Grant Muller

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:

Ring Modulator Strings

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

Ring Modulator Notes

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

Ring Modulator Chords

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

Ring Modulator Tremolo

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:


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


And how about a silly FM bass chord:

Bass Scale

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.

Inside The Korg DW-8000: Don’t Put Solder on a Battery

DW-8000-1 Recently a friend asked how hard it was to replace the CMOS battery in Korg DW-8000 keyboard. I assumed it couldn’t be that hard, looked up what kind of battery it accepted (CR2032) and said “yeah, 5 minute job”. I failed to take into account early 80s circuit construction. Sure, its no Maestro PS-1B, but I certainly discovered some “opportunities” upon cracking open the case…

Getting it open is easy enough. As with any device built before the iPod age there are far too many screws… 2 in each corner, several straight up the middle, a bunch to hold the rather flimsy keyboard tray in place. etc. NOTE: To open this thing, you need to turn it upside down, make sure you support the bottom right corner so the joystick doesn’t get smashed.

Once open, you’re probably presented with a ton of dust and 3 filthy circuit boards populated with far-from-RoHS compliant components. Dead center in the 2nd board is the battery…




which is soldered to the board.





DW-8000-7 This probably only sounds ridiculous to me, but really, who solders a consumable part directly to the board. Honestly, this kind of thing calls into question the entire circuit design. I got the dead little bastard freed from its pinholes and went to grab another battery holder. I was pretty certain I wouldn’t be able to find a holder with the same pin out, so I opted to get whatever CR2032 battery holder I could find and shoehorn it in there.


I found some suggestions on the internet for doing this, one involved adding some extra wires to the pins of the holder and running them under the new holder. After examining the board I decided a more stable replacement would be to drill another hole inline with the positive lead circuit trace. This is better explained with pictures:



Grab the tools you need. The battery holder (RS #270-009), a couple of reamers, and a tiny drill bit. You can go with just the drill bit, but if you need to widen any holes I like these little reamers.






Look for a spot around the battery circle that is still sitting on the positive lead on the reverse side of the board. Its easy to see through the board to spot the lead, and for me the hole was right next to the T in “BATT”. In this picture the hole has already been drilled.







Start drilling on the reverse side, at least enough so that the lead won’t tear when you drill through on the other side.







Mount and solder the battery holder (making sure that the polarity is correct), then insert the battery. Done.




No sweat, but certainly not a 5 minute job. The moral of this story? If you’re designing a circuit with a replaceable part (like a battery), please don’t solder it directly to the board. The amateur that has to repair it in 25 years will never thank you, but they’ll still appreciate it.

Make Your Apple Pro Speakers Useful

appleprospeakers-2I am convinced that everything Apple develops fits in to one of two categories: awesome or suck. In the awesome category you have stuff like the iPod, iPhone, and OS X. In the suck category, iTunes, iPhotos, and *ahem* their font management. Many years ago, when Apple started putting an ‘i’ in front of anything to make it the future, you could get an iMac. This was a neat little machine, underpowered but fun, that my wife bought to handle some graphic design. This little machine came with a pair of speakers that only sounded alright, but were appropriately labeled ‘Apple Pro Speakers’. It was only recently that I realized which category they fit in

A while ago I decided I’d use these speakers in the workshop so I would have some tunes while I’m working on stuff. I have an old computer down there, so I hooked them up and pressed play. Nothin’. I tried them on a few devices, and realized they only worked on the iMac. Had Apple really made proprietary speakers. I did some research and sure enough, the Apple Pro Speakers will only work on a Mac. This is unacceptable.

I figured at the end of the day, a speaker is a speaker, so with the right kind of modifications these should work anywhere. I looked at the jack and found that it had three sleeves instead of the standard two. The third ring must carry a signal of some kind, meaning somewhere between the jack and the speakers there had to be logic device to prevent sound if this signal wasn’t present. This is the offending piece:

appleprospeakers-1Where the speakers meet there is a tiny round enclosure, if you cut the speakers off at this point, and rewire them to a two ring stereo jack, you have salvaged your Apple Pro Speakers. You could take it a but further though…

I had broken the head band on a pair of noise canceling headphones a while back, and they were just sitting around taking up space. The noise canceling circuit of these headphones contains an amplifier, so I figured why not use the broken headphones to power the speakers, then I can just plug my iPod in and go.

I didn’t get real fancy with this, just pulled the speakers out of the headphones, and wired up the Apple Pro speakers directly to those now unused outputs. Cheapest iPod speakers ever (as long as you don’t count the price of the iMac or the headphones that were previously broken).

So if you have a pair of old Apple Pro Speakers, don’t just throw them away because they suck, put them to good use.

Maestro PS-1B Teardown

maestrops-1b-1 A friend of the band recently had a problem with his Maestro PS-1B, a phase shifter from the early 70’s designed by the famous Tom Oberheim, and asked me to take a look at it. Based on the problem, a loud humming at the output with no sign of any other signal, I figured it should be an easy fix. It was probably just a ground fault or a bad output or something, so I took it home with me and cracked it open.

After getting the case off (which was bolted together with rarely used square head nuts), I flipped it and took a look at the circuit board. I was a little awed, first by the simplicity, then by the age of some of the components. The board was downright sparse. I thought to myself “they could have made this a lot smaller if they had just move everything closer together…”. Here is a shot of the top board:


I noticed that the ground pin on the plug was broken. I spliced in a new power cable and turned it on, plugged in a guitar and my Ritz amp and had a go. Seemed fine. Nice and wobbly, quiet and phase-shifty. Ha, Knew it’d be an  easy fix. I ran the new cable into the back, soldered everything into place and turned it on to test one more time. BUZZZZZ.

That’s odd, I just tested it a second ago and it seemed fine. After checking the solder joints I just made, I turned the unit on again to see if it was still buzzing. Yep. Then I noticed an anomaly, there were two components on the board that didn’t look…native. At the top left there were two capacitors that looked far newer than the rest of the components:


This has been repaired before.

So I did the naturally stupid thing and poked them. No buzz. For once, doing something stupid yielded a solution. The problem was a loose solder joint, one of the caps was literally falling out of the socket. I must have inadvertently been pressing the capacitor back into its slot when I had the cable spliced in. Lucky, lucky catch (though I would have found it anyway, after poring over the other side of the circuit board).

I pulled the board out and turned it over to fix the joint, and was greeted with the strangest circuit board traces I have ever seen in a production machine:


It looks like the solder traces were drawn on with a pencil. Like the design was made casually by hand, without a ruler on a cocktail napkin with a crayon. Incredible. I looked closely at the verbiage on the board (hoping to find a production year), and instead found the name Oberheim


The same Tom Oberheim of Oberheim synthesizers, who went on to create the first polyphonic synthesizer, the OB-X, and the DMX drum machine (not the rapper). Neat. I wonder if all circuit designs during this era we as freeform and organic.

After sitting in awe for a moment, I bundled everything back up, tested it a couple of times and called it fixed. I did a little research on this unit and came up with this site, which has plenty of history on the PS-1, and other effects, not to mention Maestro and Tom Oberheim.

Lessons learned

  • Don’t assume there is only one problem
  • Do something stupid every so often, it might work.

And here is an unnecessary shot of my test setup: