My midi controller is acting up (electronics help!)

Thread Starter


Joined Dec 13, 2012
So I have an ableton APC40

It started acting up about 3 months ago (i've had it for 3+ years). Basically

- a couple of the knobs on the part highlighted in orange below, weren't functioning properly, and were sending commands to the midi interface when not being used, and also when they were in use they wouldn't respond properly (skip a trigger, or not respond to a trigger at all)

Areas in BLUE are for the faders

Areas in ORANGE are for the knobs

Now I took the unit apart (being an elec eng student i thought....what the heck) and noticed a lot of dust built up. I pulled out my toothbrush, iso-alcohol and got to work. I scrubbed down both ends, scrubbed down the area highlighted in blue (the faders) and the knobs. Also to note, i did scrub in and around the diodes, and even the potentiometers.

I put it back together, and now:

- The knobs in orange won't even deliver a signal past a quarter rotation (so if an effect has a dry/wet of 0-100%, i can only trigger about 25-30%, the knob physically turns, but the response will max out at 25-30%)

- the faders (minus the master fader, most right one), do not respond at all....nothing.

I contacted ableton (the manufacturer of the midi controller), and they sent me a link for a midi audio interface software. It recorded the inputs from the midi controller, and clearly showed me that the knobs weren't responding fully (hex values would max out at about 2F out of a total 7F) and the faders weren't even sending a signal. When I got back to them they kindly directed me to a local authorized repair center.....maybe I can do something about it before I throw the towel in.

So that being said....I've attached some pictures. FYI

one pic for each, the backside shows the circuitry right underneath those pieces of hardware. Thanks for your help!



Joined Jul 17, 2007
Isopropyl alcohol is hygroscopic, which means that it absorbs moisture out of the air if left sitting open - which means that even if it was 99.9% pure when you bought it, as soon as you open the bottle, it starts absorbing moisture. When it evaporates from your PCB, the water is left behind, and that can cause problems.

One way to speed up the evaporation of water from PCBs is to put the board in a warm oven which has been preheated to the lowest setting for at least 15 minutes; and let the board sit in there for a couple of hours. If you don't know how hot your oven gets, a safer method is to put your PCB in a sealed plastic bag with a cup or two of uncooked white rice for a day or so; the white rice acts as a dessicant. Leaving it in a sunny location will help dry it faster.

You haven't mentioned how you handled your PCB. Do you have an ESD-safe workstation? Did you use an anti-static wrist strap connected to a known good ground? Did you test the wrist strap prior to touching the board? All it takes is one static discharge to a CMOS part, and it's fried - about 25v would do it - and you won't feel static discharges below around 3KV.


Joined Sep 17, 2013
Go over the board with a magnifier. All that work with a toothbrush may have dislodged one of those tiny SMD components.


Joined Feb 11, 2008
The "knobs" are standard cheap quadrature encoders with mechanical wiper contacts.

They are a very common source of failure in musical equipment, due to wear, excess force, and corrosion. Replacing them is the only proper fix, although sometimes a squirt of electrical cleaner might give you a few weeks/months more use.