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

[Dancorp]

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.





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.goo.gl/xbHbbDasybICB7aG2

 

[Superduper]

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.

 

[Dancorp]

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 !

 

[Superduper]

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.

 

[Dancorp]

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.

 

[Superduper]

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.

 

[BoomboxLover48]

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!