SD-1  circuit board


When I did my original "Shoehorning a Strat" article, I was more focused on doing the job than documenting it: I wanted to be sure it worked, and that the effort was worthwhile. I've had years to play around with the mod and I'm convinced that it's a useful and desirable thing... for me, at least. I like it so much that I've decided to butcher another Strat. This time, I took a few pictures (but got into the process and then didn't).

If there's any stompbox effect that I'd want built into a guitar, it's overdrive. It lets you travel light, plug into any amp (even a Twin Reverb at low volume levels) and always get some beef from the guitar. Sure, clean is great, but a little dirt is sometimes a welcome ingredient. Even if you've got pedals, it's like having a little "more" available at your fingertips.

It's great to have access to that at your fingertips. While a stompbox does let you kick in and out without moving a finger, it's relatively difficult to tweak parameters while you're playing. An onboard stompbox takes no more effort to kick in and out than it does to change pickup selection. It's also great to have on-the-fly control over the gain and tone parameters as effortlessly and instinctively as you control the guitar's native controls. In other words, the OD becomes an integrated part of the guitar, not an external box that you have to pre-set and have nearby to stomp on.

The Boss SD-1 is a proven candidate for this operation. For one thing, it's not as sought-after as the Ibanez Tubescreamer popularized by Stevie Ray Vaughan, so it's significantly cheaper. The circuit designs of the "Tubescreamer" type overdrives are pretty similar, and modding SD-1s to become Tubescreamer or Fulltone sound-alikes has become something of a cottage industry. Ebay lists plenty of modded SD-1 pedals and mod kits. It's fun to do the mods, although I don't think that they make a hugely dramatic difference or improvement that I couldn't live without: There are a lot of other factors that make a great difference in tone and feel, like the amp, the guitar, the volume level & even your mood. (Most importantly, it's your playing.)

For what it's worth, Boss seems to put just a little more "goop" - thick, elastic adhesive - on their circuit board as time passes, which makes the job of modding their circuit board a little more aggravating. Maybe it makes them more vibration resistant? In my most recent purchase I see that they've expanded that treatment to include transistors and resistors, some nearly covered in that goop (oddly enough, right where the Keeley mod calls to replace some capacitors and resistors). This stuff is thick, tough, and sticks really well. Although you may be able to peel small areas, generally speaking, it has to be scraped/nibbled off. It's a little aggravating and adds extra time to the job.

For this application, the SD-1 pedal has some features that make it a good choice: It's basic, with only 3-control parameters, it's constructed "old-school", with discrete pots & jacks (not circuit-board mounted), and none of those tiny surface-mount components. It also has low power consumption: 4-6 mA (even less since you don't use the LED). This means a longer battery life than Boss' Blues Driver (9 mA) and Digitech's Bad Monkey (17 mA). While cork-sniffers may shun any overdrive pedal without a 12AX7 tube or fewer than 6 knobs, that's really not a practical option for an onboard unit powered by a 9-volt battery. This is one case where you DO want to use a battery instead of a wall-wart or AC power cord, and you want the battery to last a long time.

It's not an either/or proposition. You can switch it off and run your guitar through your monster pedalboard, or a computer console. Or you can dial in just a little boost and do the same thing. Some effects like wah-wahs prefer to be before distortion in the signal chain, so the onboard OD grinding at full tilt isn't really appropriate there. Fortunately, it's legal to buy as many tone-modifying boxes as you feel you need and arrange them however you want. You can even set up several stages of cascading distortion boxes. Having fingertip control of onboard OD is just icing.

The one thing you do give up by doing this mod is the second tone control. I've always considered that to be an unnecessary extra, and prefer having one volume and one tone control (even on Les Pauls). I adjust them on-the-fly, rather than using two tone knobs as tonal preset options for pickups. For presets, I use a multi-effect pedalboard so that I can step on one switch to change bunches of preset effects at once. Another cool benefit is that it can equalize the difference in output levels between the single coil Strat and guitars with humbuckers: You don't need to mess with having separate presets, or switching input setups for pedals or multi-effect boards to accommodate the different native output levels of the two types of guitar. Just plug and play.

Anyway, that's my rationale for doing this oddball thing. I figure that it's similar to scalloping a guitar's neck-- it's not a mainstream thing to do and such mods kill the resale value of a valuable guitar-- but if you're open to off-the-well-beaten-path stuff, you may find that there aren't any functional downsides to either mod.

SD-1  circuit board


In order to install an SD-1 circuit in a Strat without cutting a huge cavity under the scratchplate (unfortunately though, you do still need to cut smaller one, unless maybe you use one of those oddball low-profile pickup selector?), the board needs to be cut and folded in half. Planning a circuit board cut-line isn't something you could do easily from an electrical schematic; a component placement & wiring diagram would be ideal, if you had one. However, having the actual physical circuit can be almost as good, if you've got X-ray vision.

A computer can help out here. This series of pics shows what the the board looks like when you flip it over to the traces side and hold it up to light. In order to plan how the board should be cut, it would be helpful to see both sides of the board at once, which is pretty difficult if you don't have X-ray vision.

The top pic shows the circuit traces and a shadowy blur of the components as they're connected to the traces. (Notice that it's vertically flipped from the first component-view picture on the page-- that's what happens when you flip something over to look at the reverse side.) This view is helpful, but you'd still want to confirm component connections before planning your cut line.

The third pic shows the component side held up to light and flipped vertically through the magic of Photoshop.

With two pictures, you can align the traces in Photoshop layers so you can vary the transparency of the top layer and see how things connect (center pic). The center picture is only an example of a mid-point blend: The real value is in dynamically varying the transparency so you can follow the connections of specific components. It's like an X-ray of the circuit board, which is ideal for deciding where you want to cut the circuit board, where to move components, and where to attach jumpers. It's much easier than flipping the board over and over and making the mental transposition from the vertical flipping as you try to figure out where components connect.



onboard SD-1 guitar

Ideally, the cut line would be placed somewhere near the center of the board, to make two layers of approximately equal size. This optimizes the reduction of the length x width dimensions; The reason we're doing this is to make the board as small as possible to fit within the cavity of a Stratocaster's controls routing without removing a lot of additional wood. The cavity's already pretty deep; it's just too narrow for the stock board to fit.

The cut line is shown in red, jumpers are shown in magenta, and one end of the resistor at the top will need to be moved to allow a fairly straight cut across the board. The only tricky part of the cut is just below the middle. Similarly, there's a jag in the cut line above that to accommodate the circuit trace pattern and avoid using an extra jumper. It's actually easier to cut straight across and jumper the trace.

The jumper on the far left of the cut line is to override the momentary contact footswitch and permanently turn the circuit on. That lets you wire a DPDT paddle switch to the input & output for true bypass operation. (With true bypass, you can operate the guitar normally without a battery or with a dead battery.)

onboard SD-1 guitar


Even though it's easier to do the work with the circuit board out of the enclosure, it's much easier to test it in the enclosure. As you can see, I did most of some version of a Keeley mod, so I did a lot of testing throughout the process. (I later removed the gigantic 1 uF capacitors and played around with swapping in some 1N34A Germanium diodes in the clipping section.)

Despite the straight-forward nature of the job, I did have some doubt whether the board was going to work-- I never know if I've done something stupid, like frying a transistor or diode by dwelling too long with the soldering pencil. I was relieved when I plugged it in and got that familiar grit of distortion.

Before cutting, R2 was pivoted and the detached lead was relocated to a new hole, drilled using a pin vise. This happens to be one of the Keeley mod resistors that I didn't bother changing that was gooped onto the circuit board. The goop just makes it harder to pull the lead from the board because you're trying to pry it up while heating the soldered bead, with the lead end slightly bent over on the trace side of the circuit board. On the trace side of the circuit board, the insulating green stuff has to be scraped off around the hole to expose the copper so that you have something to solder onto.

The cutting is the dramatic climax to the job, but with a jeweler's saw it's easy and goes quickly. It's a good idea to drill holes where there's a change in the direction of the cut because the blades are extremely delicate and easy to break. It's also a good idea to periodically check the component side of the board, just to make sure that nothing's been accidently pushed in the path of the blade.

After the board is cut, the insulation on the severed traces needs to be ground down to copper and tinned with solder. I used a ball-tip bit in a Dremel for that. The multi-strand jumper wires were also tinned, then soldered to bridge the circuit board halves. I considered drilling holes for the jumpers but decided that it wasn't necessary since the wires are relatively ductile and solidly soldered to the board. The board will be folded over, strapped together in place and covered with shielding, so there shouldn't be any movement to stress the connections. Before doing that though, don't forget to jumper the board for true bypass operation.

I was hoping to be able to shoehorn this into the existing controls cavity, so I bought an "S-Model Megaswitch" (mfg. by Schaller) from Stewmac. This pickup selector switch uses a PC board, so it's possible to trim the depth of the switch significantly and solder leads directly to the traces. Unfortunately, it was still a no-go; I don't have X-Ray vision to see exactly where it wasn't fitting, but I felt it was best not to force it. Although I was hoping not to, it wasn't that difficult to route a cavity similar to the one Fender puts in their Eric Clapton Strat's body for the circuit board. Another cavity was routed in the backside for the Gotoh battery box.

When wiring the circuit into the switch and pots, it's important to keep lead lengths short, and to use separate shielded leads (coaxial, mono), especially for the gain pot and the input/output lines. This is particularly important if you've done a gain increase mod; without shielding, you're likely to get nasty feedback oscillation when the circuit is turned up full tilt. The shielded leads should be grounded at only one end (to avoid ground loops), and the other end should be insulated with heat-shrink tubing to ensure that no stray ground wires accidently short against something important.

Highway 1 Stratocaster onboard  SD-1 overdrive


The SD-1 circuit was supposed to go into the Yngwie Malmsteen strat, but my attempts to get a replacement body took waaaay too long. I'm impatient, so I put it into this 2006 Highway 1 Strat. I considered moving the YJM scalloped neck onto this guitar, but then there wouldn't be much left of the YJM Strat. Besides, I've got another SD-1 circuit board for a future project since I liberated one from my wah-wah pedal (see below).

I like the Highway 1 Strat with its late 60s/70s-style big headstock and its funky thin finish. The matte finish shines up pretty nicely with polish (despite the fingerprints in the pic). I replaced the string trees with butterflies, the Ping tuners with 'F'-style, and the blank neckplate with an 'F'-logo to give it more of the flavor of that time period. I'm content with the ersatz late 60s/70s-styling; I much prefer the truss rod adjustment at the headstock instead of the heel (although I would have preferred 21 frets instead of the 22-- makes it hard to remove the pickguard). Also, I prefer the Highway 1's hybrid bridge with standard 2-3/8" screw spacing + narrow 2-1/8" string spacing: It helps to keep the high-E string on the fretboard. The stock pickups were much too noisy for my tastes, so I replaced them with Kinman Woodstock Plus pickups-- In a perverse way, it was funny to dump so much money into such a cheap guitar. (Despite the naysayers, Kinman's pickups rock...)

The SD-1 mod required adding the bypass toggle switch between the pickup selector and volume knob. The switch is oriented perpendicular to the pickup selector switch so that it's unlikely to accidently get flipped while switching pickups. It's operated by the pinkie, as an extension of the motion you'd use to operate the volume knob.

The stacked TBX pot (middle) is a great fit for this dual-mode application, after it's been taken apart to lessen the detent feel and loosen the travel. The bottom pot has a cut trace, and from 1 to 5 of its travel, it's a 250K tone control (in normal mode), which happens to be around where the 1MEG SD-1 drive pot cleans up. After 5, the tone control has no connection to ground, so it's a no-load pot. This tone control is wired to the bypass switch, so it's only active in normal mode.

The lower SD-1 tone pot is an old 10K Alpha rotary SPST switched pot that cuts power to the circuit. You could do this with a push-pull pot, or an S-1 (overkill, considering the rarity), but the rotary switched pot is really slick. It's redundant with the stereo output jack so that you can cut power (to save battery) without unplugging the guitar. Although the circuit design calls for a 20K pot, I didn't notice any significant difference using the 10K pot, and the convenience made it worthwhile for me.


I considered using a ProCo Rat 2 for an onboard mod since it's got a more dramatic range of distortion. It can do a fairly subtle and articulate overdrive, a super flabby fuzz sound, or a shrill and snarly lead tone, with a huge volume boost. It's a lot of fun to play around with and in comparison, it makes the SD-1 seem really tame and limited.

The circuit board is also relatively simple and compact. Once the board is trimmed of the stomp switch and pots, the board is smaller than the SD-1 circuit board, approximately 1.5" x 3". You could probably get it a little smaller by replacing some of the traces that run along the edge with wire jumpers. My guess is that this circuit could be shoehorned in a Strat's controls cavity, to avoid the need for routing wood.

WARNING: Don't attempt to remove the circuit board from the enclosure unless you're willing to replace or repair the pots! (see right) Although the knobs are inserted on knurled split shafts, they're super tight, and I don't know of a tool to fit under the knobs to grab the shaft while you pull or pry. If you try to pry the knobs off, the entire shaft will likely detach from the pot's internal wiper before the knob comes off. This happened on all 3 of my Rat 2's knobs; I've never seen this happen before and wonder if it's a tamper-proof feature exclusive to the Made-in-China version...? At any rate, you've got to loosen the pots' nuts on the enclosure to remove the circuit board, and the nuts are under the knob. Your only other option is to desolder them while pulling-- all 9 pins on all three pots at once.

There are several reasons why I didn't choose this for an onboard mod. The Rat 2 uses 3 100K audio-taper pots, which aren't a standard guitar pot value. To set up the guitar so that it could be used in bypass mode (battery-less) and retain the standard 3-knob configuration, you'd probably want to get some ganged 250K/100K pots with a knurled shaft-- I don't know where you'd find those (although you might be able to assemble them from parts of two stereo pots).

Another reason is that the Rat 2 is such an unruly beast. The circuit is much simpler than the SD-1's, and the 3 controls (Distortion, Filter, Volume) are extremely interactive. If you change the volume or distortion, the tone changes. If you change the input volume, the tone changes. If you change the filter setting, the output volume changes. While some of that isn't unusual for distortion circuits in general, they vary in the degree of interactivity. The Rat 2's controls do give you a wide range of sounds (some of which are unusable, IMO), but you do have to spend more time tweaking all three parameters to get it just right. Unfortunately, that makes it less useful for onboard control, and more of a set-and-forget/stomp-in & out circuit. In addition, the circuit is very sensitive to the input volume, so you'd probably want 4 knobs on your guitar.

In contrast, the SD-1 circuit is much better behaved and more predictable: When you dial down the distortion, the tone remains the same. You only have to concentrate on dialing in the output volume level or tone, which is a fairly instinctive thing to do.

Philosophically, it makes good sense to have the SD-1's more conservative range of distortion onboard since it retains much of the pickups' characteristic tones and gives you an extra upfront boost for your amp and other devices down the chain; you don't need it all to come from your guitar. If you need more hairy distortion, you can do it in stages by having the onboard SD-1 plugged into a Rat 2 stompbox. That gives you 4 stages, from clean to dirt (all off/SD-1/Rat 2/SD-1+Rat 2). Insert your wah-wah in between for a super Hendrix-y sound.

In fact, that's what I did with the circuit board (after destroying the pots, I didn't want to go back). The Rat 2 circuit was a better choice than the SD-1 circuit I'd installed in my mutant Crybaby/Roger Mayer wah-wah pedal precisely because it offered a much wider range of distortion. Once it's dialed in, it's a great stomp in/out addition to the wah-wah and compliments it nicely. The circuits are low powered, so you can power both off of a single 9-volt battery and avoid using a wall-wart. Basically, it's just fewer wires to attach and less clutter at your feet.