Blog

1. A JX-8P with just one issue?
2. Freeze!
3. Fixing the VFD
4. Replacing some keys
5. Conclusion
6. Documents

1. A JX-8P with just one issue?

The last two projects went so well. Time to get a bit overconfident. I saw a Roland JX-8P polysynth listed for €250. According to the seller, the only issue was a dead display. Without doing any real research on this particular issue, I decided to buy it. Let’s see if I’m going to regret it.

The synth arrived very well packed, and I powered it up right away to see if it worked. It turned on and produced sound. So far so good. The display was indeed completely dead. I played around with the patches for a minute or so, and then everything suddenly froze. When I turned the JX-8P off and back on, it worked again. I tried to reproduce the issue, and I could. After about a minute, it froze again. If I didn’t touch it for a while, it stayed fine. I started to suspect the power supply.

Another thing I noticed was that the aftertouch seem to work but it needed more force then I was used to in comparison with other synths. I don’t know if I will repair this however as it is playable how it is. I do want to replace five keys which are damaged from the bottom. So that’s something else to look into.

That gave me three issues to start with:

1. Dead display
2. Freeze issue
3. Replace keys

2. Freeze!

I decided to tackle the freeze issue first.

I started by inspecting the power supply and measuring the voltages. I measured 24 volts on the 15-volt rail, which seemed way too high. I noticed the transistors had already been replaced at some point, but it looked like they weren’t proper equivalents. I replaced them with the correct parts: a 2SB507 in place of the 2SB1015, and a 2SD313 in place of the 2SD1406. I also replaced the M5230L regulator IC, since I had read reports of it failing in this synth.

After the replacements, I measured the voltages again. They were a bit lower, but still nowhere near 15 volts.

While inspecting the board more closely, I noticed a burn mark under one of the W02 bridge rectifiers. After reading up on it, I found that it’s best to replace these with higher-amperage versions. I chose a Vishay VS-KBPC602PBF, rated for 6 amps at 200 volts. Unfortunately, they where a bit bigger than expected and the legs didn’t fit through the holes in the PCB. I carefully widened the holes with a 1.5 mm drill bit. They fit now, be it a bit oversized.

I also decided to measure the ESR of the electrolytic capacitors. They all tested fine, so I left them in place. Removing them had been quite a job , as they were all glued down to the board. I think that was done to prevent vibration. I need to read up on how to remove this glue the easy way.

Time to power it up again. I switched it on, and the fuse immediately blew. What could that be? I figured it must be something with the transistors. I took another look and, aargh, I had forgotten to install the mica insulators between the transistors and the heatsink. That caused a short at the 2SD313. I replaced it again and made sure to install mica insulators on all transistors this time.

I powered it up once more, and it worked. I rechecked the power rails, and now they read exactly 15 volts. Success!

The unregulated 7-volt line, however, was still reading 12 volts. Even after replacing the UA7805, which is responsible for that rail, it stayed at 12 volts. After browsing a few forums, I found that many users experience the same thing and that it doesn’t affect the external PG-800 controller, which uses that voltage. So I decided to leave it for now.

But the big question: were the freezes gone?

After several sessions, I couldn’t reproduce the problem anymore. The issue seemed completely resolved!

Next up was the display.

3. Fixing the VFD

I’ve been playing with the JX-8P for a while without a display, and I have to say that with the help of an editor, you really don’t miss having one. I used an online version (https://pg-800.com/), but there are also various hardware models available.

The only thing you still need to do blindly before it works is to turn on the Sysex mode. You can do that by:

1. Pressing the MIDI button
2. Pressing tone numbers 2 and then 6 to enter the MIDI menu
3. Pushing the EDIT slider to max to turn Sysex on
4. Pressing the MIDI button again to exit the menu

But what’s cooler than an ’80s VFD display…? Not much. So I decided to see if I could repair it after all..

While researching the display issue, I quickly realized this wasn’t going to be an easy fix. The JX-8P uses a vacuum fluorescent display (VFD), and the components for it are nearly impossible to find.

The most likely cause of a dead display is the coil driver that supplies power to the VFD. It consists of several fine windings, and over time, one of them can break. I desoldered the coil driver to check it for continuity. During the process, pin 9 literally fell off. I didn’t even touch the thing! . Great… Now I had no choice but to replace it. Or did I?

According to the coil driver’s pinout, pin 9 isn’t connected to anything. In the JX-8P schematic it’s also not connected to anything.

To properly test everything, I measured the other pins for continuity. Pins 1 and 2 were completely open, while the others checked out. So it was definitely faulty after all. Which meant I still needed a replacement.

I started looking for a replacement and found a seller on eBay asking €70 including shipping for a NOS part. That felt steep, so I kept looking. It turns out Sumida had done a new production run of these coil drivers some years ago. (Sumida LC-15) Sadly, those were also sold out everywhere, except in Russia. Several Russian websites listed hundreds of them in stock for just €0.20 each, but none of them shipped internationally. So that option was out. I briefly looked into winding one myself, but in the end, I went with the eBay seller. I had gotten the JX-8P for a good price, and having a working display would make it much more valuable.

The replacement arrived quickly. I tested it for continuity before installing and it checked out fine. The coil driver sits on Switch Board 1, which is hidden under the mainboard. I had to remove the mainboard to get to it.

While reinstalling the display, which is mounted to the case, I noticed the VFD window on the housing was dirty, so I cleaned it with a bit of isopropyl alcohol. That was a big mistake. The display has a black-coated plastic cover with a clear viewing window in the middle. The IPA dissolved the black coating, and my neat rectangular window turned into a blotchy mess. Ouch..

After some more research, I fixed it by applying black electrical tape to recreate a clean frame that blocks out the excess light. It doesn’t look as nice as the original coating from the inside, but from the outside, you really can’t tell the difference.

I reassembled everything, making sure to take good photos of all connectors and cables along the way. Finally, the moment of truth. Would the display work?

I powered it on, and right away, text appeared on the screen. It worked! Another issue solved! That €70 part was worth every cent.

4. Replacing some keys

The only thing left to do was replace five keys. It looked like a mouse had gnawed on them, and when you looked at them from underneath, you could clearly see the damage. This seemed like a nice simple job to finish with.

I had a quick look around and soon found a seller offering keys suitable for the JX-8P. Keep in mind that the JX-8P has small weights under its keys. There are keys from other Roland synths that also fit, but they don’t have weights underneath. So be sure to ask about that when you order them.

To get the keybed out, you first need to remove the side panels (6 screws) and the left side section, which can be detached from underneath with two screws. On the underside, you also need to undo four screws that belong to the keybed. Once inside the JX-8P, you will see three more screws securing the keybed; these can be removed as well. I also disconnected the connectors (three on the main board and one to the left of the aftertouch). This gives you more freedom to work on the keybed.

To remove the keys, you first need to take off a transparent plastic strip on the underside that holds them in place. This can be removed fairly easily. Then, the springs need to be detached. Since I wanted to clean the keybed, I removed all the springs from the keys. Note that the black keys have different springs than the white keys. After that, you can slide the keys out easily.

All keys got a wash, and I cleaned all the contacts with IPA before reinstalling the keys. When I got to number 10, I noticed some did not move smoothly. It later turned out I was pressing the key against a contact, which made it sit differently and prevented proper contact. The result was a key that produced no sound. I had five that stopped working. So be careful and make sure the key sits above the contact.

To reposition the contacts correctly, I used two small hooks. Initially, the contact rests on one strip. When the key is pressed, the copper strip moves to another contact. Both need to be able to make contact. On the keys I had mounted incorrectly, the copper strip was not touching the strip it should initially rest on. By using two hooks to slightly bend the middle strip, I was able to get it back into its original position on the contact.

So if you have keys dropping out, which I understand can occasionally happen with the JX-8P, it may well be that the strips are not making proper contact. I also noticed that they can get dirty or that dust can get between them, which could also cause this issue. Note to self: do not use cotton swabs to clean these sensitive contacts, as fibers can come off and cause more problems.

Anyway, everything looks fine again now.

5. Conclusion

This project actually went quite smoothly. If I had known beforehand that the VFD coil driver was almost impossible to find, I might have made a different choice. I was probably lucky that a NOS one was being offered for sale on eBay. Nevertheless, it can be solved if you have the right component and are willing to wait until one is offered for sale.

It did not go entirely flawlessly, considering the forgotten mica insulator tabs, the blunder with the display, and accidentally bending the contacts on a few keys, but all of that was nicely resolved as well.

Job done!

6. Documents

1. Purchase
2. Preparation
3. The faulty oscillator
4. Replacing the capacitors
5. Conclusion
6. Documents

1. Purchase

During a weekend trip to Antwerp, I received a notification from an auction site: a Formanta Polivoks was up for sale. The Polivoks was listed as faulty, as one of the oscillators no longer worked. What also stood out were several marks on the unit, as if a soldering iron had been dropped on it.

That said, it was complete, none of the knobs were missing, and the cover was included as well. I could buy it for €400.

The Polivoks, a relic from the Soviet era, had been on my wishlist for some time, so I didn’t have to think twice. I decided to pick it up on the way back.

2. Preparation

During my previous project I really missed an audio probe. I really wanted to have one for this project so I ended up building one using instructions from this website. I also decided to invest in a bench power supply. While reading up on electronics safety, it became clear this was a good purchase if I wanted to take on more projects in the future.The idea behind a bench power supply is that you can connect its output voltages directly to your project’s power rails, so you don’t have to work with 230V mains, eliminating another risk!

I went with the Korad KD3305D. It’s an entry-level model that can supply three different voltages, which should be more than enough for most gear.

3. The faulty oscillator

The Polivoks has a modular design. When you look inside, you’ll see a mainboard with various plug-in cards.

Each of these cards has a specific function. For example, there are two VCO cards, a mixer card, and a filter card. This modular approach makes troubleshooting a lot easier! So naturally, the most logical place to start when diagnosing the issue with oscillator 2 was the VCO board.

This gave me the perfect opportunity to put my bench power supply to use. I removed the board and connected it to the bench supply. The required voltages were -12.5V, +12.5V, and ground.

I had to take a moment to figure out how to configure the negative voltage. Here’s how I solved it.

I grabbed my new audio probe to check whether a waveform was being sent to the connectors at pins 7, 8, and 9. One of the VCO boards was outputting a signal, but I couldn’t detect anything from the other one.

I had a hard time finding the source of the audio signal, because there wasn’t one. 😉

I had already replaced a few capacitors, and even reinstalled some that turned out to be fine. At one point, I suspected a faulty IC and ended up ordering a replacement all the way from Erica Synths in Latvia. I’ve realized that in some situations, I still struggle to judge what values I should expect from certain components. That’s led me astray a few times, thinking a part was faulty when it actually wasn’t.

Fortunately, the extra components didn’t cost me much this time. But next time, I want to make a better effort to understand the exact function of each component and its role in the circuit. That should also help me interpret measurement values more accurately.

I decided to do a visual inspection as the oscillator still wasn’t working. It looked like one of the transistor legs was broken!

I decided to replace it, and sure enough, the oscillator started working again! Sometimes the solution is surprisingly simple. I have no idea how it happened, but this just goes to show, it can really pay off to take a good, careful look before you start measuring.

4. Replacing the capacitors

The Polivoks actually worked quite well. All the controls were functioning, and I didn’t notice anything unusual, apart from the keyboard, which is the worst keyboard I’ve ever played on. Apparently the keyboard action was as it should as I read on the internet.

The only thing I still wanted to do was replace the power supply capacitors. You can swap them out with two 2000µF 50V capacitors. The capacitors are mounted in a rather awkward way inside the case. The plastic bracket holding the capacitors was cracked, so I made a temporary replacement and secured it with a zip tie. That worked well.

5. Conclusion

Another classic synth saved! This time it took a bit longer than expected to pinpoint the fault. Unexpected, since it was clear right away that the issue had to be on the VCO board.

In the end, I learned once again that a good visual inspection can be just as important as measurements.

So, is the Polivoks fully working now?

Yes, it’s working but I’d still like to do a modification. The Polivoks doesn’t have CV/Gate inputs, so I can’t control it from my DAW. .Luckily, there’s a mod for that, I already have the schematic, and at first glance, it looks doable.

So, in short, another project might be on the horizon!

6. Documents

Polivoks English schematics

1. Purchase
2. Cleaning the Cat
3. The first run
4. J-wire madness
5. Fixing the oscillator
6. Replacing the trimpots
7. Fixing the keybed
8. The finishing touch
9. Conclusion
10. Documents

1. Purchase

Alright, I’ve got everything I need at home to get started, so it’s time to look for my first project! I came across an ad on German eBay for an Octave Cat synthesizer in terrible condition, listed at 1500 euros. According to the seller, it still worked, but he couldn’t test it because it was a US model running on 120V and he didn’t have the right fuse. I decided to make an offer well below the asking price, and in the end, I managed to get it for 400 euros. Great! That gave me some room to fix it up, because as you can see, there’s quite a bit wrong with it. And something tells me that fuse didn’t blow without a reason…

2. Cleaning the Cat

After the Octave Cat arrived, I decided to give it a good cleaning right away. I didn’t have a step-down voltage converter and the correct fuse yet, so I couldn’t really test it anyway. I peeled off all the stickers and carefully scraped off the leftovers with a plastic spatula. The rest came off easily with some sticker remover. I also cleaned everything, including the keys, with a degreaser. The result was satisfying!

I went ahead and gave the inside some attention too. There was a lot of dust in there, a whole lot!

3. The first run

Alright, the step-down voltage converter and fuse have arrived. Time to start testing! I decided to first measure the voltages at the PSU with the connectors disconnected from the power rails. That way, if something was off, the rest of the Cat would be spared. The voltages were just as shown on the schematic. Perfect! The ESR values of the capacitors were still reasonably within spec too. Everything held up, so I reconnected the PSU and powered it on.

As soon as I turned it on, there was a lot of noise. Maybe the noise generator is active? Yes, that was the case. I tried to get some other sounds out of the Cat, but only the top 8 keys were working. I decided to measure the CV at the keybed output. The top keys showed activity, but the ones below didn’t respond much. Time to take a look at the keybed.

4. J-wire madness

The Octave Cat has a Pratt & Read keybed with so-called J-wires. These are nickel-plated steel wires that act as electrical contacts, closing the circuit when a key is pressed. There’s one for CV and one for gate. The idea is that they touch the bus bars in a specific order when a key is triggered. The CV should connect first, followed by the gate. If this happens too early or too late, the pitch can glide, like portamento is on. That was exactly what was happening with the top keys.

I couldn’t clearly see whether the lower keys were hitting the bus bars in the correct order, but I did notice they were corroded. I decided to carefully clean all the J-wires with isopropyl alcohol. I used small strips of paper sprayed with IPA and gently rubbed them under each J-wire. They were extremely dirty. The notes slowly started coming back, but not all of them worked properly yet. Lots of glides, way too many glides.

This part of restoring the Cat gave me the biggest headache because adjusting the J-wires is very precise work. I found a great video by George Terbush that explains it well. Below is an example of how to best bend the J-wires in relation to the bus bars.

But even after doing all that, it still wasn’t always working properly. There had to be something else wrong. I took a closer look at the keybed and sure enough, one of the mounting screws was missing. There should be four, but mine only had three. I didn’t realize that could have such an impact, but it really did. After fixing that, adjusting the J-wires became much easier.

5. Fixing the oscillator

Oscillator 2 was not working. I did a lot of measuring with my multimeter and pretty quickly caused a short circuit. I had damaged a metal-can CA3140. I probably touched several pins at once. Unfortunately I had to replace it with a modern one and placed it into a DIP socket. After much testing, I traced the problem to the CD4066 chip. I didn’t have a CD4066 on hand so I used a CD4016 instead. According to ChatGPT, this can sometimes work well as a substitute. At least the oscillator was working again. I plan to replace it with the correct CD4066 later.

Later in the process, while tuning, I noticed I couldn’t get VCO2 properly set to a frequency. After a lot of searching, I came back to the CD4066. I had forgotten to swap it back after replacing it with the CD4016! Apparently, that substitute didn’t work very well after all. Once I switched it back, everything worked perfectly again.

II also replaced all the electrolytic capacitors. Later, I thought back on this and wondered if I had done the right thing. My reasoning was that they were already 50 years old and due for replacement. On the other hand, the Cat was working fine and this might have affected the sound. What do you think? I’d love to hear your thoughts in the comments.

6. Replacing the trimpots

I had already replaced a number of components and felt like tuning the Cat would make it better. Unfortunately, the old trimpots were sealed with lacquer, and no matter what I tried, I couldn’t get them to move.

I then decided to replace the trimpots. Also because users had reported that the Cat is difficult to tune and the trimpots are quite sensitive. I bought Bourns 3362 trimpots for this. They’re not direct replacements as they’re not being made anymore. The pin count and the footprint were different, so I had to make an adapter myself using stripboard.. Below you can see how I did it. It might not win any beauty contests, but it works well and I can even access it from the top of the Cat through the trimpot openings.

I still found the Bourns trimpots quite sensitive. I was able to tune the Cat properly, but next time I might consider using multi-turn trimpots. That might be even more practical. When buying trimpots, also pay attention to the resistance values. There are several different values, and you can refer to the schematic for that.

By the way, those hole covers for the trimpots on the Cat’s chassis can only be opened from the inside. That didn’t seem very practical to me, so I want to replace them with rubber hole covers. That way I won’t have to open the entire Cat and remove PCBs just to get access.

7. Fixing the keybed

There was still a broken key I needed to replace. I tried to find a single replacement key, but that seemed very expensive to me. So I started looking around for similar keybeds. The Octave Cat uses a Pratt & Read keybed. On eBay, I saw a complete 44-key Gulbransen keybed that looked very similar for 20 dollars. That was worth a gamble.

When it finally arrived, I noticed the color was a bit different from my Cat’s keybed. That was disappointing. I decided to replace the entire keybed. It fit perfectly! For now, I chose to leave it like that. If I find keys in the right color later, I can always swap them out.

When I first got the Cat, I noticed the keybed sagged a bit in the middle. As I still had some trouble adjusting the J-wires in the center of the keybed, I decided to take the keybed completely apart. I also still needed to replace a trimpot that was positioned on the pcb of the keybed and wanted to replace the bushings. Dismanteling the keybed was quite a bit of work because everything had to come off, even the PCB.

There are a few things to watch out for if you decide to do this:

• When you remove the keys and brackets, lay them out so you can put them back exactly the same way later. That will make reassembling much easier.
• Watch out for the springs! I spent an hour on my knees looking for one, only to find it lying right on my desk. You can easily remove the springs with a small flathead screwdriver. The springs have different colors for the black and white keys (silver for white, red for black).
• Also, check whether the keybed is properly aligned. Use a long ruler to see if there are any significant deviations between different points.

After aligning the keybed, it seemed the J-wires in the center had more clearance. This made it easier for me to position the J-wires correctly. I also replaced the bushings of the keybed during this process. They were quite dried out and needed replacing.

8. The finishing touch

The inside of the Cat is now in good shape, but the outside could still use some love. The Cat is missing a few knobs, and the ones that are there are very discolored. I found good replacement knobs at Digikey. Below are the part numbers:

Unfortunately, the silver-colored knobs are no longer being made. I was able to buy them from a forum member on the Dutch Synthforum. Now the knobs are complete again!

There was one last cosmetic issue: a bare spot on the bottom left next to the keybed where the paint had worn off. I lightly sanded it and used black hammered finish paint. It looks great again. I also used it to touch up some small spots. Below is the result!

9. Conclusion

I’m quite proud of how everything turned out in the end. The Cat can last for a few more years, and I really feel like I saved it. I also learned a lot about how synths work and how to best test certain components. My first time soldering went perfectly after watching some videos. Would I have done some things differently in hindsight? Maybe, but that’s all part of the learning process. A few points:

• When adjusting the J-wires, make sure the keybed is in good condition and properly aligned with the Cat’s chassis. Positioning the J-wires can be a time consuming task, but keeping this in mind will make the process much easier and less frustrating.
• Buying components was pretty tricky. You need to pay close attention to values and specifications, and the overwhelming number of brands doesn’t make it easier. Does anyone have tips for this? Buying single components wasn’t ideal either—sometimes the shipping cost was higher than the component itself.
• Finding an audio signal is difficult if you don’t have many tools at your disposal. For my next project, I’m thinking of making an audio probe. That might make things easier.
• Sometimes I got a bit too enthusiastic with probing. Some ICs didn’t like that. I have to be more careful next time.
• After a while, I noticed that the step-down converter I was using was a bit noisy. I think it should be easy to convert the Cat to 230V so I won’t need the converter anymore. A project for later.

All in all, a successful first project. On to the next one!

10. Documents

1. Goal
2. Safety
3. Disclaimer

1. Goal

When I started this blog, I had no experience with electronics repair. I always wanted to fix equipment but I didn’t know if it was doable. My interest in vintage analog gear made it clear that learning to fix it myself would be quite useful. After all, old equipment often breaks down due to aging components or simple wear and tear.

I have a background in computer science and some experience with programming, where I dealt a lot with debugging. I tried to take a pragmatic approach by focusing only on repairing equipment, not on designing circuits from scratch. That felt like a somewhat different skill set. I tought if I could manage that, It would really benefit me.

I decided to dive into the basics of electronics. I bought a book (Electronics For Dummies) and took a short course with Laudius.

To get started with repairs, there are a few essential tools. The list below isn’t meant as buying advice, but more as an overview of what I thought would be necessary or useful. It’s not state-of-the-art gear, but I didn’t think that was necessary for a beginner anyway.

Plus Soldering accessories like: 60/40 tin, soldering braid, flux, wires, pliers etc..

I think I spent around 400 euros in total, including the course and the book.

2. Safety

Working with electronics requires care. I’ve had some experience at home with wiring and electrical installations, but repairing equipment involves different kinds of risks. I decided to focus only on low-voltage circuits at first, avoiding tube gear or power amps. While devices like synthesizers may be connected to 230V mains, the voltage is usually stepped down to safer, lower levels after the transformer, making these circuits less hazardous to work with.

3. Disclaimer

I am not responsible for any damage, injury, or loss resulting from the use or misuse of the information provided. Anyone attempting electronic repairs does so at their own risk and is solely responsible for understanding and managing the hazards involved.