I recently had a friend ask me to add some knobs to his
Small Clone. For those not familiar, I've
done mods to the small clone before, in particular turning it into a chorus tremolo hybrid, which I documented
on the forum.. I found that this mod, which was more of a circuit bend to begin with, was creating biasing problems for a lot of people, so I made a more "safe" (and more involved mod), which is
documented here.. Since the better mod, and also the depth and vibrato knob mods I add, are non-photodocumented, I figured I'd take some pictures as I went along so that everyone else could get a slice of the pie. This is an extremely great tasting pie, too. You can listen to a poorly-played guitar demo
here.(I apologize that the first few of these images are orange tinted, I had my camera on the wrong lighting setting) The first step is to get the board out of the case. The knob is a real pain to get it, see if you can pull it off with your fingers to avoid damaging it or the case with pliers etc (the screenprinting is VERY easy to scrape). Also, the wires are pretty wimpy - if you're up for it, I recommend replacing them all. It's fairly likely that you will break at least one throughout the process.
Mod #1 is replacing the depth switch with a pot. A lot of people recommend using a 100k, or 50k, or even 20k linear pot, but I found that the best "feel" and range is with a 100k audio taper pot. To each his or her own. So, find the depth switch, desolder the wires...
...then solder in the pot, as a variable resistor. One wire to the center, one wire to the side. If you want bonus depth, try messing with the clock capacitor (it is the 150pf one next to the 4047). Note that I replaced the wires. So, now that we have an extra slide switch and slot in our enclosure, what should we do with it?
On this small clone, I used for a "fast LFO" mod. This involves switching out the 2.2 uF capacitor seen here, right below the LM358. It turns it into a pseudo (VERY VERY pseudo) ring mod, but also allows for some nice subtly shimmer.
The first step is to replace it with a .1 uF cap. (I think that's the value I used... you might want to play around. I should have written this down!) We'll be switching the 2.2 uF cap back in, in parallel, adding back up for the normal "slow" setting.
This is how you wire the cap on the switch. Pick a set of two lugs other than the middle two (doesn't matter which) and solder the capacitor's leads. Then, solder one wire to one of the center lugs, and the other wire to the side lug on the OPPOSITE side. Hopefully the picture helps here. If you think about it, it is clear that this will add the cap in parallel, so long as you solder the wires to the board...
...like this. Try it out, see if you like it. It's more effective with the tremolo mod, too. Also, you could make it super-slow (I can't imagine why you'd want to do that) by using a large (22 uF maybe) cap in place of the .1uF one. Personally I would use the switch as vibrato/chorus, rather than a doing a vibrato/chorus knob (more on that later) but it's not my pedal!
To do the vibrato mod, lift up one of the lugs of this 22k resistor. This is the resistor that sets the level of dry signal in the output. A chorus pedals output is the dry signal, combined with a vibrato'd version of the input (that is, the pitch raises and lowers periodically, very slightly). So, if we remove the dry signal, we get vibrato. To do that we can add resistance in series, because this 22k resistor is setting the gain in a noninverting op amp pattern.
Wire the resistor's hanging leg to the center lug of a 100k pot, and one of the side lugs to the board where you removed the resistor from. I also recommend hot gluing the resistor to the board, so you don't inadvertently break it off while moving the pot around.
Moosapotamus does this with a switch (SPST in place of the pot), which is not a bad idea at all. The difference between the chorus and vibrato is pretty subtle, though if you are a tweaker and you want an extra bit of control over the depth a knob is the way to go. Personally, I would use the spare depth switch as chorus/vibrato and not do the superfast mod.
To do the improved chorus vibrato mod, you will need an optocoupler of some sort. You can
buy one if you want (range of 5k light -> 200k+ dark works well) but you can also roll your own. Forumite DiyFreaque posted a
really nice tutorial and para posted some
very useful info on vactrols, but for the sake of pictures, here's the way I do it (though to be fair I use
H11F3's 95% of the time).
So, get a superbright led, a photoresistor, some heat shrnk, and some hot glue. Any color works, superbright tends to do the job better however. For the photoresistor, as I said, 5k->200k+ works well, but it's not super important. It needs to be the same diameter, or close to the same, as the LED. The heat shrink should be big enough to fit around the LED quite comfortably.
I find it's helpful to glue the LED to the photoresistor first. That way you aren't fussing with dumping hot glue into the heatshrink, which will make it shrink and make putting the photoresistor in there pretty hard. You just need a tiny bit, be sure not to put so much on as to make the whole thing fatter than the heat shrink.
Cut off a piece of heat shrink a bit longer than the LED and photocell combined (not the leads), slide it over, and use your soldering iron to shrink it. You'll get a nice little package like the one here. Make sure you don't glue together the photocell or LED leads within the assembly.
Find the 10k resistor that is perpendicular to the 22k one we found earlier. This determines the overall level of the output. It's the feedback resistor for the op amp associated with the 22k resistor. Changing its value will change the output level, which is exactly what we want for a trem effect.
Replace this 10k resistor with the two leads of the photocell from your optocoupler. The cathode (negative end, shorter lead) of the LED needs to be grounded. A convenient place for this is leg four of the nearby 4558 opamp chip. Bend the optocoupler over, cut the lead, and solder it on. This should also secure it in place pretty well. If you want to ground it elsewhere, feel free. Leave the anode free for now.
Here's the tremolo pot. You need to wire a 10k resistor to one side lug, and a 1k resistor to the other side. I recommend choosing the lugs in the order I have them (1k to the right)- that way as you turn it up, more tremolo will be added. Try to really secure the resistor on, maybe wrap the leads around where you are soldering a few times. Also make sure to keep the resistors overall below where the pots will be touching the enclosure - if any lead shorts out with the box, you will be screwed.
Here's where to put wires for this pot. Use a 5k linear pot. You want one going to the cathode (positive side, the longer lead that you didn't just ground), one wire going to pin 1 of the LM358, and one wire going to pin 8 of the LM358. Make sure not to inadvertently bridge together any leads of the LM358.
The wire from the cathode goes to the center, from pin 1 goes to the 1k resistor, from pin 8 goes to the 10k. The 1k resistor is now connected to the LFO output, so when you turn the pot all the way to that side, the LED will pulsate in speed with the LFO, modulating the gain. The 10k resistor is connected to V+, so when you turn it all the way that way, the LED just lights up, giving a constant signal with no modulation in gain/level. If you find that your output is too quiet or too loud, adjust the 10k resistor. If you notice that the LED is blinking a little even when you turn the pot all the way to the 10k resistor side, it's OK, you won't be able to hear the tremolo.
Drill some holes and you're done! Drilling holes is probably the worst part, the EHX enclosures completely suck for drilling - you will probably bend it a little, so bend it back before you screw it back together. Be careful not to scratch the box at all (this is very hard not to do). Test all the mods before you put it in the box of course, and enjoy! Please post any questions. These mods are for noncommercial use - if you want to sell them, come up with your own!
-Colin
