Rheostat Lug doesn't conduct.Drill out Lug, or Scrape mask from Trace? Rheostat Difficult to replace.

Thread Starter

roarkh

Joined Apr 3, 2020
9
Keyboard Sustain Pedal is not working because the [default] Right hand Lug on the Rheostat does not conduct.
[After several tests,including continuity] [Rheostat is for partial pedalling on MP11 Keyboard, Sustain Pedal,Model F-30 ]

Rheostat is labelled"203B"

NOTE: The Rheostat Ground lug & the left hand lug are both conducting! [But the left hand lug won't work for this style Keyboard]

There is visible,but very tight to access to: 3 very thin Pale Green Traces,3/32 in long to each of the lugs,
but no surrounding dark green solder mask, on a chip of brown Bread Board, ½ in square.
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The Rheostat looks difficult to replace as it's connected to a series of gears & I would have a lot disassembling to reach it .

Before replacing the Rheostat should I :

1] Drill out the right hand lug to get access to the trace & pad & make an alternate connection
I would guess there's a pad,but I don't know if there's solder joint, or copper coated hole -- and Drilling could damage all of these.

2] Scrape off the green mask layer of the very thin Pale Green Trace--but this could damage the copper Trace!
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How would you suggest I proceed?

In your experience is it likely there's a pad ,solder joint, or copper coated hole underneath the lug?


Sorry these are the best photos this camera can make!
 

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Thread Starter

roarkh

Joined Apr 3, 2020
9
Addendum: original poster, roarkh: I have tried 2x applying a solder iron to the RIGHT terminal eyelet in hopes of resoldering the joint,but no luck!

I have tried using a very small vicegrips to hold the the RIGHT terminal eyelet tightly against the board & then testing for proper function,but no luck!
 

drc_567

Joined Dec 29, 2008
844
... Have you tried a spray of two of electrical contact cleaner? If the problem is an accumulation of dirt or debris, the spray may wash it away if it is accessible. While spraying, operate the pedal several times. ... Allow a few minutes to completely dry. Don't use the WD-## brand. Purchase Electrical Contact Cleaner'. ... No guarantee, but worth a try.
 

Thread Starter

roarkh

Joined Apr 3, 2020
9
... Have you tried a spray of two of electrical contact cleaner? If the problem is an accumulation of dirt or debris, the spray may wash it away if it is accessible. While spraying, operate the pedal several times. ... Allow a few minutes to completely dry. Don't use the WD-## brand. Purchase Electrical Contact Cleaner'. ... No guarantee, but worth a try.
Thanks,Maybe I'm wrong,but I don't see how dirt could be the problem: the rheostat works fine with the Left lug & the Ground connected --I also checked this arrangement using the other style of keyboard, which uses the Left Lug.
 

Ohmlandia

Joined Mar 2, 2020
32
The advice by drc 567 is good! The contact cleaner should be directed to the interior of what you are calling a rheostat (these are more commonly called potentiometers, especially if all three terminals are wired). Attach the fine tube supplied with the spraycan to its nozzle, and then spray a small amount inside the body of the potentiometer, while at the same time operating the pedal up and down.
 

drc_567

Joined Dec 29, 2008
844
... Another wild guess ...
There is a readily available copper 'weave type' material sold as Solder Wick. It is sold in various widths, in small rolls. The possible fix would be to use a short, thin piece of the Solder Wick to reinforce the trace going to the faulty lug. A few drops of liquid rosin solder flux should improve the flow qualities of the heated solder.
 

Thread Starter

roarkh

Joined Apr 3, 2020
9
The problem started when I was adding a switch to the pedal : I accidentally bumped the Right Lug & bent the tip up, 90 degrees. [I also just noticed today that there's a tiny bit of play between the Lug & the board.]That's why I tried resoldering the joint by heating up the Eyelet from the outside. And I also tried to hold the the Lug Eyelet tightly against the board [with small vicegrips] to test if that solved the problem.
 

Thread Starter

roarkh

Joined Apr 3, 2020
9
... Another wild guess ...
There is a readily available copper 'weave type' material sold as Solder Wick. It is sold in various widths, in small rolls. The possible fix would be to use a short, thin piece of the Solder Wick to reinforce the trace going to the faulty lug. A few drops of liquid rosin solder flux should improve the flow qualities of the heated solder.
Thanks!--that's good advice.
 

Thread Starter

roarkh

Joined Apr 3, 2020
9
I have a Kawai MP11 Keyboard Triple Pedal [Sustain Pedal /Soft Pedal /Sostenuto Pedal ] ,Mdl F-30. I have to replace the Sustain Pedal Pot/Rheostat.
I need advice as to what spec’s I need for the replacement Pot/Rheostat.

The Rheostat is present to allow partial pedalling. The Rheostat knob rotates [sweeps] 180 degrees when the pedal is depressed fully .

When the Rheostat was working properly: my ohmmeter showed: Pedal in the Up position [No Sustain]: infinite resistance ;
& when the pedal is depressed [Notes are Sustained] reads : 6 Ohms [or .6 Ohms?].

The attached schematic seems to show a variable resistor of 500 Ohms. Is that a Correct interpretation?
[I also attached a 2nd schematic for the other 2 pedals: Soft & Sostenuto]

Does the amount of sweep [180 degrees? ,300 degrees?] matter for a Pot like this?

What else do I need to look for?


PS: Kawai Tech Support is closed due to Covid19.
 

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Thread Starter

roarkh

Joined Apr 3, 2020
9
I have a Kawai MP11 Keyboard Triple Pedal [Sustain Pedal /Soft Pedal /Sostenuto Pedal ] ,Mdl F-30. I have to replace the Sustain Pedal Pot/Rheostat.
I need advice as to what spec’s I need for the replacement Pot/Rheostat.

The Rheostat is present to allow partial pedalling. The Rheostat knob rotates [sweeps] 1800 when the pedal is depressed fully .

When the Rheostat was working properly: my ohmmeter showed: Pedal in the Up position [No Sustain]: infinite resistance ;
& when the pedal is depressed [Notes are Sustained] reads : 6 Ohms [or .6 Ohms?].

The attached schematic seems to show a variable resistor of 500 Ohms. Is that a Correct interpretation?
[I also attached a 2nd schematic for the other 2 pedals: Soft & Sostenuto]

Does the amount of sweep [180 degrees? ,300 degrees?] matter for a Pot like this?

What else do I need to look for?


PS: Kawai Tech Support is closed due to Covid19.
 

Thread Starter

roarkh

Joined Apr 3, 2020
9
The voltage in the schematic appears to be 3.3 Volts.

I forgot to mention: as the pedal is depressed: the volume of the sustained notes increases steadily [& the resistance decreases steadily from infinity down to 6 Ohms.]
 

Alec_t

Joined Sep 17, 2013
11,295
In the posted schematics the input LED of each opto-isolator is shown shorted, so I don't see how the pedal pot there can do anything useful.
500 Ω is the pot value, but it is unclear whether this is a linear or logarithmic type, or whether the track is a wirewound, ceramic or carbon one. The pot symbol is unusual, which could indicate some proprietary type.
A pot limited to 180° of travel would likely be a custom one.
 
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