#1
Like the title says:
I want to repair an old Marshall lead 12.
I've taken it apart, and I've done some "research" to find where the problem lies.
There seems to be a bad contact with the buttons "Volume" and "Gain".
Is there something I can do about it?
#2
Are you certain this is where the problem is? How do you know those controls are bad? Do they make a "scratch" noise when rotated? Are they totally inop? I'm an electronics tech - tell me more. Let's fix this thing.
#3
Well, it's like you said. When I rotate the knobs I hear scratch noise and sometimes there's only noise and no guitar sound.
#4
Cool. This is a relatively easy fix. As long as the controls aren't physically damaged, it's possible to repair this amp. Do you have a Guitar Center nearby? If so, you're going to need to purchase some Caig CaiLube MCL. It comes in a small spray can. You'll need to gain access to the inside of the chassis, so you'll have to disassemble the amp. When you get it apart, examine the back side of the pots. That's just a fancy word for control. "Pot" is short for potentiometer, or variable resistor. Most pots have small, metal cans around them. This protects them from dust, which would also cause the scratchy sound you're hearing. Most of the time, these cans will have small holes in them, which are located on the side or top of the can. The bottom of the pot will definitely have a hole for the 3 leads to exit. Chances are, you won't be able to get to those. What we need to do, is spray some of the CaiLube into one or all of the holes. The CaiLube cleans and lubricates the pot's wiper and resistive substrate. You don't need to spray a whole bunch into the pot - just enough to get the inside wet, wash away the worn stuff and lubricate it. You can use a kitchen wipe to clean up any that gets on the circuit card, or anwhere else you don't want it. Do not use WD40. It will only attract dust and become gummy after a short while.

Over a period of time, as the pot is rotated and used, some of the resistive substrate wears off. As it wears off, it gets in the way of the wiper arm, causing the scratchy noise you're hearing. If the pot isn't totally worn out, cleaning it this stuff will revive it and it should last quite a bit longer. Eventually, you may have to replace the pot with a new one.

If you don't have a local Guitar Center, you should be able to order the Caig product online. You should be able to order it from Guitar Center, Mouser, DigiKey or any other electronic supplier.

Good luck!
#5
Ok, thanks for the explenation.
I had allready taken the amp apart, so that wasn't a problem.
And I think I've located the wholes in the Pots.
This might be a stupid question, but spraying a little bit of water in it, would that help?
And another question: Should I, somehow, make it dry after spraying, or do I need to wait untill it has dried by itself?
#6
No. No water. Water and electronics are a bad combo. Water, air and metal are a formula for corrosion. We don't want rust. The wiper of the pot is most likely metal. If it gets corroded, it's game over for that pot.

If you use the stuff I recommended, there's no drying period required. Spray it, wipe up the excess, turn it on and go. In fact, you want to leave a film of it on the inside of the pot.

Hope this helps get your amp running again.
#7
Yes, I will try it.
Thanks for the help:p

//edit

Sorry, but I was searching for this product: http://store.caig.com/s.nl/sc.2/category.183/.f
This page lead me to this one:
http://store.caig.com/s.nl;jsessionid=nWVLMyfdj8Lvv5p1XRr7PM29N1yZTpWpjX3nhL6K5LJt1kZL4Yc1wlQmk7yGvGpNcHdgGpVT11cNFTTBXcXp9GyQywzftP1hGhfJGyfDGzjH3XnY2FJ1pbvQ3X250G0B!1909807007?sc=2&category=293
And here there several spray's, so which one do I need, makes it al lot of diffrence?

And one more question:
What is exactley inside the spray, that makes it so special? What is it that I will be spraying inside my pot's?
Last edited by MVmatthver at Aug 16, 2010,