Question about the M70 bass/treble sliders

[koleloi]

Hi all, my new purchased M70 have problem with the bass and treble slider. They travel up and down smoothly but make no effect on the sound. I read few topics here and assuming that the problem is broken slider pots (missing/loosen feelers).

I look up on internet and found this potentiometer that looks very similar to the original pots. It has right dimension, looks almost the same from outside, and it even has mid point tap. Before I order them, I have few question need your guys help.



1. The bass/treble potentiometer on the M70 write A100k, while this one is B100K, will it makes much difference in sound? Assuming that the new one can fit on the board nicely.
2. Regarding the answer of the above question, would you suggest me to replace (swap) the entire pot, or is it better to only take advantage of the moving assembly (the feeler and handle). Still assuming that the inner part are interchangable.
3. Is the bass/treble pots on M70 and RC-838 the same, interchangable?

Much appriciated any help. Thank you.

 

[caution]

The loudness tap is useless for bass/treble adjustment. That's only for volume pots, where you're trying to simultaneously reduce the (boosted) bass as you turn up the volume.

A (audio/logarithmic) and B (linear) tapers are quite different, and you will struggle with accurate tone adjustment.

Normally, on an M70, the bass and treble sliders arrive at 15% levels if you turn it up halfway with their "A" taper.

If you replace with a "B" taper, you arrive at 15% levels when you turn up bass/treble sliders only 15% of the way!
(Linear = 1:1 ratio between position and level).

I'd suggest keeping your existing resistive track, if you are able to, in order to preserve the "A" taper.

Check out these graphs from Panasonic's slider pot datasheet.

 

[koleloi]

Thank you for your detail help Caution.

As your mention, I also think of using only the moving part from the pot I am going to purchase. But just in case the carbon strip on the pot was wear off, like I often see in rotaty pot, what is our best solution here? As I have realize after many hours of searching that a replacement spare slide pot with indentical specs (shape and resistance) are impossible to find. Thanks.

Also anyone can confim if bass/treble pots on M70 and RC-838 are the same. I am considering to buy 4pcs of the pot (quite expensive and high purchase fee as it come from oversea).

Thank you very much.

 

[docs]

I have repaired m70 broken traces with conductive paint successfully.
When you do it, it is better to do the same action to both left and right in order to create the same level of resistance.
The conductive paint is applied in layers after each layer has dried, I found that about 3 to 4 layers was enough to restore continuity but I would safely go with 5.
In my case, the disconnection was between the end point of the trace to the pin, and I have seen this quite a number of times to be the usual point of deterioration. You can test this with you dmm continuity test.

 

[koleloi]

I have repaired m70 broken traces with conductive paint successfully.
When you do it, it is better to do the same action to both left and right in order to create the same level of resistance.
The conductive paint is applied in layers after each layer has dried, I found that about 3 to 4 layers was enough to restore continuity but I would safely go with 5.
In my case, the disconnection was between the end point of the trace to the pin, and I have seen this quite a number of times to be the usual point of deterioration. You can test this with you dmm continuity test.

Thank you. I already purchased that pot. Things will be more specified once I get my time to open the box. And I think I will need further helps.

 

[koleloi]

Update on this topic:

I received the spare bass/treble pots then put them on volume board and they just worked perfectly. The bass/treble come out smoothly regardless of the Linear nature of the pots. I am so satified.




The next task is searching for volume pots. My question is that what is the use of the center pin (the pin marked by number 4 on last picture) on M70 volume pots? Seem very rare to see a potentiometer has that pin so just curious about its use and wondering if I can just ignore it.



Thank guys.

 

[caution]

Loudness taps (or midpoint taps or center taps) were common on stereo systems in the 70s and 80s. Since unfiltered audio has very weak bass at low volumes, a low pass RC filter is added, to boost it. As you turn the volume up, the resistance between the wiper and the loudness tap diminishes, to the point where by the time you arrive to 50% volume, the extra bass boost has disappeared, because you don't need it between 50% and 100% volume settings.

They probably still exist today in some products. Normally you would have a "loudness" button on your device, but most of the time it's just set to "on" internally. That's how most boomboxes are.

Now, if you replaced an M70 slider without the tap, you have two choices, both of which are not very good.
1. Leave the loudness filter disconnected:
No more extra bass on volumes under 50%; bass setting may not be enough anymore.
2. Hard-wire the loudness filter, 100% at all volumes:
Since there's no loudness tap anymore, the filter cannot gradually fade out as you turn up the volume, because you no longer have the resistive track on the potentiometer (between loudness tap and wiper). You will have tons of bass at all volumes - turning down the bass may not be enough.

 

[koleloi]

Loudness taps (or midpoint taps or center taps) were common on stereo systems in the 70s and 80s. Since unfiltered audio has very weak bass at low volumes, a low pass RC filter is added, to boost it. As you turn the volume up, the resistance between the wiper and the loudness tap diminishes, to the point where by the time you arrive to 50% volume, the extra bass boost has disappeared, because you don't need it between 50% and 100% volume settings.

They probably still exist today in some products. Normally you would have a "loudness" button on your device, but most of the time it's just set to "on" internally. That's how most boomboxes are.

Now, if you replaced an M70 slider without the tap, you have two choices, both of which are not very good.
1. Leave the loudness filter disconnected:
No more extra bass on volumes under 50%; bass setting may not be enough anymore.
2. Hard-wire the loudness filter, 100% at all volumes:
Since there's no loudness tap anymore, the filter cannot gradually fade out as you turn up the volume, because you no longer have the resistive track on the potentiometer (between loudness tap and wiper). You will have tons of bass at all volumes - turning down the bass may not be enough.

Thank you alot. It seem to be impossible to find replacement part for those Volume pots. Some mono box has nearly exact pots but without the tap.

 

[Superduper]

In my experience, it's rarely that the resistor board fails in a manner that can't be repaired. If the carbon traces have an issue, it is usually at the rivet. However, one time I did see a fracture in the actual resistor trace that was closer to the middle and repairing it did affect the balance between the 2 channels so you'd have to perform the exact same process on the opposite channel to "balance" them. As for the broken feelers, they are all the same whether on the mono (volume or record level) or stereo (bass/treble) sliders. I've always repaired/rebuilt them using spares that I cannibalize from original Alps NOS sliders from the same series but with different tonal characteristics. Unlike what you did, even though those NOS Alps sliders (different values) would be a direct drop in, I disassembled them instead to use the bits to fix the original sliders. Like Caution says, it's not that pots with different curves or even values can't be swapped in and work, rather it is that it will alter the original characteristics of the boombox. It is absolutely true that at the (mid point), the original 100k pot will show 15k/85k ohms balance whereas a linear taper pot will have a 50k/50k balance. If this was on the volume, the difference will be remarkable. However, because it is on the bass and treble, you may not notice how your boombox slides are sounding differently unless you A/B yours and an original one side by side. I definitely understand, however, how a working replacement can be a godsend, even if it doesn't perform exactly like it did. As long as it's working, it's far better than (not) working. As for your volume pots, no need to replace them if they are working. In a pinch, your bass/treble pots can be disassembled to reuse the feelers.

 

[koleloi]

In my experience, it's rarely that the resistor board fails in a manner that can't be repaired. If the carbon traces have an issue, it is usually at the rivet. However, one time I did see a fracture in the actual resistor trace that was closer to the middle and repairing it did affect the balance between the 2 channels so you'd have to perform the exact same process on the opposite channel to "balance" them. As for the broken feelers, they are all the same whether on the mono (volume or record level) or stereo (bass/treble) sliders. I've always repaired/rebuilt them using spares that I cannibalize from original Alps NOS sliders from the same series but with different tonal characteristics. Unlike what you did, even though those NOS Alps sliders (different values) would be a direct drop in, I disassembled them instead to use the bits to fix the original sliders. Like Caution says, it's not that pots with different curves or even values can't be swapped in and work, rather it is that it will alter the original characteristics of the boombox. It is absolutely true that at the (mid point), the original 100k pot will show 15k/85k ohms balance whereas a linear taper pot will have a 50k/50k balance. If this was on the volume, the difference will be remarkable. However, because it is on the bass and treble, you may not notice how your boombox slides are sounding differently unless you A/B yours and an original one side by side. I definitely understand, however, how a working replacement can be a godsend, even if it doesn't perform exactly like it did. As long as it's working, it's far better than (not) working. As for your volume pots, no need to replace them if they are working. In a pinch, your bass/treble pots can be disassembled to reuse the feelers.

Thank you for your detail explanation and suggestion.

Actually I was first trying to clean and fix my orginal bass/treble pots but it came out not working after all. I disassembled them, clean carefully with Isopropyl, the feelers are all still in nice condition so I though the prolem must be in the carbon trace. Then I put them back on the board but no luck. From the look, the carbon trace seem to be in good shape (I do not have a multimeter though).

Then I remove them from board again then put the new parts in. I am still keeping the orginal pots as I think they might be used as part somehow for my next M70.

As for the volume pots, they are still working but I am sure they fill eventually fail one day not too far, because it is very dusty here, and I am intending to use this M70 as my main listening box, because it is 220v and the tuner is so good. I am not very impressed by its sound. My Sanyo 9994 (220v too) sounds much better but It has some problem with tape deck that I can not fix.

Here are pics of my original pots carbon trace.

 

[Presto]

Anybody have and spare bass and treble pots for sale?…. Working or for rebuild

 

[koleloi]

Anybody have and spare bass and treble pots for sale?…. Working or for rebuild

I am selling many boombox parts/accessories in my local website, including those bass/treble pots that I just did replace on my M70. I am planning to open a store on Ebay but not familiar with selling oversea yet.

If you need the bass/treble pots for your M70, I can send you two for free. You only need to pay for Shipping which I am not aware of how much. But I guess it will not too much due to tiny weight. Fell free to contact me if you need them.

Here is my small store on our local commercial platform. They are only available inside South East Asia.

Linh Kiện Radio Cassette | VN

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www.lazada.vn

 

[Presto]

Very cool!!!…. Im in…. Let me know what you need
43 Windham Dr
Simsbury CT 06070
so you can calc shipping

thanks! Phil

 

[AE_Stereo]

Update on this topic:

I received the spare bass/treble pots then put them on volume board and they just worked perfectly. The bass/treble come out smoothly regardless of the Linear nature of the pots. I am so satified....

Would you kindly provide the source address for the Bass/Treble sliders that you bought?

 

[Superduper]

IF you are in the USA, I offer these NOS Alps sliders for $13.95 plus S/H. 3+ and I can include shipping. They are 50KWX2. I would recommend simply using the internal bits to rebuild your existing sliders to preserve the original characteristics. Physically, they are the same size as the original Alps sliders in the JVC M70 and 838. The knob shaft might need modification to fit the knobs depending on whether it is the M70 or RC-838. Look at photos and decide for yourselves.

 

[AE_Stereo]

Would you kindly provide the source address for the Bass/Treble sliders that you bought?

No worries, I got the info.
Pl. ignore this post.

 

[Presto]

Awesome!…. Do you have a pic of the internals of the nos sliders?
are the brushes the same?… I am afraid some of mine may be missing but I won’t know until i am deep into it.

 

[koleloi]

Awesome!…. Do you have a pic of the internals of the nos sliders?
are the brushes the same?… I am afraid some of mine may be missing but I won’t know until i am deep into it.

Everything is exactly the same, except for one thing: Characters of the resistance. One is Audio Type (original), One is Linear type. I ignore that and just simply swap the whole thing. Work beautifully.

 

[koleloi]

No worries, I got the info.
Pl. ignore this post.

Ok bro.

 

[Presto]

Cool …. I will take them.
one point the 2 center through hole leads? What are they for?