Grant Muller

Casio MG-510 Midi Guitar

Back when I documented the repair of the Casio PG-380 MIDI Guitar, I had no idea that this post was going to dominate the traffic patterns to my little home on the web. Fully 1/3 of all visitors to this site come to that post, asking questions, posting comments, and requesting repairs. One request I’ve gotten over and over is a repair on the Casio MG-510.

The Casio MG-510 is like the little brother of the Casio PG-380. The base functionality is similar, but the 510 lacks some of the extra features that the PG-380 offers. The 510 has no space for an expansion slot, and no internal synthesizer, which for most software synth users is just fine. The biggest differences you’ll notice between the 380 and the 510 are hammer on sensing and the ability to perform pitch bends. The 510 is strictly chromatic; when you bend it assumes the same pitch until you bend far enough to change notes, in which case a note off and note on message are sent and interpreted. The 380 will perform a pitch bend at even the slightest pull of the string.

The 510 and 380 share one major flaw though: the electrolytic capacitors used for the pitch envelopes. These heinous little surface mount caps tend to leak over the years, especially on the 510, leading to corrosion and in most cases total failure of the MIDI capabilities in the guitar.

I finally got around to repairing one of these guitars, and the process is so similar to the PG-380 that it would be a shame not to document it. If you’re new to this you should probably refer to the post on the PG-380 before getting started.

You will need:

  • 6 x 1 uF non-polarized electrolytic capacitors
  • 4 x 10 uF polarized electrolytic capacitors
  • 1 x 22 uF polarized electrolytic capacitor
  • 1 x 4.7 uF polarized electrolytic capacitor
  • 1 x 33 uF polarized electrolytic capacitors
  • Anything you need to unsolder old capacitors and solder on new ones

First, crack open the back and take a look at the boards:

(1 of 5)

You’ll see two double-stacked and plugged into three header cables.

Take both boards out (unlike the PG-380 you have to operate on both):

(2 of 5)


Take a look at the capacitors on both boards below:

(3 of 5)

Top of PCB 1 – C9, C18, C33: 1uF non-polarized electrolytic


(4 of 5)

Bottom of PCB 1 C42, C52, C63: 1uF non-polarized electrolytic


(5 of 5)

Top of PCB 2

  • C4, C22, C29, C12: 10 uF polarized electrolytic
  • C30: 22 uF polarized electrolytic
  • C31: 4.7 uF polarized electrolytic
  • C48: 33 uF polarized electrolytic

Basically, for both boards, replace the capacitors with the caps above using capacitors of identical value. It shouldn’t matter if you use polarized caps for the entire repair, since the frequencies are not high enough to affect response times, but use non-polarized where needed above if possible.

You will find that you have 2 "extra" caps (seems like 2 for each string plus 2). I know that one capacitor is used for CPU reset (C30), but I’m not entirely sure what the last one is for. I replaced it anyway.

Some notes:

  • The traces on the top board are very small. You might find yourself pulling them while unsoldering the old caps. Not to worry, there are plenty of places to solder the new caps.
  • Corrosion makes for crappy contacts. If you find that your caps have corroded, particularly on the lower board, you will need to sand the corrosion down with steel wool or other light abrasive until you can expose some copper to solder to. On the guitar I repaired the corrosion was severe, and I spent a lot of time scraping out leaky capacitor guts.

That’s really all there is to it. Plug the boards back into their headers, screw them back into the guitar, and adjust the trim pots as needed to calibrate the guitar again.

Another Schwinn Prelude?

After a leisurely lunch on the Marietta Square yesterday, I came across this:


Another Schwinn Prelude!

It seems I’m not the only one who was unimpressed with the original rusted gunmetal gray look.

The owner of this Prelude painted theirs similarly to mine. A base coat of cream with an accent color for the lugs and forks. Observe:



This Prelude owner went with green, no doubt as an accent to the sexy leather Brooks seat and handlebar tape. I also like the fork paint:



No detail was left:



All in all a very impressive redux! Anyone else have a Schwinn Prelude Redux to Share?

Ring Modulator: Prototype to Final Build in One Ridiculous Step

RingModulator-16 If you’ve been to this site before you know that I’ve been building a ring modulator for Bill Graham to use with his Rhodes for the better part of a year. If I had enough time to do it right it’d take me less than a week, it’d be stable, and I wouldn’t be worried that a 3 foot tumble would render it useless. But alas, Bill had some gigs coming up, and I wanted to put this project to rest in the interest of getting some of my time back, so I resorted to some rather ridiculous means to complete it. What follows is not to be emulated or admired, merely witnessed.

RingModulator-8 Having assembled the circuit on a breadboard in the previous post, I had a working prototype that if you pinched the alligator clips just right, would produce the effect I was looking for. I had a day or so to get this thing boxed up and stable enough to work at some gigs, not nearly enough to design the PCB, etch, reassemble, test and ship. What’s a time-starved designer to do? Box up the prototype breadboard and all, right into the oversized power supply box, wiring up the controls right to the front panel.

RingModulator-10 I started the process by first removing the breadboard strips from the substrate they were attached to. The entire breadboard wouldn’t fit assembled into the case so I simply transferred the screw hole locations from the substrate to the new case so I could attach the strips to the inside. After drilling and attaching the strips it looked something like the picture to the left.

RingModulator-11 Moving the controls from the board to the panel was easy enough. As you can see I had to settle for using the PCB mount pots I ordered expecting to mount this on a board, rather than the panel mount ones which are much easier to solder wire to. Live and learn I guess. I drilled holes for the 4 controls knobs (Frequency, Depth, Pre and Post Gain) in a row on the panel, along with two more for the input and output. I tried to keep everything on one detachable panel so that if I ever did get around to designing and etching a board I could replace the breadboard strips with it and not have to make any other modifications, I did, after all, order two of everything. I do think ahead on occasion.

RingModulator-13 With the wiring soldered to the pots I jammed the other ends of jumper wire where the pots used to reside on the board and ran a final test. Everything appeared to be working, so I wired up the power directly to the terminal strips and packaged it all together. There, done. For now.

RingModulator-17 Not that it sounds any different than the last audio samples I posted, here are some new samples from the last test run before Bill came and fetched it for a gig:

Audio clip: Adobe Flash Player (version 9 or above) is required to play this audio clip. Download the latest version here. You also need to have JavaScript enabled in your browser.

Audio clip: Adobe Flash Player (version 9 or above) is required to play this audio clip. Download the latest version here. You also need to have JavaScript enabled in your browser.

Audio clip: Adobe Flash Player (version 9 or above) is required to play this audio clip. Download the latest version here. You also need to have JavaScript enabled in your browser.

So I’m calling this project done. I may come back to it and do it right some day, but that won’t be anytime soon…

Ring Modulator: Prototype Take Two

RingModulator-3 About a year ago I posted an article about a Ring Modulator prototype I had created using 2 audio transformers and some matched diodes. The design was beautifully simple, and I may return to it someday, but it had a number of shortcomings. The circuit I started with would have needed a preamp for my input signal and a separate oscillator. In addition, I would have probably needed some means of amplifying the output signal, and mixing the effected and un-effected signals together. I’m not quite good enough with circuits to throw all of those disparate components together on the fly, so I sought out another circuit that had this integrated into the design.

I came across this design based on the AD633 chip and a reference to a design by Roman Sowa:


I actually found several different circuits based on Roman Sowa’s design, but I liked this one. It was clear and concise, easy to read, and split the components up into easy to understand modules. You can clearly see the input stage, the oscillator (with waveform selector…another bonus), the multiplier and the output stage. I got to work in the basement prototyping this design to see how it sounded.

Most of the components I used were whatever I had on hand, with the exception of the very expensive AD633 chips (8 bucks from digikey). The pots I used were whatever linear equivalent pots I had laying around. I figured that would work for the prototype testing, if I liked how everything was turning out I could pick up the real pots as part of a second order, and design the PCB while I waited for them. The power supply is an old kit I built up about a decade ago from Craig Anderton’s book…still delivers 18 V as steadily as the day I built it. You’ll note the schematic calls for 15 V, but the TL072 and AD633 chips this circuit is based on can easily handle 18 V, so I just used what I had.

Since I had an input stage to work with this time, I tested with a guitar.

Here is just a quick run up the strings, once with modulation, once without…and an accompanying sweep of the frequency:

Audio clip: Adobe Flash Player (version 9 or above) is required to play this audio clip. Download the latest version here. You also need to have JavaScript enabled in your browser.

I just goofed off with this next test, playing some scales and random notes with the frequency mostly held steady:

Audio clip: Adobe Flash Player (version 9 or above) is required to play this audio clip. Download the latest version here. You also need to have JavaScript enabled in your browser.

Then I played some chords, usually changing the carrier frequency after each strum:

Audio clip: Adobe Flash Player (version 9 or above) is required to play this audio clip. Download the latest version here. You also need to have JavaScript enabled in your browser.

Finally, here is an example of using a ring modulator as a seriously tremolo:

Audio clip: Adobe Flash Player (version 9 or above) is required to play this audio clip. Download the latest version here. You also need to have JavaScript enabled in your browser.

So there we have it. Very little carrier leakage (to be resolved with tweaking the null pots), steady oscillation without the need of a separate carrier instrument, everything integrated into one circuit. Maybe I’ll toy with putting the oscillation frequency knob into a pedal…

Look out for the PCB design next, hopefully it won’t take me another year.

Casio PG-380 Midi Guitar

1218315849_5b5784b61f_o Several months ago Jason asked me if I could fix a MIDI Guitar. I didn’t have the slightest idea how to fix one, and had only speculative knowledge about how they work, so naturally I said “yeah, sure, piece of cake”. If I’d have known at the time the kind of gear lust this project would create I might have turned him down at the outset.

The Casio PG-380 is a guitar that translates the notes you pick on the strings into MIDI Notes. Think of it like a Keytar, only its ACTUALLY a guitar. You may think that the name brand somehow reduces the quality of this particular instrument, but you’d be mistaken; this baby is top of the line. It can translate amplitude, hammer-ons, and string bends with very little latency. I’d soon find out how hard to find, and how expensive, buying one of these would be.

The problem sounded simple: only the bottom two strings of the guitar were producing notes. At first I figured this must be a calibration issue or something, so I tweaked some of the pots on the board, messed with action height, etc in an effort to get the MIDI pick up to hear and translate the notes. This effort proved fruitless so I turned to the web.

How do you translate audio into midi? I had a vague idea how you could do this with envelope followers and some basic filter networks, but I wanted to understand how this thing actually worked before I could say with any certainty what was wrong with it. I looked around for a long time on the web and turned up nothing related to the technical aspects of converting the output of a guitar pickup to MIDI. In the end I relied on the premise that there must be a filter network to divide the audio by string, and a logic device to convert that analog value to a digital stream of bytes. Since 2 of the 6 strings were working, I could assume that the logic device was probably ok. I turned my attention to what I assumed was the filter network.

I cracked open the case and had a look around. I followed the traces from the pickup back to the 6 calibration pots to the series of capacitors that make up the filter network. I didn’t see anything visibly wrong so I returned to the internet to see if there were any already reported issues for the PG-380. Sure enough I came across this post, which identified a common problem as deteriorating electrolytic capacitors in the filter network. It turns out that electrolytic capacitors go ‘stale’ if left unpowered for a long stretch of time. So, just replace the caps, right? Almost.


MidiGuitar-3 I’m usually pretty reckless (or overconfident), especially with my own gear, but when it’s someone else’s very expensive stuff on the line I tend to be a little more cautious. I prefer to stick with old PCBS, with large thru hole components. Think of your grandpa’s large print books. This was a modern board with tiny surface mount components, something I’ve never dealt with before. I searched around for some techniques I could use to get these little caps off the board and settled on the “hot tweezer” method. This is essentially taking a blow torch to a pair of tweezers until they’re hot enough to melt solder, then gripping the cap and pulling it off the board. This worked for the most part, though there were some persistent ones that I ended up just jamming a soldering iron under an pulling off. That “technique” ended up being a little messy; there is a plastic separator under the caps that melted all over the place. Those tweezers came in handy for scraping that crap off.


MidiGuitar-4 As for the replacement caps, I went with the smallest long lead electrolytic capacitors I could find. I had some of these lying around already and ordered the balance from Mouser. Along with some other stuff for future projects (and posts). Replacement was easy. Cut the leads short, flux the pads, tin the soldering iron, and tack one lead in place. After tacking one lead solder the other post, then fully solder the tacked post. Just like thru hole only you’re tacking the caps on top of the board. It looks a little goofy, but not as goofy as playing a keytar…

Capacitors in place I plugged the guitar in and went to work. Whoa. I hadn’t imagined using a guitar to trigger a synthesizer would be so fun.

Here is a drone sound, with a completely unnecessary string bend at the end:

Audio clip: Adobe Flash Player (version 9 or above) is required to play this audio clip. Download the latest version here. You also need to have JavaScript enabled in your browser.

Here are some chords, which I thought the pg-380 did a pretty decent job of sensing:

Audio clip: Adobe Flash Player (version 9 or above) is required to play this audio clip. Download the latest version here. You also need to have JavaScript enabled in your browser.

And how about a silly FM bass chord:

Audio clip: Adobe Flash Player (version 9 or above) is required to play this audio clip. Download the latest version here. You also need to have JavaScript enabled in your browser.

I’m addicted and I have to give this thing back at some point. Looking around on the internet, these puppies go for upwards of $1500. So, if you have a less than perfect PG-380 for sale, perhaps one that needs some new capacitors, I’ll take it off your hands.

Schwinn Prelude Redux: Complete?

The Schwinn is all back together now with the new paint job, decals, handlebar tape, and cabling: SchwinnPrelude-70 Here’s a before pic for reference: schwinnprelude-5

The red and white striped bar tape was a nice final touch: SchwinnPrelude-71

So what’s next? I might replace the seat with an old Selle Itialia Flite saddle if I can find one, but other than that, we’re pretty much done here. Anyone have a $20 steel bike they want to sell me so I can do this again?

Schwinn Prelude Bicycle Restore: Clear Coat

schwinnprelude-63 It’s been a while but I finally found some extra time to slap a clear coat on my Schwinn redux. I saw “I”, but really my father-in-law took care of this one. Its delicate work, and I’m just not in to that. Maybe next time.

Clearcoating is really to hide and protect. It masks any blemishes that might be left on the bike, and protects the coats of paint you slapped on previously from rust, nicks, and other marring. It serves a third purpose of course; it makes everything “pop”. The red on against cream look was already a nice contrast, but with the clearcoat applied the red really went from bright to ostentatious…in a good way.

I don’t have any pictures of it to describe it, but the chemicals involved in this process are a bit more noxious than anything else we’ve used in this project. Respirators were a must, and the manual was consulted to get the exact chemical composition nailed down. Rick was extra-extra careful on this one:


The clearcoating itself went on in 3 stages, all applied with the same airbrushes we’ve been using thus far. First a “dust coat” was applied, to fill any gaps and lay down a rough surface for the rest of the coats to stick to. Next a thick layer of clear was laid on. Finally, we wrapped it all up with some touch-ups and gap filling to complete the job

The results are excellent. I don’t think I expected the bike to transition from nice to sexy quite as much as it did. You heard me. Sexy. See the photos.

Next up, we’ll re-cable and rebuild the components on the bike. It’s going to be ready to ride very soon…

Schwinn Prelude Bicycle Restore: Decals

With all the painting done its time to move on to puttin’ fancy stickers on the bike. It occurs to me that I didn’t post a picture of the bike with the paint all finished, so here is a shot:


Looking pretty slick. I ordered two sets of decals for the bike from eBay. If you do a search for whatever bike brand and decals you’ll come up with hundreds. In retrospect I should have looked for Roll-Royce decals, hindsight is often hilarious. I kept it real though, and ordered a set of Schwinn feathers and a set of Schwinn Black Phantoms. The Prelude (what the bike formerly was) came with some pretty 80’s decals, replete with scan lines and big bold beautiful Helvetica, so even if I could find some of replicas of them, I would have gone another direction. I ended up not liking the Phantoms. They were a little big and not as elegant as the the feathers. But, the Phantoms did come with a nice red and gold Schwinn badge, which I ended up using.


Putting decals on is pretty easy. Cut them up into sizes that are manageable, soak in lukewarm water for about 30 seconds or so, then set them into the area you want them on the surface. Oh yeah, clean the surface first. Once you get them where you want them, peel back the backing layer while holding the clear decal layer, making sure that the decal stays where you want it. This being detail work (which I’m miserable at) I just let Rick take care of the three decals


It looks a bit, or exactly, like this with all the decals on:


Next, slap a clear coat or two for protection (and of course to cover up some blemishes), then put it all back together and get it on the street.

Schwinn Prelude Bicycle Restore: Painting II

Its been a few weeks, but I finally moved on to the last of the painting over the weekend. In the previous post we painted the frame in its entirety with a primer and the cream colored base coat. We did a test with the red paint for the lugs and details on the fork, and it looked excellent, so we continued using the same process on the rest of the frame.

Cary spent a week’s worth of evening taping off the frame, afterwards I realized that selecting cream instead of “blue-painters tape” for the base coat was a good idea. We hung the frame up in the paint room and went to work on it with the small Iwata airbrush. Maneuvering around the fork was a bit easier than the whole frame, but overall things went smoothly.


I’m not fantastic with the airbrush yet, so Rick had to clean-up for me from time to time:


Some of the details work was delicate, but with Cary’s knock up job taping everything off, I could pretty much apply my usual brand of recklessness without being too worried I’d screw it up. Here’s how it looked all taped up with paint attached:


Cary is currently referring to it as the Superman bike until the tape comes off (Rick took the tape off a day ago and as I understand it, it looks incredible, I’ll get a picture posted soon).

Next post is decals and clear coating, which should wrap up the decorative portion of the redux. After that will be the bike reassembly, recabling, etc.

Ring Modulator: Initial Prototype

ringmod-1-2A few months ago at band practice Bill mentioned ring modulators, and asked if I knew how to make one. I had seem some circuits and knew they were pretty easy to construct so I said yeah, no problem, give me some time. Since my life is basically just going from project to project, I had some other stuff to clear out before I could get to this one. I wrapped up some other stuff and started fleshing out the project after Christmas.

A ring modulator is a device that multiplies two audio signals together. Usually you have a carrier frequency, a sine wave or something like that, and another audio signal like a guitar or piano or something. Bill sent me a really sick example of the Mahavishnu Orchestra playing live, with one hell of a ring modulated Rhodes solo somewhere in it. Most modern ring modulators are actually balanced modulators, but the effect is basically the same. The ring in ring modulator comes from the shape of the diode configuration.

I knew of a couple of circuits off the top of my head, but after a little research settled on this design: which consists of a couple audio transformers, four diodes, and a signal generator. With the exception of the signal generator, the circuit is passive, and if you use an incoming carrier signal instead of a generated sine or square wave, it requires absolutely no power. I could have used the much more complicated “modern” circuit, but decided to stick to the analog approach. Here is the parts list:

  • 2 x TM018 Audio Transformers
  • 4 x 1N4002 Diodes (I used matched pair germanium diodes instead).
  • 3 x Mono Audio Jacks

ringmod-4-2I assembled the circuit on breadboard, using an atari punk console as my signal generator (since it happened to be sitting on the breadboard already), and ran a few tests. To be honest it sounded like shit. At first I was testing with a guitar and just couldn’t get a good sound out of it, it was scratchy at best, and the apc was overpowering the guitar. I figured the output level on the guitar must be too low, then after a few minutes the atari punk console (signal generator) gave out, so I walked away for a while out of frustration.

A day or so later I came back to it with a new strategy. Rather than test with a guitar and the apc, I could just test with two synthesizers, one generating some simple carrier tones, and the other controlling the melody. The XP-50 was already hooked up, and the Casio SK-1 had some batteries in it, so what the hell, I gave it a shot. This was working much better, and after some tweaking I was starting to get some quality sound out of it. Here is an example of using the Roland to play some stupid melodic lines with the SK-1 ‘flute’ tone, and a lot of portamento:

Audio clip: Adobe Flash Player (version 9 or above) is required to play this audio clip. Download the latest version here. You also need to have JavaScript enabled in your browser.

Not too bad, sounds like a ring modulated keyboard. Surely a more talented ivory-tickler could do better. I thought about it for a few minutes and realized that the signal levels of the carrier and source signal would have to be at the same level (not volume) for the output to sound correct. This mismatch in level is why the guitar APC combo didn’t work, but this did, since the keyboards were both at line level. When I build the final circuit I’ll have to remember that. There is also some leakage in the carrier (nobody wants to hear the modulating frequency) which I’ll have to address, its probably due to the voltage drop across the diodes (1.6 volts?), but that should be no big deal. I might also through an opamp on the output to drag the signal level back up.

On the other hand, since I had the thing up and running now, I screwed around with it for a while. The SK-1 is a terrible little synth from the 80’s that samples. So I recorded a ‘whooping’ noise and looped it, then used that as the carrier:

Audio clip: Adobe Flash Player (version 9 or above) is required to play this audio clip. Download the latest version here. You also need to have JavaScript enabled in your browser.

Since the carrier signal was now a vocal sample instead of a sine wave, I could make pretty much any stupid noise I wanted and it would modulate the XP-50 synth…So I made some siren sounds:

Audio clip: Adobe Flash Player (version 9 or above) is required to play this audio clip. Download the latest version here. You also need to have JavaScript enabled in your browser.

and finally, I put the keyboard in polyphonic mode and hit several keys at once. Since the SK-1 changes the speed of the sample to modify the pitch, you end up with the same sample played back at different speeds. When used as a carrier frequency you get this:

Audio clip: Adobe Flash Player (version 9 or above) is required to play this audio clip. Download the latest version here. You also need to have JavaScript enabled in your browser.

Once I box this thing up for Bill I’ll have to build one for myself…