations, so even in the best situation, you will have to glue the pot into position and jumper the pins, and maybe also add a riser so it still sits at the right height.

 

As far as I know, no other boxes used the M70's sliders, unlike the M90 pot, which Toshiba and Panasonic (and maybe others) used on many models.



#14 Dancorp

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Posted 20 October 2017 - 04:51 AM

Thanks a lot for your comments guys. Finding a well sized pot with all features looks close to be impossible.

I can expand search results by removing features...

Can someone explain the purpose of the midpoint ?Is this feature mandatory or can i fit a legacy pot with jumpers?

Is the B curve mandatory too ? if i found a A curve pot, will the lever still friendly usable ?

Thanks in advance

 

Dan



#15 Dancorp

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Posted 15 November 2017 - 03:10 PM

What do you think about these pots ?

A little bit larger, without midpot, but...

http://s.aliexpress.com/7fmuUnQb



#16 caution

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Posted 15 November 2017 - 10:05 PM

Nope

Needs to be 50K and have a loudness tap.



#17 Dancorp

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Posted 16 November 2017 - 03:18 AM

Volume sliders are 550k.

Is the loudness tap the famous mid tap mentionned by superduper ?



#18 caution

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Posted 16 November 2017 - 03:43 AM

Nope. 50K.

38454971841_be74525b16_o.jpg

37568455175_cd9b81ef71_o.jpg



#19 Dancorp

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Posted 16 November 2017 - 05:09 AM

You're right, I do not remember where I read 500K.
It is easier to find 50K potentiometer, it's cool.
But I have to find some who have the loudness tap.

 

Size : 45mm x 8mm with a 15mm shaft.



#20 caution

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Posted 16 November 2017 - 09:29 AM

Good luck finding one, nobody else has. You'd be better off finding a parts unit and scavenging from it.



#21 Dancorp

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Posted 06 February 2018 - 03:54 PM

FIXED !

In advance sorry for my english
 
I propose you today my system D to repair a volume pot from M70.
Of course, it will have to be unsoldered from the volume board, and dissassembled.
 
On the diagram below, you must measure a resistance of 50kOhms between points A and D.
In my case, this resistance was infinite because the resistive band was cut between B and C. A simple multimeter allows you to find the cut-off zone.

image_imagesia-com_1fqic_small.png
 
 

For information, this cut causes the volume to whisper. just unplug the volume board from the boombox to have both channels whispering.
 
The location of this cut is not surprising, it is the minimum volume position that was oxidized, and it ended up tearing off the tape.
 
My idea was the following: connect point B and D with a small metal wire (cut on a component) and invert the potentiometer on the volume circuit so that the area repaired corresponds to the maximum volume.
That's why I simply and delicately twisted some pins to tip them on the right side.
And it works perfectly!
In addition, the metal wire blocks the potentiometer properly just before the end.
 
I don't know if this trick will save a lot of volume pots, but I wanted to share it with you:)
 

 
Somes pics :https://photos.app.g...HbbDasybICB7aG2



#22 Superduper

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Posted 06 February 2018 - 08:39 PM

Just for your information, those pots are polarized.  If you look at the side of the pot, you will see a number 1 stamped on one corner.  That corresponds to pin #1.  Also, on the tone board, there is a #1 marked which tells you which way to orient the pot.  The reason that the pot is polarized is because the taper is not linear.  It is logarithmic.  So if you invert the pot, then the characteristics change.  If you don't believe me, check the resistance with DMM.  A 50k pot wil read ~50k from end-end of the resistor, but from wiper at center location, it will read about 45k to one of the pins and 5k to the other pin.  That means that at the midpoint of the pot measured to end,  will measure about 10% of the total resistance and to the other direction of the pot has 90% of the resistance.  Will it work if you reverse it?  Yes.  Will it work the same?  No.



#23 Dancorp

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Posted 07 February 2018 - 01:37 AM

 

Just for your information, those pots are polarized.  If you look at the side of the pot, you will see a number 1 stamped on one corner.  That corresponds to pin #1.  Also, on the tone board, there is a #1 marked which tells you which way to orient the pot.  The reason that the pot is polarized is because the taper is not linear.  It is logarithmic.  So if you invert the pot, then the characteristics change.  If you don't believe me, check the resistance with DMM.  A 50k pot wil read ~50k from end-end of the resistor, but from wiper at center location, it will read about 45k to one of the pins and 5k to the other pin.  That means that at the midpoint of the pot measured to end,  will measure about 10% of the total resistance and to the other direction of the pot has 90% of the resistance.  Will it work if you reverse it?  Yes.  Will it work the same?  No.

Your remark is quite interesting, but in the case of volume pots, it's a linear curve. (B50K > B Curved)
The resistance between each pin and the middle pot is about 25kOhm.
Of course they are not perfectly linear, but will it work the same? Almost !

 



#24 Superduper

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Posted 07 February 2018 - 02:29 AM

It’s linear? Well I’ll admit that I presumed that the volume control would be A-taper for audio. If indeed linear, then I suppose it will work for you. I think I must be getting confused with the stereo pots which are the ones that most frequently fail (bass/treble).  You found a solution that works for you and sometimes, we must improvise although it is still better to retain the original orientation if possible.  That's because when you reverse the slider, the center slide assembly is also reversed.  The feeler tips are probably not perfectly centered so if one is installed in one direction and the other the opposite direction, then the center position might possibly be a little "off"? between the L and R.  The other reason is that the slide feelers have a push and pull direction.  Perhaps it doesn't make that much difference but frankly, I just like to stick with and not second guess the orig design.



#25 Dancorp

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Posted 07 February 2018 - 03:16 AM

I agree with you

However, this is a simple method to bring back to life  a component that is very hard to find.
If someone has a more elaborate method, using graphite powder or something like that, I will be happy to test it.



#26 Superduper

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Posted 07 February 2018 - 03:42 AM

Ok, I took a closer look at what you did.  It's probably too late now but I have done repairs before using carbon (graphite) resistor board repair compound.  That would probably have been better because it will not inhibit slider movement.  I don't think you will do that because you already know about the repair to your slider, but someone not familiar with why the slider doesn't have full travel might try to force the slide (thinking it was stuck) and maybe break the feelers or center slide block.  Also over time, if that wire repair blocks the slide assembly, and it keeps hitting it, will probably cause future problems.  If it's possible to undo that wire bridge, maybe you can try the other method, although I can't say how durable the repair will be since it depends upon how well the compound wears.  One last thing.  That slide is missing a pretty large portion of the strip.  It must affect the total resistance (lower right?) when you bridge it.  If you use carbon trace repair, you can control how much resistance by the thickness of the application to better match original value.



#27 BoomboxLover48

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Posted 08 February 2018 - 02:00 PM

Years back I had the same issue with a nice looking M70 and Superduper and Badboybill were of great help with good advises and suggestions. Great site to get help from fellow members! 

I used a conductive paint at the broken (tiny cleavage) carbon strip. It works like a charm!