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 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:

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

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

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

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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:

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

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And how about a silly FM bass chord:

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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.

Schwinn Prelude Bicycle Restore: Painting I

In the last post I wrapped up all of the prep work on the bike frame. It was taken apart completely, sandblasted to remove rust and sanded from top to bottom. Now it’s time for painting!

Most of the tools I got to use on this project belong to my father-in-law. In the past he has painted cars and motorcycles, so using this kind of equipment to refinish a bike is probably a lot like using a firehose to fill a bucket, but its a treat to use this high-end stuff.

The first step was to get a coat of primer on the bike. I forgot the name of the primer we used, but its a high-end steel primer that prevents rusting on cars, and requires a two chemical mix to activate it. We hung up the frame in a dedicated paint room, put on some respirators (the chemicals in the paint are REAL safe), and went to work. We used this Iwata spray gun to put both the primer and first coat of paint:


With primer and the base coat on bike, it looked a little like this:


As you can see most of the pitting around the top tube no longer shows:


We pined for a few days about how to do the detail work. Since the lugs, drop outs, bottom bracket, etc were all going to be painted red,we needed a way of getting the paint on there without ruining the base coat. There are at least a half dozen ways to do it, with a brush and a lot of patience perhaps. In the end we elected to have Cary (my wife) tape off the sections we were going to paint with plain ol’ painters tape, and attack it with this small scale airbrush, also from Iwata:


Cary is an expert paint taper, not to mention painter. She warned us about things like leakage, less-than-perfect-lines, but agreed to do it anyway. We elected to use the fork for the first test, so Cary taped the lugs and drop outs and we set to work. There is a small detail inside the lugs on the fork which Cary couldn’t get any tape into, but in a moment of last minute genius, Rick crammed a wad of clay into the detail to block paint from entering. Very nice. We mixed up a very brilliant vermillion paint mixture and hoped for the best.

After a couple hours when the paint had dried, we peeled back the tape…Behold!:


p style=”text-align: center;”>schwinnprelude-22 schwinnprelude-20

It looked great! The little filled in spot on the lugs near the top is the clay Rick used in that divot. The system works. Cary is currently taping off the lugs on the main frame of the bike, which will take her much longer. Thanks wife.

In the next part we’ll wrap up the main paint job, hopefully apply the new decals, and slap a clear coat over the top for a nice professional shine. After that we’ll get to the fun bike stuff, recabling, recalibrating, etc.

Schwinn Prelude Bicycle Restore: Prep Work

schwinnprelude-5Cycling might be the perfect sport for an engineer. The mechanics of a bicycle are beautifully simple, and the gear porn aspect of owning and operating a performance machine may be paramount to the health benefits of actually riding. Data is key as a cyclist as well. Altitudes, cadences, heart rates and watts determine my performance on a ride. I can criticize or praise myself in a quantifiable way, and I have my bicycle to thank for much of it. As much as I like the act of cycling, I still think my main interest in the sport is the device itself rather than the ride.

For years I have been looking for a bicycle to work with. Not a modern carbon/aluminum cycle like the one I ride in races, I’ll let someone else make sure it functions (thanks Cam), but an old steel relic from 70’s or 80’s. About 6 months ago my father-in-law Rick found the perfect bicycle for me, a steel Schwinn Prelude circa 1980-something a co-worker was selling for $20 dollars. You heard me, $20 dollars. He left me a courtesy phone message and bought it anyway, which is fine, I would have paid twice that and still felt like I stole it.

An old Prelude is by no means an amazing bike, but it was exactly what I was looking for. For starters, it was my size, which is rare since I ride something between a 59 and a 61 (tall people don’t ride bikes apparently). It was old, steel, and had exactly the geometry I was looking for. Since I bought it in the Summer, Rick recommended riding it for a while before actually making any changes to the bike. I spent the summer riding it back and forth to the pool, gym, and library, and the occasional training ride. Steel gives differently than aluminum and carbon do, so a ride on the often rough roads of Marietta seemed smoother than usual, and the fit of the bike was perfect. By the end of the fall the Prelude had become my favorite bike.

Once winter arrived it was time to start work on restoring the bike.

If this were an old Colnago or 3Rensho, ultimate care would have been given to restoring it to look exactly how it was meant to look from the factory 30 years ago. But since this wasn’t an amazing bike, it gave us the liberty to do whatever we pleased to the bike in terms of paint and aesthetic. We decided on a preliminary paint job, Rick did several tests and pulled the entire bike apart, and we got to work on the frame.

This would be a good time to mention that most of my in-laws are cyclists, whether it be bicycles or motorcycles. In addition to being an accomplished cyclist, Rick, my father-in-law is an incredible mechanic and has restored (among other things) a Norton Commando and a Triumph, not to mention a Corvette and a Karmen Ghia, to factory floor condition. Some creative painting to a bicycle frame was going to be a walk in the park. However, this is a trade I wanted to learn, so I’m doing the work under the watchful eye of someone with the experience to say “no, that’s just not right”. So far this has been a lot of fun, and I’m already looking for more bikes to restore or otherwise impose my will upon.

Before doing anything we assessed the frame. Whoever owned the bike before me must have had acid sweat from hell, the amount of pitting and rust on the top tube right around the seat post was incredible. Here are some details of the rust we needed to remove:



In order to get these spots out and get the frame in a condition to be painted, it would have to be sandblasted. Sandblasting is easily the least glamorous part of this process. For something this large you pretty much have to sandblast in the open, whereas for smaller pieces you can use a hood. This is what it looks like when I sandblast a bike:


Every hole in your skull is full of sand by the time you’re through. Not to mention your clothes, hats, or whatever is in a 10 ft radius. Its painful too, in a very hard to describe kind of way. Kind of like loading a leaf blower full of sewing needles and blasting them at your face. Awesome. It didn’t take very long, and after little while I was done. Rick inspected my work and it seems that everything went just fine:


The lugs are clear of rust, to the point that you can see the brazing used to join the metal:


The steel was pitted heavily in a few spots due to the rust. For that we used some of this:


Bondo, the same stuff you use on cars. When you apply it, looks like this:


After a significant amount of sanding and reapplying Bondo, the frame was ready for priming and painting, which will be the next post in this series.