Replacing Boss BX-8 volume pots with another value than original

[tohtorizorro]

EDIT:

I tested swapping the potentiometers both ways and couldn't hear such difference that would matter either way (the BX-series aren't exactly HIFI gear ). Although since both of the master pots only work at around 80%-100% range, the 50k -> 10k (master pot in channel) configuration couldn't be tested at low levels where the difference would probably be most pronounced. With 10k -> 50k (channel pots in master) everything seemed OK at least to my barbarian ears.


Hi!

I am trying to find fitting slide potentiometers for an old Boss BX-8 audio mixer. The only hit so far is a listing at Aliexpress for a lot of 10pcs of either 10k or 50k pots. Conveniently the mixer has 10 faders in total and I'd be happy to replace them all, but the eight channel faders are 50k while the two master faders are 10k (all with a log taper) and I wouldn't want to order any more than what is needed for the project. So, here's my actual question:

In such configuration (see attached excerpts of the schematic) how critical do you think the actual resistance of the potentiometer is? Could I use 10k instead of the 50k ones or vice versa and if so which way might be better ie. would you prefer replacing the 50k channel faders with 10k ones or the 10k master faders with 50k ones?

Here's a link to the full service manual:
https://www.synthxl.com/wp-content/uploads/2019/10/Boss-BX-8-16-Service-Manual.pdf

Here's the Aliexpress listing:
https://www.aliexpress.com/item/1005006285283014.html


Original potentiometers info:

Panasonic EWA-NK6 S10 A54 (A50k, channels)
Panasonic EWA-NK6 S10 A14 (A10k, master)

taper: audio / log
travel: 30mm
shaft: 10mm

W: 8mm
H: 7mm
L: 45mm

See attached photos for more info.
Note that the photo with dimensions and pinout marked has a different shaft than the originals, the plain photo of a pot is the actual master pot from the BX-8.




PS. My original intention was to dismantle the slide pots and clean them but unfortunately the carbon tracks are totally worn out in couple of them so I don't think there's much else to do than to replace them.

PPS. The pots at Aliexpress are stereo but the dimensions seem to match. I'm optimistic about the pin layout as well since in the (mono) pots in the mixer have only pins on one side connected.

thanks in advance,
Mikko

[Roehrenonkel]

Hi tohtorizorro,

make them all 50k, else you will mess up the tone-controll-network.

Good luck

[RoGeorge]

Should work with all 10 of 50k log, but I wonder why the original schematic is using a different value for master volume.  Usually the manufacturers prefer to have as few distinct items as possible in the BOM, so there must be a reason.  My best guess is the master volume is made with more rugged potentiometers (longer life), or maybe they have slightly different form factor than the other 8.

Unless the original potentiometers are defective, I wouldn't hurry replacing them.  Chances are the Aliexpress potentiometers will be of a lower quality.

In my experience, buying some Arduino, or some other devboard/chips from Aliexpress is OK, but mechanical items tend to be very bad quality.  For example, I've bough some mains power switches, used 2 out of 10 in different boxes and one already failed 3 times, and the other sometimes toggles unreliable.  Same for cables with alligators:  unusable alligators and iron wires instead of copper, very bad 3mm jack connectors, etc.

Make sure the seller is reputable, and don't replace the originals unless you have to.

[wraper]

None of them should be replaced without adjusting other component values. VR1 10k to 50k replacement is possible with increasing R2 resistance, however it will probably result in some performance downgrade.

[SeanB]

60l in the main volume will have a slightly different response, ans a little more attenuation when lowered, but should not really affect much, as you normally have enough excess gain, and run off VU display in any case, and thus 50k all round will be best. Order 20 though, and have spares for when others fail, or you have a noisy one, or somebody drops a drink on the console again.

[Nominal Animal]

For reference, the original slide potentiometers are Panasonic EWA NK6 S15 A14 (50kΩ audio taper) and EWA NK6 S10 A14 (10kΩ audio taper), both single-gang, but with a funky pinout.  Someone on eBay claims to have some new/old stock for the 50k ones, selling them for almost 24€ apiece.

[tohtorizorro]

Thanks a lot for the insights and effort for all!

Should work with all 10 of 50k log, but I wonder why the original schematic is using a different value for master volume.  Usually the manufacturers prefer to have as few distinct items as possible in the BOM, so there must be a reason.  My best guess is the master volume is made with more rugged potentiometers (longer life), or maybe they have slightly different form factor than the other 8.

Unless the original potentiometers are defective, I wouldn't hurry replacing them.  Chances are the Aliexpress potentiometers will be of a lower quality.

As far as I can tell, the only difference is the resistance value, the pots are clearly of the same series (same form, dimensions and pin layout). Both master pots are worn out beyond repair, along with channels 1 & 3. This makes sense since often the first channels get used more than the last and there's a good chance that ch2 is on it's way too. Anyways it wouldn't be too far fetched to suppose that the first channels have seen about the same amount of use as the master so there seems to be no difference in the longevity either. Thus, I'd guess there might be a functional reason for the differing values and very interested in understanding what it might be!

I see your point and share your experiences of Aliexpress quality. However the listing is the only one I've found selling log pots with the right foot print so it's either those or downgrading the mixer quite heavily.

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[Nominal Animal]

However the listing is the only one I've found selling log pots with the right foot print so it's either those or downgrading the mixer quite heavily.

Can you tell us the exact dimensions of the body of the slide pots (case width, case height, case depth; slider height above the case; maximum slide travel)?

You see, another option would be to create replacement 50k audio taper slide pots, using one or two layers of PCB material (made at JLCPCB, PCBWay, etc; they're cheap!).  At the bottom, you'd have the Panasonic EWA-NK6S15A14 or EWA-NK6S10A14 footprint, but at the top a standard slide pot footprint, say Bourns Pro Audio PTA3043-2015CIB503 (50kΩ audio taper, 15.70€ for 10 at Mouser) or PTA3043-2015DPA103 (10kΩ audio taper, 1.79€ apiece at Mouser), which are 9mm wide, 5.7mm tall, 43.5mm long, with a 30mm slide travel.  Two layers of PCB material would lift the slide pot by about 3mm, so would make the replacement slider 9mm tall (9mm × 9mm × 43.5mm body) with the slide tab extending 15mm above the case).   With just one layer, the slider body about 7.5mm tall, but there is a small risk the pins might overlap; two layers would make it easier and safer, and allow perfect alignment.

If that would work, all that would be needed is a careful measurement of the relative locations of the holes in the PCB from the bottom side, so the footprint can be replicated in KiCAD or EasyEDA; and measurements of the EWA slider bodies, so that the alignment can be ensured to be correct.

[themadhippy]

This makes sense since often the first channels get used more than the last

Really ? in the days of analouge  it was almost industry standard to have the  channels that need tweaked the most closest to the masters or on larger desks the center section.

[tohtorizorro]

This makes sense since often the first channels get used more than the last

Really ? in the days of analouge  it was almost industry standard to have the  channels that need tweaked the most closest to the masters or on larger desks the center section.

I might be wrong but the pattern on this unit seems to suggest so. Also BX-8 is not a public address console but a compact line-level mixer, commonly used by synth people and the like, in such use cases I'd personally start from ch1 but could be the exception of course, obviously room for further studies exists! 

[tohtorizorro]

For reference, the original slide potentiometers are Panasonic EWA NK6 S15 A14 (50kΩ audio taper) and EWA NK6 S10 A14 (10kΩ audio taper), both single-gang, but with a funky pinout.  Someone on eBay claims to have some new/old stock for the 50k ones, selling them for almost 24€ apiece.

Thanks! I think there's a slight mishap with your model numbers, according to the service manual the pots are:

EWA-NK6 S10 A54 (A50k, master)
EWA-NK6 S10 A14 (A10k, channel)

it seems that the last three characters stand for taper, resistance in kOhms and 10^x multiplier. The two digits before them could be the shaft length since the eBay item you probably refer to is listed as EWA NK6 S15 A14 and it seems to have a longer shaft than the ones in BX-8 which are about 10mm. Anyways, they could be fitted perfectly with a little filing etc. but sadly the price is ridiculous. I think you can get a BX-8 under 250€/USD delivered.

I might still contact the guy if there was any room for negotiation when ordering more than one.

[tohtorizorro]

However the listing is the only one I've found selling log pots with the right foot print so it's either those or downgrading the mixer quite heavily.

Can you tell us the exact dimensions of the body of the slide pots (case width, case height, case depth; slider height above the case; maximum slide travel)?

You see, another option would be to create replacement 50k audio taper slide pots, using one or two layers of PCB material (made at JLCPCB, PCBWay, etc; they're cheap!).  At the bottom, you'd have the Panasonic EWA-NK6S15A14 or EWA-NK6S10A14 footprint, but at the top a standard slide pot footprint, say Bourns Pro Audio PTA3043-2015CIB503 (50kΩ audio taper, 15.70€ for 10 at Mouser) or PTA3043-2015DPA103 (10kΩ audio taper, 1.79€ apiece at Mouser), which are 9mm wide, 5.7mm tall, 43.5mm long, with a 30mm slide travel.  Two layers of PCB material would lift the slide pot by about 3mm, so would make the replacement slider 9mm tall (9mm × 9mm × 43.5mm body) with the slide tab extending 15mm above the case).   With just one layer, the slider body about 7.5mm tall, but there is a small risk the pins might overlap; two layers would make it easier and safer, and allow perfect alignment.

If that would work, all that would be needed is a careful measurement of the relative locations of the holes in the PCB from the bottom side, so the footprint can be replicated in KiCAD or EasyEDA; and measurements of the EWA slider bodies, so that the alignment can be ensured to be correct.

This sounds interesting, I added the pot information in the original post! I'll examine the space restrictions in the chassis and PCB and report back.