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Korg Poly-800 (Moog Slayer Filter And Battery Mod)

I’m trying to improve my electronics skills. I thought modding might be the logical next step from changing internal batteries and swapping out tactile switches. I’d like to add MIDI to my Korg Poly-61 and maybe improve the MIDI on my Roland JX-3P. These mods require skills above and beyond what I have now, and I certainly don’t want to wreck them in the process, so when a cheap Poly-800 became available on Yahoo Auctions, I snapped it up in the hopes of trying the Moog Slayer Filter Mod.

As anyone who’s looked at a Poly-800 knows, there are no knobs on the front panel, just a few buttons and a lot of teal. I couldn’t do much about the teal but I could add two knobs to bring direct control of the filter and cutoff parameters to the fore. Seeing as it’s a Poly-800, and they sell for around $100 in Japan, I wouldn’t be too disappointed if I killed it. I could always sell it for parts and get my money back anyway.

Cosmetically, my new Poly-800 wasn’t in terrible shape. There was some mottling on the case but the buttons still had their original color, which is something of a miracle. The tact switches were tight and responsive, clear sign that they had been replaced recently (or perhaps the synth was rarely used). Everything seemed to be in working order. Perfect for modding.

When I turned it on, instead of hearing the familiar organ-like tones of preset 11, I was greeted by a whoosh of white noise and a resonance bump. The internal battery was dead. Before opening it up, I checked inside the battery compartment and there were six C batteries in there. Never a good sign. Many a fine synth has been ruined by leaking batteries. Time to crack it open and see what was happening inside.

The batteries were cheap but not leaking or even particularly old, so someone had been using it recently. There were tell-tale signs of battery leakage inside but it had been cleaned up pretty well, aside from the positive battery terminal, which was crusty and rusted. I didn’t plan on strutting around on stage with the Poly anyway so no big deal, but it’s nice to have everything in good working order. I scraped off as much orange crust as I could from the terminal but it was still pretty corroded.

Next, I looked for the internal battery to check its condition. Usually, the internal battery sits right in the middle of the board that’s screwed to the base of the case. You can’t miss it. It’s right dead center. Except that mine had no battery. It didn’t even have the surface-mount solder pads for a battery. Just an empty space, a void in the board, like someone forgot to finish their work, closed it up, and sent it off into the world.

Apparently there were a number of different revisions of the Poly-800, and the early ones were shipped with no battery. These early Polys stored patches with power from the C cells; remove them and the presets disappear. I bought a fresh set of Cs and loaded in some patches I found online with my iPhone. I turned the machine off and left it for a few hours, and when I came back, they were still there. However, the Poly wouldn’t power on with just the C batteries. I checked the voltage with a multimeter and it was pulling 9 volts but it just wouldn’t power up.

I thought about replacing the corroded battery terminal and even looked around Akihabara (Tokyo’s electronics neighborhood) for one but no such luck. In the end I decided to leave it, since a far more convenient method for saving patches would be to put in the coin cell battery it was supposed to have.

The mod involved clipping out a resistor and diode near the power supply and soldering wires onto the legs that are still protruding from the board. I used a coin battery holder so it could be replaced easily down the line, and some double-sided tape and Velcro to hold it down. A flatter holder would probably be better but this was what I had, and there seemed to be enough clearance there at the back of the synth for it. I closed the synth up, left it off for a few hours, powered it back on, and my presets were still intact. Success. Now I just have to remember to never unplug the synth with the power on, as it could drain the battery.

With the battery in place, I thought I’d move on to the Moog Slayer Filter Mod. I was nervous undertaking this, as the only other mod I’d tried—adding din sync to a Boss DR-110—ended with the poor thing dying in paroxysms with a shorting circuit. But I was determined to improve my skills, so I rolled up my sleeves (figuratively, it’s still cold in Tokyo), got out my tools, and tried to not make too much of a mess of things. Follow along as I bumble my way through it.

If you’re unfamiliar with the Moog Slayer, here are two links: the original site and another with better pictures. The basic idea is you’re disconnecting the internal cutoff and filter calibration trim pots and bringing that functionality up to the front of the synth. To do the mod you’ll need two 50K ohm (or 47K ohm) audio taper potentiometers with knobs, a 470 ohm (or 510 ohm) 1/4 watt resistor, and wire. I got my pots at a shop in Akihabara that specializes in parts for musical instruments.

I recently bought a drill and was excited to try it out. I had been using a small battery-powered one that was really meant for screws, so this new one was a real step up. Look at me, I’m a man with power tools. I had discussed the mod with my father, who has a lot more experience with power tools than I do, and he kept telling me when drilling, start with a small bit and go up one size at a time. I started with the smallest bit to get the hole started, and then because I was impatient I jumped up to a tapered bit. I got it right on the first hole but on the second one I blew past the size I needed and ended up with a hole one bit too wide. The drill tore through the plastic case like a synth fan with GAS in an unboxing video. What was I going to do now?

I hadn’t realized it before, but the each pot I bought had a small, metal tab on one side of the post, perhaps meant to slot into a small divot to hold it firmly in place. I didn’t have a small divot. It didn’t make a difference for the pot in the correct-sized hole but it made the other one crooked. I tried wadding up some thick tape to make up the difference, and ended up using a piece of cut zip tie to hold the pot straight. It’s not perfect but I don’t really mind: it’ll be a reminder to proceed carefully when cutting things destructively.

Next was the wiring. I was a little concerned, having everything happening in the same area as the battery mod, but I followed the directions, making sure the wires all went to the correct posts on the pots. I closed it up, held my breath, turned it on and… nothing. No power. I felt deflated and disappointed but at least there was no loud pop, no smoke, no fiery death. I was exhausted by this point from all the detailed work so I put everything away for the next day.

Thinking about where I might have gone wrong, I checked the bag of resistors I bought. Forty seven ohms. Wait, 47? Wasn’t it supposed to be 470? I pulled out my multimeter to double check the resistance and sure enough, they were only 47. Goddamnit.

On day two, after taking a trip to Akihabara to get the correct resistors, I followed the advice of a friend and undid the filter mod to see if that helped. Thankfully power came back on. The battery was still holding patches as well. Great news! My deflation turned to elation. It wasn’t dead! I hadn’t killed it! I redid the mod, making sure that all my wiring was indeed correct, put in the correct resistor, powered it on and… the LED screen came on for one brief second and then went off.

This was better than no power at all. “Failure is the root of success,” says a famous Japanese proverb. I was swimming in an ocean of failure but each pounding wave was bringing me closer to the shores of success. (Wow, what a metaphor.) I checked the wiring again, trimmed up the loose ends a bit on the potentiometer solder points, and I HAD POWER. I tested the pots through headphones and they worked great. The cutoff was opening and closing, and the resonance was working its magic. I plugged in an amp for further testing and promptly lost power. By now I had a raging headache from stress and inhaling solder fumes so I stopped for the day and went for a run to clear my head. I should have been discouraged at this point but I wasn’t. I knew I was on the right track.

On day three, I powered it up to see how the patient was doing and it worked. I had power, I had sound, and it even played with the amp plugged in. Well, hallelujah. Maybe it just needed some rest. I was tweaking those knobs and having a good old time until I tried programming in a test pattern on the sequencer. When I moved the write switch to “enable” the power went out. Actually, I still had power, I now realized. I could hear it humming in the headphones. But there was no sound and the LED display was dead.

Consulting my friend, he suggested that maybe having the ground in the same area as the battery mod was causing problems, so I moved the ground wire over to the C battery ground terminal. I also moved the coin cell out of the corner, as I was afraid it might have been shorting against the pots or pitch bend PCB when closed. I nervously turned it on, had glorious sound for a few minutes, posted a celebratory mission accomplished-style picture on Instagram, and then lost it again when I tried flicking the tape switch to the enable position. It just didn’t like things happening in that area. I even checked for dry jack solder joints on the board, and removed the unused pin legs that were still in there from the battery mod, just in case.

I looked again at the solder points on the pots. I compared my solder job to pictures on Google. It looked similar but I still wasn’t feeling confident. I noticed some people had used heat shrink tubing to cover their solder points. I had been careful to use heat shrink tubing when joining wires and to cover connection points on the board but not the pots. I didn’t want to desolder everything to put on the tubing so I covered each point with a small piece of electrical tape instead. Well, that was all it needed. It powered up fine, and withstood the tape and sequencer switch stress tests. I guess electricity had been making the jump across the posts via exposed wire. I put a bigger piece of electrical tape across the pots, closed everything up, and it continues to work great.

(Thinking back to my dead DR-110, bad solder points on the pots was probably the cause of death. From now on I’m going to solder on the wires before I screw in the pots so I have more room to work, and I’m going to use heat shrink to protect it. Lesson learned.)

Lastly, to make sure I didn’t blow my speakers (or my ears), I went through and set the resonance in all the patches to 13 (out of a maximum of 15). At 14 it really started to scream but in a non-musical way. Thirteen was hairy enough. I also experimented with the cutoff settings and settled on 60 as a good upper limit (out of 99). If I wanted it open more I could always reprogram the patch.

In the three days(!) doing these mods, I learned a lot. I still have a long way to go but I feel a little more confident in trying some of the other mods I have planned. I’m sure if an experienced technician saw what I did to this Poly-800, he’d post a picture to a Facebook group and say, this is what happens when inexperienced people try to do mods. But how else can we learn? Anyway, it’s not like I’m going to sell it after this. We’ve been through too much together. He never stopped encouraging me through the whole process. And now it’s time to make music together.

Thanks to The Outsider for all the coaching on this mod. You should visit his site, My Synth Fetish. There’s lots of useful stuff there.


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