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WIGGLING 'LITE' IN GUEST MODE

replacing potentiometers
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author replacing potentiometers
zerosum
Have any good links handy or words of wisdom?
I have to replace pot in my sherman,the Bypass/Effect pot, dual 100K lin.

Have you done this before?
Hmmm....
Muff Wiggler
hey, shouldn't be hard at all 8)

(sorry I'm crazy busy, i've got loads to reply to on this forum, hopefully find time later tonight, but wanted to get to this quickly for you....)

Hardest part will probably be taking the Sherman apart and getting to the pot

if you are lucky, the pot is NOT mounted to the PCB, and it's mounted to the chassis and wired to the PCB with 'flyout' wires

if this is the case, it'll be dead simple, all you need to know is:

1) Is it a linear or audio-taper (logrhythmic, or 'exponential response) pot? Sounds like you already know that it's a linear pot.

2) What value of pot is it? Measured in the maximum resistance that the pot will induce at it's full setting. Sounds like you know that it's a 100K pot, good work

3) What size pot is it? Measure in MM, usually the length of the shaft. Typically you want to get the same size and brand of pot to maintain 'feel', but this isn't technically essential if you can't find the right pot

4) clip the wires, remove the old pot, put the new pot in place, solder the wires back on. No need to worry about heatsinking the pot. You should solder it with organic no-clean solder, don't use acid core solder or washable for a pot

---

If the pot is mounted to the PCB, life will be more difficult. It can be tricky to replace a PCB mount pot without hurting the PCB. You may find it is easier to forget about desoldering it (when you may damage the traces on the PCB), and instead simply destroy the pot, leaving the connection pads (where it contacts the PCB) in place, then you can solder wires from these pads to the lugs on the new pot. Problem with this would be if the PCB is needed to keep the pot 'in place', and the pot cannot be secured to the chassis by a nut on the back of the pot. You'll just have to look and see and scope this out.

Finally, not all 100K Linear pots are equal! not at all! Sealed Bourns pots will last basically forever, but are a LOT more expensive than most.

I usually use Alpha brand pots, they are very good quality, and not too expensive!

let me know how it all works out - hope this helps!

cheers
zerosum
Quote:
(sorry I'm crazy busy, i've got loads to reply to on this forum, hopefully find time later tonight, but wanted to get to this quickly for you....)

Hey no problem smile Thanks for the reply

Quote:
Hardest part will probably be taking the Sherman apart and getting to the pot

if you are lucky, the pot is NOT mounted to the PCB, and it's mounted to the chassis and wired to the PCB with 'flyout' wires

It wasnt as hard as I thought(opening it up), I thought that i was going to have to take the knobs off. Luckily 6 screws fasten the bottom in place and everything is connected to the top of the unit.
Unfortunatly the pot is mounted to the PCB :shock:

Quote:
Finally, not all 100K Linear pots are equal! not at all! Sealed Bourns pots will last basically forever, but are a LOT more expensive than most.

Good to know.
Ill look into those.
I did also find out that BigCityMusic(tee-hee) has Sherman replacement pots in stock.

Yeah this is interesting, the pots on the PCB and it has to come off to put a new one on, therein lies the skill.

Cheers!
Muff Wiggler
unless they are crazy expensive, i'd buy a 'certified' replacement pot, for a couple extra bucks you'll get the same feel and size (and can use the same knobs)

sounds like you'll have to desolder the old pot. uh-oh.

first, can you remove the PCB enough to get a good work surface? you don't want to be messing around inside a tight box to do this. Ideally the PCB is right out, sitting on it's own on your workbench.

For desoldering the pot, I'd proceed really carefully. First, look around the PCB where the pot is mounted. Is it really crowded? Are there other components that you may damage? More to the point, are there many close traces on the PCB that may be damaged? Is it a dual-sided PCB? (components and/or traces on both sides?) Familarize yourself with what is happening on the board in the area where you have to work.

Whenever I've removed things from PCBs (usually DIP packages on my Blacets, as I like to mod them to replace internal DIPs' with panel-mounted switches), I do it like this:

1) destroy the package before unsoldering any legs. With a DIP this is easy to do with a pair of toenail clippers ( ! ), then you have the legs sticking out of their holes, but no component. This gives you more room to work (and you can easily do the legs one at a time), and you usually wont hurt anything except the component this way - just watch that you don't scratch a trace.

NOTE: If you do scratch, break, or otherwise hurt a trace - DON'T FREAK OUT! Take a break. I mean it! Go chill for a bit. Don't rush through this stuff. And know that you can repair any damage that you do.

If you inadvertantly damage a trace (I've done it before!) it will probably be with heat from a soldering iron. Anyway, you can repair a trace by reconnecting it with a piece of solder. No worry. But you want to familarize yourself with the traces around where you are working, so you know where they go, can check them for damage, and can repair them if you break them. A trace is just like a wire - if it is broken it can be rewired.

2) Now that the package is gone (perhaps you can use some little clippers to remove the pot's body from it's three legs), you'll simply have three legs soldered into three holes. You will want to remove these one at a time, leaving the hole behind for the new component. Realize that the hole is probably 'normally' filled with silver conductive material, plus around the hole will be a small 'pad' made of more of this material. You will damage some of this when removing the legs - don't worry, but understand this is conductive material that your new pot will need to have a soldered connection to. You can build a pad out of solder when you install the new component, just make sure you understand what traces the pad connects to. Also, check both sides. On a two sided board there may be pads on both sides, leading to different places. Everything has to connect when you get the new pot installed (it may look messy, but as long as the connections are made, you'll be good).

Also, you probably should have a DMM (digital multi meter), just so you can 'trace the trace' and make sure that current gets from one side of the component to the other - a quick way to check your work, and also to test any trace to make sure it is conducting across any possible breaks, areas you've worked on, etc. No need to spend a lot, I paid $9 (yes, nine dollars) for the cheapo Radio Shack one that I use.

Ok, onto removal of the legs...

3) I think your best bet is to try to 'grab' the leg with something, like a small pair of needle-nose pliars, or hemostats. Apply some force 'pulling' on the leg, so that as soon as the solder softens, the leg will pull out. This should give you the cleanest removal. Heat from one side, pull from the other

You don't want to leave your soldering iron on the board for more than about 3 seconds at a time, then let things cool off. Hopefully there isn't a ton of solder making you have to heat both sides. You may find it useful to get a solder sucker, so you can heat, and suck, repeat 'till enough solder is gone to remove the leg. I've never had a good time with solder suckers though, heat-and-pull seems to work for me.

When the leg is gone you'll need to inspect the hole. For one, it may be sealed with solder that hardened when you pulled the leg out. Obviously you need to have the hole open to mount the new pot. In this case, you'll need the solder sucker. Don't just stick your iron in the hole, hoping to bore it out through applied heat - you'll probably damage traces around the hole as well. perhaps stick a wire into the hole, and heat the wire, and it should push through.

Once the hole is clean and open, examine both sides of the board, looking at the pad(s) around the hole, and looking at the traces that connect the pad. Chances are the pad is burned off - this is probably going to happen, so be ready. as long as your component goes into the hole, you can use solder to rebuild a pad around it, and connect that to your traces.

repeat for the other two legs

a couple warnings -

when desoldering/removing/pulling/soldersucking, you will probably get bits of crap that fly around the PCB (that you won't notice happening at all). You may get a tiny blob of solder land somewhere, and then solidify.

When I modded my first Blacet EG1, everything was fucked after I was done. I thought I had destroyed the module. Figuring I had fryed some IC's, I ordered new ones, and was heartbroken when they didn't solve my problem. I thought I'd have to send it (my only envelope generator at that time) to Blacet for a 'professional' repair.

Anyway, the next day I inspected the board, and I found this tiny rogue bit of flux quite far from the area I was working, and it was shorting out a couple of traces on the PCB. I cleaned it off and the module worked perfectly! D'oh. Solder suckers in particular will throw bits of solder and metal and crap all over the place - pulling out a leg will throw some little bits of metal around. So inspect the WHOLE board after you are done, make sure you didn't leave a little mess somewhere

and.. again... if things are fucked, DON'T FREAK OUT!!! It's so easy to think you have killed your awesome piece of gear, and it's a horrible feeling. Any professional tech or EE or other knowledgable cat will do the same things you do - take a little break, inspect the board and the area around where the work was done, and repair/rebuild broken or shorted connnections. You can do this too. If you fuck something up, take a break, come back to it when you are chill, TAKE YOUR TIME, inspect and test with your DMM, and you WILL find the short or the broken trace. and then you can fix it.

When the old pot is out, the board is clean, and you have a place to mount the new pot - mount it. Use organic no-clean. It's about the consistency of toothpaste, very thick, but you do NOT want acid core, and you can't use washable solder once the pots are mounted (in other words, you don't mount pots untill AFTER you have washed a new board, and you install pots with no-clean solder). If you have a cheap soldering iron, it will become TOO HOT, and you'll be able to tell as the solder will become a bitch to pickup and use. Let it heat to a good point, do what you can, unplug it, let it cool, repeat. This is the only way to keep from getting crazy frustrated. If the solder is difficult to work, your iron is too hot (if the solder won't melt, it's obviously too cold!). If you have a pro iron and can set the temperature, you don't have to worry - mine is cheap, and you plug it in, it goes from too cold, to just right, to 'far too hot'.

If you have a double-sided board, make sure that the component is connecting the pads on both sides of the board. Your DMM will make it very easy to test this. If current flows, you are good.

Hope this helps! Be careful. It's a great feeling when you realize that you can do this stuff on your own. It's scary when you are new to it, but it's not rocket science. A bit of patience goes a LONG LONG way in this kind of work, but it's nothing you can't learn, and nothing to be afraid of trying.
zerosum
Great post man! grin
Thanks!

I feel confident that i can do this, i just have plan ahead and know every step i plan on taking before i start.
I have soldered plenty of wiggly guitar jack inputs, I put my own PC's together, after opening this up it is a good feeling knowing that anything that can break in here can be fixed...well i dont want to fuck up the PCB but pots and jacks can all be replaced.

Im going to pick up an ohmmeter as well as some other tools,
and post some updates when i get started.
This thread will definitly help.
Muff Wiggler
be sure to let us know how it works out!

good luck
$uperpuma
see..thats love... smile
Muff Wiggler
damn right

always good to help a brotha out when you can
JohnLRice
Hey, any advice on repacing the wet/dry mix pot on a Electrix Fiter Factory? hmmm..... ike at least what part number? Pettest of peases??? Mr. Green
Randaleem
zerosum wrote:
Great post man! grin


Agrred. That was a killer post, Muff!

One thing to add. A digital camera (or any camera actually) can be your best friend in doing repair work. Take a LOT of pictures. That way you know where the wires were connected, what the traces looked like (it's easy to add solder and make a new connection where there wasn't one before. A few *before* pics will help you see this...

If you don't use a camera, at least cover the basics with a pencil and paper...

Kind regards, Randal
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