Open up the effect, taking care to note how it came apart so you can successfully put it back together. Carefully examine all of the wires inside the box. All wires should be firmly soldered at both ends. If there is a wire with one end loose, carefully examine all of the places that it might have connected to, perhaps with a jeweler's loupe, ($5 at most hardware stores, very handy) to find where the wire broke off as it left the soldered joint.
Wires almost never break in the middle of the wire, as there are tiny nicks in the wire where the insulation was stripped away. The wire breaks at the nicks when it is flexed too much. Melt the solder at the joint an push a wooden toothpick into the hole to push out the old broken bit of wire and to make room for the wire to be put back in the hole and soldered. Carefully strip a little insulation off the remaining wire and push it into the hole, then solder it.
If you find no broken wires, suspect power problems. This will sound a little simplistic, but measure the battery voltage, both with the battery out of the circuit and with it clipped in and the effect working. The battery may be dead, or alternatively may not have enough oomph left to run the effect. The battery voltage should be at least 7 volts when it is actually running the effect. Measure this by poking voltmeter leads on the battery terminal, poking it between the battery and the battery clip (easy for a 9V battery). If the voltage is low, the battery may be dead; try a fresh one. If a new battery does not help, there may be a short on the board. This is especially true if the battery gets hot!
Sometimes the isolation of the circuit board from the case is messed up somehow, and the board shorts out to the case. If this is true, the effect will work when the case is open and the board pulled loose from the case. The short may be a signal short, in which case the battery testing will not find it, or a power short, which will be caught.
There may be an open wire between the battery and circuit board. Connect one voltmeter lead to the case ground at the input or output jack, and measure the voltage at the place where the battery lead goes onto the circuit board. Note that for effects which use either the input or output plug to switch power on, you'll have to stick a plug into the appropriate jack to connect the battery up. This is very common, so you may just want to plan to stick a plug in each jack.
If you don't read battery voltage going onto the board, something is open between the battery and the board. The easiest way to find this is to stick one ohmmeter lead on a battery terminal, poking it between the battery and the battery clip and the other on the ground or + voltage respectively at the circuit board. There should be almost zero ohms from the battery terminal to the board. The path from battery to board may go through a jack (often the (-) terminal does this), or through a power jack for external power (frequently the (+) terminal does this). You can rapidly go down the wiring chain from terminal to terminal until you find where the connection stops; you will have just passed the break.
Sometimes the battery clip has a broken wire inside the plastic holder that fits on top of the battery. This is why you should connect directly to the battery terminals.
If one of these tricks doesn't catch it, there is probably a fault on the circuit board, and you'll need to be able to debug it from the schematic, or trace out the board to make a schematic to work from. This is much more complicated, and you should only try this if you already know how. If you don't already know how to do this, get someone who does to fix the pedal for you.
If the effect is not totally dead, just sounds wrong somehow, it is
probably a circuit board problem, and needs the circuit board tinkering
to fix it.
This is one of those rare cases where you can have an effect that may not be fixable, at least not easily. If the analog delay chip is the Reticon SAD512 in the Micro Chorus and a Reticon SAD1024 in the others, the parts are not in production any more. While you can find leftover SAD1024's fairly easily (GEO has them, so do other places) there were never very many SAD512's made, and the 8 pin part is simply not available that I know of.
The only easy options are cannibalizing another box that contains one, finding a leftover one, or hacking in one of the Panasonic MN3xxx series delay chips. This hack is almost certainly possible, but I have not seen one fixed this way yet. The MN3xxx parts are NOT pin-for-pin compatible with the Reticon chips, unfortunately. Since I originally wrote this up, Matshushita (Panasonic's parent company) has removed the MN3xxx series of delay chips from manufacture. There are still some stocks of them remaining, but they will become harder and harder to get.
If you have a Reticon R5106 (may still be available), it can be plugged directly into
the SAD512 circuit, but bias supplies will need tweaking to function. This is
not a task for the weekend pedal fixer, though, as you need to understand the
circuits to know what to change.
If the pot is sealed, no holes into the interior, you may have to replace it. The manufacturer may have the actual part with the little gear, saving you a LOT of time and effort. If not, it is possible to get a new, same value pot and cut the shaft to length, drill a little hole for the pin that keeps the gear from slipping on the shaft, and reassembling. This requires some significant mechanical aptitude, though.
A slam-dunk solution is to refer to Craig Anderton's article from Guitar
Player entitled "Clean up your Volume Pedal" , only use the LED/LDR pair
to replace the pot on the Wah. I have done this for a friend who wanted
to have a remote pedal, the actual Wah circuit back in his rack. Worked great!