Potentiometer Rebuilding (Not for the faint of heart)

[Eric D]

As many of you know, a lot of parts for our old PG amps are no longer available. Both the gain and bass control potentiometers tend to fail on PG amps over time, and they can be nearly impossible to source.

I recently purchased some gain pots for a ZX amp, and they turned out to be the wrong part. Since I am pretty sure I will not be able to find the correct part, I was left with only one option...

...rebuild the original parts.

Below is the bad pot...



Step one, shear off the plastic pins which hold the pot together...





Step two, push the knob out the back of the metal housing...



Step three, remove the knob and set it aside if it is in good shape.



Step four, using a sharp tool, release the tab holding the two halves together...





Notice the bad pot on the left, and a worn but cleaner one on the right...



Next I will show how I took good parts from a new, but wrong style pot and used them to rebuild this one.

[marko]

Brilliant stuff Eric! this is what these Forums are for! I look forward to the rest of the thread

is there no way of cleaning the existing inner gubbins? like with cleaning fluid or something abrasive?

Mark.

[Eric D]

I am confident they can be cleaned, but my concern is finding a good grease to use once they are clean. The contacts need something to protect them from oxidation, however, it cannot be conductive (conductive would defeat the whole use of the pot).

[shaheen]

you sir deserve a medal ..........

[Eric D]

I would not pass out any awards until I have another pot working correctly. Right now I have just taken one apart, and that is not too challenging.

(I should add though, I did already do this and put one together. I just did not have a chance to hook up the amp yet to see if the fix worked or not, and I did not take any photos of the process.)

[oldschoolfan]

I am confident they can be cleaned, but my concern is finding a good grease to use once they are clean. The contacts need something to protect them from oxidation, however, it cannot be conductive (conductive would defeat the whole use of the pot).

Couldn't you use dielectric grease?

[Eric D]

I am confident they can be cleaned, but my concern is finding a good grease to use once they are clean. The contacts need something to protect them from oxidation, however, it cannot be conductive (conductive would defeat the whole use of the pot).

Couldn't you use dielectric grease?

There are lots to choose from. Which would you suggest? I think clear would be better than the white stuff. Last thing I needs is white goo leaking from a pot on an amp.

[tonym]

thats some good shit there.....keep the pics comin....

[Jacampb2]

My employer (Dow Corning) makes some good stuff. I'll get you the name off my tube when I get home.

Later,
Jason

[ttocs]

not sure you would really want to grease it. That will just attract dust/dirt and eventually lead to it failing again I would think. There was no grease originally was there?

[Eric D]

Yes, they have grease in them originally. I would guess it is to protect the contacts from moisture, which will corrode them.

Here is an update.

The left is a new part, the middle is a worn one, and the right is a corroded one.



Here are the parts to make up a good pot...



Here is an assembled pot, but now I need a way to keep it from falling apart. Since I cut off the plastic pins, if you pushed on the knob the back would eventually fall out of the assembly once in the amp...



I decided to use a piece of lead from a capacitor to form a retainer...





Which I then soldered in place...



And, it works! I did a ZX500, and two in a ZX450. No more scratchy gain controls. And they feel smooth to the touch again.

[itchnertamatoa]

woaw .... good stuff Doc ...

[Eric D]

woaw .... good stuff Doc ...

I hope you like it, it was done to your amp

[ttocs]

I would have just put a couple little dabs of CA glue back in the place to hold it together. Now the real question, does it work now?

[oldskoolmseriesfan]

I would have just put a couple little dabs of CA glue back in the place to hold it together. Now the real question, does it work now?

read the post stoner...... he stated that it works




ttocs.... its the D.O.C.....ofcourse it works!!!!

Good job eric

[itchnertamatoa]

woaw .... good stuff Doc ...

I hope you like it, it was done to your amp

I kinda figured that ... of course I will like it

[Eric D]

I have CA glue, but chose not to use it for a few reasons.

-I don't think CA would hold well to the greasy and oily plastic of the case on the pot.

-All the parts to the pots have paths leading to the center in one way or another, so CA might wick its way to the guts and lock it up.

-In the future if these needed to be rebuilt again it would be very simple to de-solder the wire on the back and then they would just fall apart with no effort.

But, none of this means CA would not work. I think if CA were used, thick CA would be the best choice.

[edman79]

I admire people that can do stuff like this. I only know a bit of the theoretical stuff.

Eddie Ed

[longboard]

so the question is whats the part nos and specs of the pots you can canabalise to make the old ones good??
awsome work i always think that everything is put together so there is always a way to get stuff apart and repair it just takes some effort!!

[Eric D]

Look for this part number...

RK09K12A0A2K

Mouser has them.

These pots might be a functional replacement for the bass pot on PG amps, but they are not the right taper on their resistance. In other words, if you used one for a bass control you might not get any change in the bass until near the end of turning the knob and then it would all happen at once. But, I am just guessing. Either way, they are not the OEM part, even though they look the same.

[ttocs]

They don't sell them with a linear taper?

[The Golden One]

i may try this on my 430 even though its all put together and in my car right now, ah i need to cycle my 0.3 in to its spot anyway. one thing you may want to consider is the spring tenstion on the pot gets weak in some case's i think that's the main problem with the ones on my 430 as they feel real loose when i turn them. also you may notice i havent been on here much its because i messed up my wireless adapter and had to buy a new one, i still dont have it all the way right but im still trying.

[Eric D]

They don't sell them with a linear taper?

Yes, they do sell them with a linear taper, but that is the problem. Audio generally requires a logarithmic taper.

I know the gain pot is a logarithmic taper, but maybe the bass is linear. Either way, the ones on Mouser are not the same as the originals.

[Jacampb2]

They don't sell them with a linear taper?

Yes, they do sell them with a linear taper, but that is the problem. Audio generally requires a logarithmic taper.

I know the gain pot is a logarithmic taper, but maybe the bass is linear. Either way, the ones on Mouser are not the same as the originals.

Eric,


The Ti amps I have worked on use a linear taper 50K pot for gain, so use of log taper is definitely not exclusive to gain controls, even though I have seen this written as an absolute (yes, I know you said generally ) It all depends on the input circuit design.

Also, that part number you list comes up as a 10K pot-- is this correct? I know you are not using directly, but isn't the one in those amps 50K as well? I was fairly certain it was...

Are you still looking for good dielectric grease? I haven't had a chance to dig my tube out of the shop, but I definitely will if you are still looking.

Later,
Jason

[Eric D]

Jason,

Yes, that is why I used the word "generally".

The ZX amps use a 10k pot for bass, and a 50k pot for gain. The 50k is logarithmic, with a reverse pinout, which is hard to find (at least it has been for me). The 10k pot might be linear, but I don't know for sure.

I am reusing the old resistive element, so really getting a 10k, or 50k or any k pot does not matter. I don't use the new resistive element, as that is the part which is not compatible with the amplifier.

I am pretty sure the 10k pot is pin compatible with the OEM 10k bass control pot, but just has the wrong taper, so it won't behave correctly. It should still allow for min bass, and max bass, and any point in between, just the middle of the rotation will probably not be half of the bass boost (this is hard to explain, so I am probably not doing a good job of it).

Since the 10k pot is pin compatible and will function, I figure it is better to buy them instead of a 50k pot which cannot be used in the amp. At least if someone needed their bass control fixed, this 10k pot could have a secondary purpose filling that role.

I am curious what good grease might be available, as I would like to repack these pots when I rebuild them to keep them going even longer.