Blog Archives

Korg Polysix Battery Repair

Oh, the Korg Polysix.   A wonderful piece of vintage awesomeness, except for the unfortunate tendency for the on-board battery to self-destruct, taking the programmer board with it. I’ve done several of these in the past; this was by far the most challenging to work on.

This unit was not unusual in that the battery had indeed leaked, and I could immediately see several components were damaged. It was unusual in that not only did it have MIDI, it had an original Korg MIDI retrofit installed from the factory. I honestly didn’t know that Korg even made an interface for this. I also had no idea how difficult it was going to make the repair.

The first hint came in trying to access the KLM-367 programmer. The retrofit sits on top. Two of the larger CMOS chips that are normally on the programmer are instead installed on the retrofit, with ribbon cables leading back the the original sockets. There are also hard-wired (soldered) connections to the front panel controls, the key assigner, and the voice board. I pulled the ribbon cables, but had to leave everything else connected.


Factory installed by terrifying space monkeys.

When I first looked at it while the customer was present, the damage didn’t look too bad. With the board out of the unit, though, it was as bad any I’ve seen previously. One of the quad logic chips and one electrolytic capacitor were completely shot.  The cap would be removed for the battery modification, but the chip had to be replaced. In addition, many of the traces were bad.

These repairs are not normally difficult, just time-consuming. I removed the old battery and the damaged components, then cleaned the board. Using the component diagram from the service manual, I marked the traces that would need to be repaired. For the new battery, in addition to removing the capacitor there’s a resistor that has to be replaced with a diode. I also installed a socket for the new logic chip. Once all that was in place, it was time to start adding the wires to bypass all the bad traces.


And this is why I’ve started wearing my glasses more often.
“Oh you need little teeny eyes for reading little teeny print…”

Double-checked everything, installed the new components, put everything back together, hit the power, and…success! Sort of.

The programmer seemed to be working. Kind of. The synth section, though, was not. There were several dead keys. Some of the controls worked and some didn’t. I could create a patch and it would store, but they all sounded horrible. Powered it off, pulled the board, and went over it again. I found one wire that wasn’t soldered correctly, and also noticed that the metal mount for the retrofit was shorting a couple of resistors. A quick dab of solder, a minor change of position, and after I pulled the keyboard and took care of the bad keys it  seemed to be working – almost.

This time, the synth section was perfect  and it sounded fantastic. The arpeggiator didn’t work, though, and while I could still store patches the behavior of the programming section seemed odd.  Realizing that the retrofit might be causing an issue, I tried to locate documentation. A web search turned up a user’s manual, but no schematics.  At least I was able to disable the external sync for the arpeggiator, so now the whole synth side worked as it should.

Ignoring the odd behavior, I tried reloading the factory patches from known-good WAV files. No dice. They’d partially load, but then I’d get an error. I ended up entering the missing patches manually. I also verified that not only does the MIDI interface work, but the additional patch storage locations are accessible.

In the end, the customer picked up the unit and was extremely happy, even with the quirky issue saving patches. It doesn’t affect playability at all, or patch selection. I told him that if I can locate the service information for the retrofit, he can bring it back.

And of course the other positive note is that after this one I’ll never be afraid of doing a Polysix battery repair again.


Prophet & Loss

Just had an interesting repair.

Customer had a Prophet 600. The unit powered up correctly, ran through the tuning cycle, and appeared to be perfectly fine – for about three minutes. Then the control panel would lock up and pressing buttons had no effect. If you turned it off and back on, the same thing happened. Obviously, unless you’re in a band playing REALLY short songs, this rendered it pretty much unusable.

The symptoms would generally lead me to think it was either a power or thermal issue. I dug up the schematics and checked all the voltages; they were all fine. The main chips – including the processor – are all socketed, so I popped them all out, checked for bent or mis-aligned pins, and reinstalled everything. No difference. It didn’t appear to be thermal – I got the same few minutes of normal operation whether it had been on for a minute or an hour.

In my day job I work with computers so at this point I’m thinking it’s a bad chip. The consistency of the failure, though, is unusual. In talking to a friend about it at some point in the conversation I was explaining that this unit is microprocessor based…which got me thinking (not as dangerous as it sounds). Once the unit finishes tuning, it has to monitor the status of all the front panel controls, not just the switches but also the rotary pots. Suppose one was dirty? It was conceivable that the CPU could read that as constantly changing, and hang up attempting to deal with it.

I removed all the knobs, removed the two control panel PCBs from the case, and cleaned & lubed all of the pots & switches. Plugged it back in, waited three minutes and then…it kept working. The first time I let it on for a couple of hours, randomly changing patches or moving controls every time I walked by. The second time, it was on for five hours – still no lockups. Re-loaded the factory patches, made sure the oscillators were properly scaled, and it was back to operating like it just rolled off the line.

First time I’ve run into this, but it’s conceivable that any programmable knobs-and-switches synth could develop this issue.

Siel DK-600

A very interesting synth. It’s one of the first (or last) “original” analog synths with both MIDI and complete knobs-and-switches controls.

As received:

No backup battery installed
Missing most screws
One broken key & one key missing
Missing left end panel
Several broken plastic pieces
Adhesive goo all over panel
Bottom case full of mouse poo

It did power up, though, and made sounds when poked at.

Despite the odd end panels, it’s one of the easiest synths to work on I’ve run across. There are acres of open space, all the cables are nicely run, and everything is labeled. The one downside is the odd shape, which made fabricating replacement end panels a bit of a challenge.

When I removed the main board, under the keybed assembly, I got completely squicked out when I realized the case was full of mouse droppings. Although that was fairly gross, the other thing I found were bits and pieces of the foil wrapper from a chocolate coin. Which may have been rodent food, but since foil is (obviously) conductive it was also a disaster waiting to happen. I’ve gotten some pretty filthy hardware in the past, but this is the first one I’ve ever had to hit with Lysol.

At this point, locating replacement keys isn’t much of a problem. I have several sources, and apart from the fact that one is a slightly different color and is missing a bit of the front apron, they went in without a problem. The velocity-sensitive key triggering mechanism is unusual. In most of the other synths I’ve seen, each key has two contacts, one set slightly in front of the other. When a key is struck, the time difference between the two contacts is converted into velocity information. Here, instead of individual contacts a pair of buss bars is used, and the interval measured is the time between breaking contact with one and connecting to the other.

The missing backup battery was easy to identify – there was a screened sqare labeled “backup battery” on the main bord, with the terminals clearly indicated. After removing the old solder and broken terminals, I tried to find the most sutiable replacement. My first choice is always a standard CR2032 socket. The pin spacing was correct, but there were a couple of resistors and a disk capacitor in the way. Thought of just mounting it on the bottom of the board, but the plastic bottom case has a reinforcing rib running exactly where it would have ended up. With a little creative lead-bending, I was able to mount a 1/3 AA socket on the top.

Downloading the factory patches was a challenge. I was able to locate a WAV file, but spent more than an hour trying to match volume levels to get a clean restore, with no success. Finally, after much web searching, I found a SysEx dump that I was able to reload using MIDI-OX. Somewhere I also dug up the original patch list. The synth came with the patch list on a label stuck to the front panel – hence the paint chips – and I laid out the new list to match the original as closely as I could.

The patches loaded correctly, and everything sounded okay. The odd numbers on the keypad, however didn’t work. I was able to determine that one of the pins on the DIP-style connector on the ribbon cable was missing. These are readily available, but rather than wait until the next time I order from Mouser I rigged a small piece of metal in the socket so that it would contact the little stump of the missing pin. It works fine.

I’d iniitally intended to make very nice end panels, but because of the odd shape decided that a basic set would do, particullary if I was reselling the unit.So instead of oak, I used clear pine, and instead of mounting them on the outside of the case and filling in with more wood, I mounted them on the lip next to the edge instead. After filling in the original screw holes and painting the lip black, it looks fine.

One of the nifty things I didn’t realize until I’d actually finished working on it is that it has a set of fold-down legs to angle the unit upwards, making the control panel easier to access when standing. Kind of neat.

Determinng the value of something like this is always tricky. I’ve seen them sell for close to $800 on Ebay. However, I’d rather trim my nails in a Cuisinart than go through the hassle of trying to sell something this large and this vintage-y that way. I’ve done my time. Instead, I posted it on craigslist with a low yet still fair price. It sold within  a couple of days; the buyer was very happy, I got some hassle-free cash, and it went to a good home. Everybody wins!

Sequential Circuits Prophet 600

This was a recent repair job. When it was dropped off, the owner said it had a bunch of non-responsive keys and he wanted to see about getting the battery replaced. Testing it, however, showed an additional problem. The patches were all really, really wrong, and it would hang up randomly. I let the customer know; he just said “fix it’.

One of the things I absolutely love about older equipment is how easy a lot of it is to work on. Four screws, fold the control panel up, everything is there in front of you.  The date of manufacture – 10/19/83 – is clearly visible here. I was in high school when this rolled off the line.

I’ve got keyboard rebuilding down to a (tedious) science. As expected, it took about an hour. Also as expected, all keys worked.  I checked the battery; despite it being the original I was still getting the correct voltage. Since there’s no way to know how long that would continue, however, I went ahead and replaced it. Although it’s mounted from the top, I had a holder that was perfect for it.

Replacing the battery, of course, completely wiped out the memory. I was unable to get the factory preset WAV file to load, so I used MIDI-OX to push the SysEx files over. One bad thing about this unit is that there’s no indication that it’s receiving, so had to unplug it, schlepp it downstairs, and hook it back up to the amp to see if it worked. And it did! The memory must have been hosed from the start, because all the patches sounded fine and the locking up problem went away. But…

Two of the voices were really, really off; the oscillators were nowhere near each other in pitch. Since the unit tunes itself, that meant that the main voice board needed to be rescaled. Fortunately, SCI built in a relatively painless procedure for doing this. It took about twenty minutes, but after that it was all working perfectly.  I replaced a few missing screws, touched up the stain on the end panels, vacuumed the inside, cleaned the outside, and let the customer know it was done.

He got back a fully-functioning synth, and I got to mess around with a classic piece of vintage gear. And got paid for it,  too…everybody wins!

Weekend Sale Finds – 6/3/2012

Didn’t actually do any yard sales or thrift stores this weekend. I did, however, stop at the Columbus Flea Market on the way to take a friend back home. Since I have several projects going right now, I really wasn’t looking to buy anything. But of course, any lost and lonely keyboards still tug at my heart – and my wallet.

Found two this time. A Casio CZ-230s synthesizer with stand, and a Casio MT-520. Both are a little beat up, but as usual the prices were way too good to pass up.

The CZ-230s is a bit of a rarity, and I’ll do a full-scale review later. It’s essentially a preset CZ-101 synthesizer, with built-in rhythms and limited editing capability. It’s also got strap pegs, so if I ever feel like completely nerding out I can dig out my cape,  put on a little Yes,  and keytar my way around the living room. Okay, that won’t be happening – but I could!

It’s missing one slider cap and one button, but is otherwise all there. The stand is actually quite cool, too. I haven’t run across one yet that can be adjusted to fit a mini keyboard.


The MT-520 is more common (in fact, I just realized I may have one already). This was a straight pity rescue. End of the day, tired seller, dusty little keyboard sitting in a pile of toys. I’ll clean it up, test it, and add it to the sale list.

Both of these were initially spotted by my eagle-eyed friend. Now, I’ve certainly had my share of unexpected finds, but lately the only credit I can take is for knowing what’s worth buying.  It’s good to know that when I go out with someone else there’s very little chance of anything sneaking by.

At current prices, both together would sell for around $140, with another $15 for the stand. Since I paid a very small fraction of that, I’m certainly not going to complain about having to clean ’em up a little.

A Tale of Two Polys

About a year ago I got a reasonably good deal on a Poly 61 and a Siel DK-600. Both were described as “working, but need TLC”. That turned out to be, at least in the case of the Korg, a bit on the charitable side. (The Siel is a future project.) Note that the picture above is not the unit in question – I really need to make more of an effort to take “before” pictures. Trust me, though, it was a mess.

Problems were, in no particular order:

  1. Half the keys were completely non-responsive.
  2. Most of the keys that did work only did so intermittently.
  3. Pressing down on the center of the keyboard caused the whole thing to go nuts – LEDs went random, the sound jumped all over the place. There was obviously a short somewhere.
  4. Most of the screws were missing.
  5. The battery was dead.
  6. A good bit of the plastic laminate on the case was gone, and the case itself looked as if it had been attacked by angry beavers.
  7. The joystick didn’t work.
  8. Although this wasn’t immediately apparent, the mylar ribbon cable that connects the two main boards was only being held in place by gravity and good wishes – it was cracked in several places.
  9. It was filthy.

It did, however, power on.  All the major parts were present; all the panel buttons and LEDs worked. It did make noise as long as you played very gently and managed to hit any of the few keys that functioned.   I added it to my “to be fixed one of these days” list & stuck it in the closet.

A couple of months later, I found another one on Ebay that had no bids on it. The case looked to be in good shape, but the seller said the keyboard didn’t seem to work and all the patches sounded the same (dead battery).  It was yet another as-is sale, as  they had no idea if anything else was wrong. So, I picked that one up too.

With two semi-functioning units my initial goal was to combine parts to come up with one working synth, but the more I thought about it the more it seemed like a really bad idea to trash a piece of vintage gear. So, I went with plan “B”.

Since I no longer have a wood shop, or even just the tools, making a new case wasn’t on the table. However, veneering the old one seemed just as good a choice.

Again in no particular order, here’s what I did

  1. Rebuilt the keyboard – I’ve done enough crappy Roland keyboards to have this down to a (tedious) science.
  2. Replaced the battery.
  3. Cleaned & re-aligned the joystick.
  4. Removed the ribbon cable and connectors, and replaced with regular wire. This was Not Fun – I had to solder both ends of something like forty wires to the headers on the logic boards.
  5. Replaced missing screws and standoffs.
  6. Removed the rest of the plastic veneer, repaired the damage to the case, and veneered with red oak.
  7. Cleaned all switches and pots.
  8. Reloaded the factory patches – which also verified the memory and tape interface work.

A couple of the presets seemed to be incorrect, but I chalked that up to the dodgy WAV file I had to use to restore the factory settings. Entering those patches manually took care of it.  The bottom line is that now I have one functional synth in a pretty sweet custom case.


And the other one? The electronics weren’t as bad. I replaced the battery and rebuilt the keyboard on that as well. The only thing not working is the arpeggiator.  Normally, I would be hesitant to dive into that level of repair, but based on the way it’s acting I know what the problem has to be – I just need to see if I can use the crappy schematics to find the chip that needs to be replaced.

Even if I sell it as-is, though,  I’m sure I can get at least $100 for it, which means I got the other one for a song. A squelchy, buzzy, pitch-bent, impersonal and oh-so-cool song.

Roland JV-80

A much more recent synth – at least for me – and another sleeper.  Still twenty years old (released in 1992), but I’d really rather not think about that. There’s simply no way the nineties were that long ago.

26 voices, 8-part multi-timbral. Sound quality is excellent. The pianos and strings are gorgeous, the basses thumpy and resonating, the brass brassy, and the percussion…percussive. The guitar patches are also standouts.

Those looking for traditional analog-style synthesizer patches should probably look elsewhere. There are a few, along with the obligatory sound effects, but the focus is definitely on more traditional acoustic instruments. That said, there are at least a dozen add-on cards out there, so if you want vintage keyboards or dance sounds, they are available.


This particular unit was another rescue. Despite my previous unpleasant experience with purchasing a keyboard in untested, as-is condition, it was inexpensive enough to be worth taking a chance on. Several keys were missing and the original ad indicated it wouldn’t power on. It’s far more likely that the owner didn’t have the requisite Roland power cord, because it powered right up for me. The interior was filthy, and half the keys didn’t work at all. I cleaned it, replaced the missing keys, checked the battery, cleaned all the key contacts, sliders, and buttons, and thoroughly tested it. The only thing I didn’t do was replace the stupid Roland 2-prong power jack with a standard IEC connector – I was out of them.

As a side note, I find working on all of the Roland keyboards from the early to mid-nineties to be quite painful. First, they need to be opened from the bottom, and I usually forget about the pitchbend lever until a nanosecond after I’ve turned it over. It’s amazing I’ve never broken one. Second, it’s difficult to remove the keyboard assembly, which is particularly annoying as bad key contacts are the most common problem. Finally, the circuit boards tend to be stacked on top of each other. Which means, of course, that the one you need to access is always on the bottom.

I spent far more time playing it than I normally would because even after cleanings some of the buttons were a still  little sticky. They did eventually all started responding correctly.  I really had second thoughts about selling it – the sounds are just that good. The keyboard, although having somewhat lighter action, was very responsive, and as always I found the aftertouch to be so much more intuitive to use than the mod wheel for introducing modulation effects.

Both the velocity and aftertouch features are used to great effect on several of the performance patches. Hitting the keys faster triggers a different sample, so for instance a slow attack will play strings, but slightly faster will add a piano note on top. There were also a couple where the aftertouch introduced another completely different patch.

Patches can be comprised of up to eight tones. The eight sliders and associated  buttons allow the relative volume of each tone to be adjusted or disabled entirely. This makes changing the character of a given patch immediate and fun – everything is in real time. There is a brightness slider and an additional slider whose function changes depending on the patch. The excellent built-in reverb and chorus can be easily switched on and off.

For more specific changes to envelope and filter settings, diving into the menu system is necessary. The buttons and sliders do make it a little easier, but without the manual locating any particular parameter would be difficult.

Even with the manual, it took me a good five minutes to figure out how to change the polarity of the sustain pedal. This is required when using a non-Roland pedal. Commiting all the settings and options to memory would have required too much brain space, which is another reason I ultimately decided to part with it.

For the sounds themselves, I can always avail myself of a JV-1080 module.

Pros: Great sounds. Aftertouch. Well-built. Many available add-on cards.

Cons: Very deep menus. Historically unreliable keyboard mechanism. Some patches sound dated. Few analog-style patches.

Roland JV-80 Manual

Demo songs. These nicely demonstrate the range of sounds this beastie can produce, and its multi-timbral capabilities. I have to admit, though, that either the arrangements or the patches themselves sound a bit “nineties”.

JV-80 Demo


New Listening

The Obligatory Cat & Synth Post

Cuz, you know, cats & synths go together. Even if cat hair & synths don’t.  Jellybean is guarding a lot of broken equipment I unloaded a while ago. Left to right – Korg DW-8000 (dead); Casio CT-310 (broken power jack, bad keys); Ensoniq Mirage DSK (no floppy, intermittent power problem); Kawai K3 (missing keys; no power). With the exception of the DW-8000 they were probably all repairable, but they weren’t worth my time.

The Korg was one of my very few unsatisfactory Ebay transactions. It was listed as “untested but working when it went into storage”. As-is, of course. When I received it,  it wouldn’t power up. My hopes for an easy fix went out the window when I opened it. It had clearly been stored in a really damp area for a really long time.  Like, say, the bottom of a pond – the case was a solid mass of rust. Contacted the seller, who denied that it was improperly stored and wouldn’t consider adjusting the price. Ah well; live and learn.

Listed the lot on craiglist as non-functioning, and still ended up getting a decent price – lots of optimists out there.

The Kurzweil K1000 – Welcome Home!

In 1987 when IBM introduced their new PS/2 computer lineup, mixed in with all the fancy new machines was a nondescript add-on card called the IBM PC Music Feature. Retailing for  about $495, it was essentially  Yamaha’s 4-operator  FB-01 FM synthesizer on a card, coupled with a MIDI interface.  Around this time I was working as a hardware tech in a computer store in New Jersey.  They had actually had a system set up with the Music Feature, a Yamaha YPR-9 piano, and whatever the heck the software was that ran the card (I know it WASN’T Play/Rec; beyond that I don’t have a clue).

Since I’ve been interested in electronic music forever, and since no one else in the store had a clue what to do with it, I talked the owner into letting me take the system home occasionally. So when they finally had someone who was interested in purchasing a complete setup, naturally they had me do the demo.  Those were the days…they sold a $3500 computer with a $500 sound card, based on my dubious selling skills and a badly programmed version of the Miami Vice theme.  I did cheat, a little – in addition to the computer, I also had a Casio CZ-101 doing the guitar parts.  Not that anyone noticed, of course.

The customer wanted a really good keyboard – he didn’t like the dinky piano we were using (well, neither did I).  Given a (literal) blank check, I went to Sam Ash in Edison and picked out the Kurzweil K1000 as a controller.  If I could never actually own one, at least I could visit with it for a while.

And visit I did – I loved it. Loved the keyboard action, loved the look, and loved the sounds. As I recall I sold at least two other complete systems, and each time the K1000 was carefully and thoroughly tested for as long as I could possibly justify it.

Flash forward to, well, now. Although the K2000 series and beyond are firmly established as classic high-end high-power machines, the K1000 series faded quickly. They would show up on Ebay from time to time, usually for a couple of hundred dollars but always with outrageously high shipping. Which only makes sense as they’re made of things like real metal and weigh a ton. But I read craigslist… a lot. And about twelve seconds after one was listed a few weeks ago, I got it.

I have an absolute limit of $100 on any single piece of kit. Although I have a lot of equipment, my cheap nature (or thriftiness) means that most of the things I buy have something wrong with them. But in this case, I got the K1000, the stand, both original pedals, a copy of the manual, and the power cord, for that same $100. The only defect at all – which I didn’t even notice until I was reading up on the unit – was the backlight was bad.

Although it’s not really a big deal, my eyes aren’t all that great anymore.  There are a number of approaches to this, but since I found a guy on Ebay selling replacement EL panels for $25, I went the easy route.

Working on this is a dream. It’s big, well-made, solid, and logical.


From left to right, the processor board, the keyboard logic board, and the power supply. There is a lithium backup battery, but it’s very intelligently mounted on the power supply PCB instead of, say, the middle of the programmer board (yes, Korg, I’m looking at you).

This is the panel board which contains the LCD display.


Sorry about the odd colors – it’s a new camera & I haven’t figured out all the settings yet. Here’s the actual LCD panel with the LCD removed from the circuit board and the EL panel visible, and the replacement EL panel. I used flush cutters to snip the two leads on the existing panel, tinned the little copper tabs on the replacement, and soldered it in as quickly as I could to avoid melting the covering.


This is the panel after the install. Which you could see in the first picture  if the flash hadn’t gone off. You may also notice the display looks a little odd – I initially installed the LCD upside down, which offset the element contacts and shifted the letters over one row.  Memo to self – magic markers are our friends.

I also took the opportunity to clean the inside and out, checked all the connections, made sure the backup battery was holding a charge, and tested all keys, all switches,  and the pitch & mod wheels.  There are no other problems.

And it still sounds awesome.