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Michael Kukat
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Weekend hack - universal keyboard scanner

To make use of this new Fatar keybed, i need a scanner for it. And as usual, it has to be flexible for lots of different purposes. As the development will happen in an iterative way, i decided to solder it on veroboard first.

STM32F103C8 does the work, 2 74HC138 to select the rows in the matrix, and some analog stuff might be added in the near future.


Velocity with configurable curve, release velocity with configurable curve, both can be disabled independently. "Turbo mode" to just use the lower contact of the keys, this implicitly disables release velocity but should allow extremely fast playing. I don't think anybody needs this, but it's not much effort to add it.

8 analog inputs für wheels, joysticks, whatever. First one will be for the after touch strip. This analog stuff is what might need some more circuitry.

Scan rate likely will be 4KHz, maybe even 8. The scanner can handle up to 96 velocity-sensitive keys (or 192 keys without velocity) and the final PCB layout will be compatible with all Fatar keybeds from 25 to 88 keys. There are still some GPIOs left over for buttons or LEDs or whatever. Or a real time clock. Or a PS/2 interface for a notebook touchbad, if you prefer this over a pitch bender.

Interfaces with USART and USB, so classic serial MIDI and USB are possible, but the USART can also be used within a keyboard synth, where it also serves to upload the firmware - even on a bare chip using the STM32 boot loader.
Adding WiFi or Bluetooth and running it on battery for a keytar should also not be a problem :)

Tomorrow: Firmware.
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Things you usually don't care about

During the day, i had a quick idea concerning synthesizer keybeds.

I already had ideas in the past to replace ugly keybeds in synths by better ones, for example crappy Roland boards with dissolving weights or unreliable contact springs by good Fatar boards. But i never really considered it because the mechanical differences might be too big and i don't have a Roland keyboard synth anyway, so not my problem. And with all the stuff i have, there aren't big problems.

Okay, my Hohner String Vox has a totally dead keyboard, but that's not a problem at the moment, because the machine sits in the archive anyway :)

But what if you have a really broken keybed like a Pratt-Read one in an ARP 2600 or Minimoog? A friend has this problem, his ARP 2600 keyboard is FUBAR and absolutely unplayable. I added a CV/Gate input a while ago, so he has MIDI and still uses the portamento stuff of the keyboard for this. The intention was to repair the keybed one day. If we get the spare parts, tons of patience and motivation.

But will this really happen? I don't think so. What if you throw away the 3-bus Pratt-Read keybet, adapt a Fatar keybed that already mostly fits, add a micro to scan the keyboard and generate the signals that usually come from the keyboard? Yes, while you're on it, you can add MIDI also. A clean solution and if done right, it's not even horribly intrusive. You might need to drill some holes for the new keybed and MIDI, solder some wires, done. So the original condition is not totally damaged and the change is mostly reversible.

No, it's not a project yet, just an idea. But what do you need to replace the keybed? Some measures. Length, width, height and so. The width of standard keyboard is pretty consistent, just the GDR had slightly narrower keys. This was what i thought until today. But i just took some measurements of the width of 49 keys (C to C, 4 octaves + 1 semitone) and was a little bit surprised.

My AAC Tiracon 6V in fact was a bit narrower with 665mm than "the rest" with 680mm. So far, so good. The rest (Fatar keybeds, SCI MultiTrak, Crumar GDS) mostly were 680mm +/- some mm, Fatar was the widest with 683mm on the Wersi stage piano (TP/40 i think) and the Kawai K5000W (TP/8S). The surprise was Yamaha. The DX21, AN1x and CS1x all had the "GDR measure" with 665mm.

So now i know that when considering such a conversion, it's a good idea to have a closer look at the keybed width, not just for GDR keyboards. And my AN1x will never get a Fatar keybed. Not that the original one is that bad. Only the CS1x has an ultra-cheap keybed, but that fits it's capabilities, i just have it as a visual accessory for the AN1x :)

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Single-Vidicon/Newvicon camera testing

Some minor miscoloring. But just a little bit. Barely noticeable :)

Thrown a test image to my TV (right) and recorded it with the SABA CV-79 (aka JVC GX-N70E) Newvicon camera. The two photos are with and without the ND filter.

Some days ago, when i had it open to check something else, i already tweaked things like scan width, linearity and so and it looks like the problem is not the electronics but the tube because changing the scanning leaves the image defects in the same position of the image (as projected onto the tube's target), not the output.

The manual tells me i need to run it 2h every 6 months to avoid that the tube degrades. Assuming the camera wasn't used for nearly 30 years, that's, hm, 120h. I'm not sure if the rest of the electronics will survive that long at all :)

The other camera shows a more uniform coloring, but it lacks nearly everything except green and a little bit of red.

At least i think i can use my TV as a substitute of the pattern box with stong lighting they require in the service manual for the adjustments. If i ever am so much bored that i really readjust this one.

Technically speaking - if you want an old single tube Vidicon based camera, don't expect to get any usable image from it. Two failures on Newvicons, the Trinicon (Sony HVC-3000P) works fine after removing the rotten IR filter. I might get a Saticon (a Nordmende branded JVC GR-C1, the one from BTTF) from a friend in several months, so another candidate, but i think all those things really are just usable as decoration those days. But for that, the SABA really looks nice. Maybe i get a Sony DXC-1820P or even DXC-1640P some day.

But with the SABA camera, i get full remote control with the Panasonic VHS recorder, a feature that wasn't possible with the two original combinations (Olympus/Panasonic camera with Panasonic recorder and SABA/JVC camera with Sharp recorder).
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SABA CVC-79 / JVC GX-N70E plus SHARP VC-220N

Another member for the vintage tube video camera collection. I wanted another Newvicon camera as reference for the Olympus/Panasonic camera i recently got. One with an available service manual, which is not the case for the Panasonic.

Then i saw this one. In nice condition. With VHS-C recorder, manuals, 10m extension cord for the camera (if your personnel to lug around the recorder is currently not available), manuals and a box of tapes. And "SABA", a brand that brings back so many childhood memories. All this for 20€. Couldn't resist.

And yesterday it arrived. I was a bit disappointed because i paid more for DHL shipping but he shipped with Hermes instead, he promised some MiniDV/music/VHS tapes as a substitute for some VHS-C tapes that he no longer had (20 were offered with the package, only 10 were left), but they were not included, instead i got a printer cable nobody needs and the manuals are trash, all the pages stick together, they definitely got wet in the past. At least it was very carefully packaged and arrived undamaged. You can't have everything.

As i was a bit concerned about the optics after the experience with the Sony a day before, i immediately tested it - usually, i first have a look at the capacitors, but as the seller said it works, i didn't do it this time. Okay, 5 minutes later the Rifa suppression caps exploded, but at least the image from the camera looked okay :)
The PSU is extremely compact, so maybe i wouldn't even have it checked that deep to find the Rifa caps, but so i was forced to do something.

So yesterday evening got a bit longer, i wanted to repair the PSU ASAP to get out the debris and smell of the exploded capacitors, then i wanted to have a look at the tube's front, especially the IR filter, and so i couldn't stop and took everything apart, cleaned it, put it back together. Just the viewfinder had to wait until today. I inspected all electrolytic capacitors, none of them is leaking, and it might look like the primary caps in the PSU are bulging, but that seems to be the photo, in real life i didn't consider them to be critical. The tube front end is in great shape. I found the front bias illumination, the Olympus/Panasonic camera seems to have only back bias. You need to give Vidicons some "bias light" to reach a threshold where they start delivering useful output, this usually happens with LEDs close to the tube's socket, and in this case with 2 incandescent lamps and a light guide at the front, close to the target. Damned. I forgot to check the lamps. Maybe this is the reason that the image shows some vignetting. It was too late around midnight to care about everything...
In the VCR, everything was fine, just some resoldering of the connectors was necesary to remove some audio dropouts.

At the end, this is a very nice package for my collection, the VCR looks heavily used when having a closer look, but from distance, the scratches are not so noticable. Maybe i'll record some test videos soon, with a (very unfair) comparison of my modern Full HD Panasonic camcorder versus this luggable vintage beauty.
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...once again.

Maybe i already told you that from time to time, not so often, at least not more often than every 3 minutes, you get into trouble with capacitors in electrronics.

Today, i got a new vintage electrronics toy and as the seller said something about the condition, i didn't disassemble it before first power up, it's too late anyway if he already powered it up within the last days. I have seen the 12V DC input but decided to power it from 230V, what could possibly go wrong?

Everything worked fine for several minutes, but then, a cracking noise and horrible amounts of smoke came from the power supply. Before i could even smell it, something went through my head: "damned suppression caps, f**king crazy plastic crappy suppression caps! caps caps caps kill kill kill!!". Everything still worked great, absolute no power problems. But due to the smell, i had to power it off, close the door of the room and open the window. For a long time. And consult the service manual on how to get to the caps. This thing is very, very evil to take apart, so in fact those exploding caps forced me to inspect the rest, i wouldn't have taken it apart otherwise.

No big deal to repair this damage, but i hate it when that happens in winter. You need days to get this smell out of the lab again. That's many, many hours of an open window with temperatures around 0°C outside.

More about the whole package during the next days. Teardowns and tests following. I involves heated image processing devices and magnetic recording/playback. Again.

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Sony HVC-3000P

And today, we put together another nice retro video camera.

After i recently got this Newvicon video camera, i read a bit about the different technologies used back then because before, i never cared much about image sensing tubes. I knew that they exist, i knew how they work and i knew that you need 3 of them for color video. Which was wrong. There were several approaches of using one or sometimes two tubes to grab a color image. And the most common ones are the Newvicon/Saticon using a stripe mask in front of the target which is designed in a way that makes it possible to synchronize the different colors from the amplitude of the signal alone. The other ones are the Trinicon tubes by Sony, which use a standard RGB mask (not verified) and need an auxiliary electrode to sync the scanning with the color stripes.

All in all, the Newvicon/Saticon tubes have weak colors and a tendency to green/purple tones, the Trinicon tubes had much better color representation. A bit like PAL versus NTSC :)

So i wanted a Trinicon camera also. For research, sure. I found a very nice DXC-1820P, but obviously, the seller had some serious issues with her mental health or simply no clue what this "professional video camera" she sold really is. She wanted 450€, normal price on eBay is 30-50€, maybe up to 100€ if it really works and produces good image. So no deal. The next day, i bought this HVC-3000P for 15€. It doesn't look as cool as the DXC, but it's enough to give Trinicon a try and it's still good enough as decoration.

Today, it arrived. And i decided to make some photos of the innermost guts, because for this one, i have a service manual. So nothing can go wrong. After removing the lens, i got a shock. The pickup tube looked very bad, like lens fungus or so. The camera in general looked like it was kept in a damp area, but this one was evil. Okay, not an issue, i bought it as defect and for 15€ i really don't complain.

So i removed the tube to have a closer look. I found a video of someone taking such a tube apart (interestingly with a very similar issue) and within the first seconds i knew what to do:

No, not cracking it open so brutally, but trying my lens tools - voila, i could unscrew the retainer. And it was a big relief to see that only the front filter is damaged, the next thing (maybe the optical lowpass filter) looks great. I didn't get deep enough to see the RGB stripes, but i don't want to damage this delicate piece of historical technology. I cleaned the filter (i at least assume it's a filter for UV or IR) with standard household cleaners like hydrochloric acid, which helped a bit to remove the dirt, but the glass surface seems to be damaged, so there still are spots i wasn't able to remove.

Anyway, while putting it together i have seen that i don't need to remove the tube, i can reach this filter through the lens channel. So maybe i completely remove it for testing next time.

Another issue was the motorized zoom no longer working - the motor was stuck. Some fresh grease through it's maintenance hole fixed the problem. Yes. The motor has a maintenance hole for this. And the belt for the rest of the zoom assembly is worn, but it still works good enough.

Well, the rest is just putting everything together. Very nice construction, all connectors (except 2) are labeled good enough that you know where they go, no real challenge taking this thing apart and putting it back together.

The EVF CRT is pretty large compared to the ones from the more modern CCD cameras i recently posted. I think it's a 1" CRT or so.

Anyway - first power up (the connector is missing, which was the reason why it was sold as defect, but i'm happy with this because i don't have a suitable recorder anyway), video appears on the EVF. The video output on the cable seems to be dead, but as the EVF, which gets the same video signal, shows an image, this can't be a big thing.

Some quick tests have shown me that there very likely is a chance that the tube can be rotated to adjust the image :) And i can clearly see the remaining spots on the front filter.

Too bad i couldn't test color quality yet due to the missing video output, but i think i'll have a look at this during the next days. Until then, another Newvicon camera with VHS-C recorder should also arrive, one for which i found the service manual, so it might be a reference to possibly improve the other Newvicon camera. And then i urgently need to stop collecting those old cameras. Or maybe i want a sony DXC-M3A (triple Saticon) or DXC-3000P (triple CCD)? They look so cool. Really. And they are not that expensive.
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Component failures in electronics

I just had the idea to put together a list of component failures i observed during the last decades of repairing lots of different electronic gear. The list might not be complete yet, but i tried to put together what comes to my mind at the moment. The age of the components also is interesting, especially with capacitors, the lowest amount of problems with them is with 1970s gear, while all 1990s gear, when they started with those SMD aluminium electrolytic caps, has leaking capacitor problems.

Most observations described here are valid for normal operating conditions. Bad design, especially thermal problems, increases failure chances significantly. Especially for capacitors next to heatsinks, but also semiconductors like linear regulators or transistors.

Those are my personal observations and might not exactly match the observations of others. I didn't add any problems i just heard about, no urban legends here, just my own experience.

Tubes (mostly up to 1960s)
- Normal aging, fail slowly, degrading current flow, rarely current leakage a while after power-up or even shorts
- Applies to video pickup tubes (until early 1980s) and CRT display tubes (until early 2000s) also
- Rare vacuum loss (detectable by getter mirror getting white)

- Rarely fail
- In tube radios, resistance might increase with age (up to 1960s)
- Found open resistors from time to time, even ones with close to zero power dissipation (even with 2000s gear)

- Tube gear - oil/paper capacitors (up to 1960s), there are many lists of bad types out there
- Pre-1980s electrolytic caps rarely fail, but might need reconditioning if not powered up for many years, can overheat and explode otherwise
- Supression caps with plastic housing cracking open and exploding (1960s to 1990s)
- Ceramic caps shorting (rarely, 1970s)
- Tantalum caps shorting (especially 1970s-1980s), often exploding or frying protection resistors (which were there for a reason back then)
- Some types of electrolytic caps leaking (1980s)
- SMD aluminum electrolytic caps nearly always leaking (1990s)
- Electrolytic caps in switchmode power supplies drying out, (late 1990s until today)
- Cheap electrolytic caps losing capacity, sometimes dead already after 3 years (today)
- Metal film capacitors losing capacity, especially in capacitor power supplies for RC power outlets and similar stuff (today)
- Metal film capacitors shorting (rarely, 1970s)

- Selenium rectifiers, aging, detectable by smell (1960s and earlier)
- Silicon rectifiers, partial failure (diodes open) - especially 1980s
- Linear voltage regulators, rarely fail, but can short and lead to overvoltage, 1980s-1990s
- Power transistors in switchmode regulators - fatal in step-down converters (overvoltage)
- Small signal transistors, germanium and silicon (1960s until today) rarely fail
- 40xx CMOS logic, especially analog switches like 4066, 405x (1980s)
- Cheap modern semiconductors more prone to failure, possibly due to counterfeits
- LEDs (today, no issue before 2000s)

- Leaking NiCd/NiMH batteries, destroying contacts and/or PCBs, even traveling through cables, lithium non-rechargable batteries usually no problem
- Early lithium rechargable batteries often dead after several years, lithium polymer cells seem to be more durable, but can swell and develop other problems (2000s until today)
- In general the charging electronics and correct handling of LiPo batteries is essential for their long life, not all cheap devices do this right, if such a cell is discharged below 2.5-3V, it can no longer be safely used.

- Relays, contacts sticking together (RC outlets) (2000s until today, rarely with older ones)
- Switches, potentiometers, sliders - dust, dirt and spring pressure degrading can lead to contact problems (all ages)

- Drive belts wearing out or dissolving
- BGA solder joints falling apart (baking PCBs is a modern trend)
- Highly integrated chips delaminating (Nvidia knows this very well...)
- THT solder joints in thermally stressed areas cracking

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Panasonic NV-180EG teardown

The cute portable VHS recorder that came with my Olympus VX-301 needed some more work, because during a first test, something blocked the mechanics.

While having it open for a first inspection before powering it up for the first time, i noticed a small plastic part that fell out of it. After searching for a while, i found where it belongs - the cover of the machine has 4 tabs to lock into the base, and 3 of them are glued to this cover instead of being a fixed part of it, for whatever reason. I just found one of them, so i assumed the others are lost.

But now, i need to inspect the mechanics. After removing the cover and tried some pushing while ejecting the cassette, everything suddenly worked like nothing ever happened. Okay. Time to take it apart. With the help of the service manual and some educated guessing (the SM isn't really accurate describing the disassembly procedure), i managed to dissect it into many PCBs and screws. And some mechanics. I found another enclosure tab, but the third one is still lost.

During reassembly, i decided to take some photos of the process of putting it back together.

I'm really impressed about the maintainability of this machine. You need some care to not break something or forget a connector, but besides this, it's very easy to take it apart and put it back together. What a fine vintage device. While it looks like it's hard to reach for example the second board behind the front panel to measure something while operating it, i don't think it's impossible. Sure, it needs some care, but the front panel itself is not required for operation, the next board can be flipped down while still being connected, so the second board is reachable. You might not want to really come into this situation, but seeing how compact this machine is built, it's not that bad.
The color-coded connectors also make it easy to get everything together without writing down where everything belongs during disassembly.

And according to a quick test with a VHS cassette, it also works fine - after more than 3 decades, it's built around 1985. So no need to repair more here.

The camera has some white balancing problems, but i'm still trying to find a service manual, it has way too many trimpots in it for blind experiments. Besides this, it also works fine (i had to replace a leaking capacitor in the EVF), but if i'm bored one day, i might also take it apart and make some detail photos of it. I'm especially interested in finding this green/cyan/white stripe filter in front of the vidicon. And learning how this works on the electronics side, i only read about some theory, which sounds plausible, but i need to understand the details. Not that it's something i'll ever need in my life, just curiosity :)

In the last photo, i put my current full HD Panasonic camcorder next to the others - just for a size comparison. Same purpose as the large luggable stuff from back then, but fits in your pocket. Not talking about what's in every smartphone today, besides the optical zoom, even this stuff does the same as the good old 1980s luggables.
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Roland Juno-60 restoration

Yawn. Another synth repair. Okay, i didn't make photos right from the start because i didn't want to bore you with the same routine maintenance over and over again :)

But on the other side. Vintage synth guts!

A friend asked me to replace the battery because it doesn't store sounds any longer. And while i'm on it, i could check the PSU also. So it really started as routine maintenance. But then there was this massive dent in the rear of the metal panel. And the volume knob was not original, very ugly. Okay, a bit more work.

After opening it, i decided to give it a full restoration because it was in pretty bad shape. Dirt everywhere, the foam rubber parts around the switches and sliders being brittle from all this dirt and age already, it just deserved some more work. While i'm not such a big fan of those single DCO Roland synths (i have a MKS 50, don't need more of them), they still sound nice. For sure the chorus takes a big part in that. And i don't know why, but i really like working on the Juno-6/60. They have some special kind of build quality that makes working on them enjoyable. Too many connectors and cables, like some Korg synths, but i've never seen a Roland get into trouble due to this - on the other side i've never seen a Poly-61 not being in trouble with the connectors. The leaked battery for sure makes the difference. Roland started using lithium batteries very early, and they nearly never leak. Many others used NiCd rechargable batteries. At the first glance, that's a great idea, because if you can recharge them, you never need to replace them, while a lithium battery will be drained some day and you need to open the synth and in most cases solder in a new battery. That's the theory. In practive, those lithium batteries deliver enough power for 30 years, while after 30 years a NiCd battery leaked in nearly 100% of the cases and destroyed the PCB it was soldered to. But in engineering, there are decisions that look bright in the first place but prove to be not so great decades later.

But okay. Let's get back to this one. First, i completely took it apart to get the cover free from anything because this needs some less delicate handling with the hammer. I'm not a specialist for this, but the result is good enough for my taste. You didn't see it before, the spot with the dents was at least 3cm bent inwards, i have no idea why the wooden base isn't completely destroyed. I wonder how this happened. Okay, With some hammer action, it's good enough again, there is not much room for more work without damaging the top side of the panel from the inside. So better keep the visible parts intact and accept some imperfections in a spot that's barely visible during normal use.

For the battery, i replaced the CR-1/3N by a CR2032 holder, i needed a good amount of hot glue here because it's soldered to the copper side of this single-sided board, it wouldn't make sense on the other side. This is a bit dangerous for the PCB traces if someone in the future isn't careful enough when replacing the battery.
And i decided to convert this to CR2032 because those batteries usually have capacities of more than 200mAh while the CR-1/3Ns i found are just 170mAh. So you don't lose anything, but win easier replacements in the future. Like in 20-30 years, when it's time again :)

The PSU got new rectifiers (i always do this in Roland PSUs meanwhile because i diagnosed half-dead rectifiers in around 50% of the Roland gear i already had in my fingers), new capacitors and a new +5V regulator. The 10mm snap-in capacitors are a bit hard to obtain, i didn't find a dealer having both the types i need, so i decided to use normal 7.5mm Panasonic caps as replacement. Some careful bending of the pins and some hot glue, problem solved.

And the rest was just boring cleaning of everything. The ultrasonic cleaner didn't do it's job right, so i had to improve the result with a brush afterwards, the rest was just toothbrush versus PCBs. And some surface cleaning while all components are removed from the front panel.

The only interesting thing here were the foam rubber parts Roland always added around the sliders and switches. Maybe as dust seal, maybe for cosmetic reasons because it looks nicer, anyway - they were no longer usable. I tried some materials and from conventional felt, modeling felt and doam rubber, i decided to use foam rummer again, 2.0mm thick. It might not live forever, but it somehow felt right to use what they originally used there. But i'm pretty sure i've already seen Rolands with those parts being felt. Anyway - instead of using glue to attach them to the sliders (that was evil to remove, really.), i used double-sided adhesive tape. Much cleaner. And the result looks really nice.

Sure, i cleaned and lubricated all sliders, potentiometers and switches before.

And with all those wires, it was a great feeling that it didn't immediately catch fire after putting it back together but worked on the second try. I really took care, but 2 connectors were swapped, so it didn't work on the first try. Anyway, nothing happened, everything worked after correcting the connections, added the remaining tons of cable ties.
And yes, i checked the PSU before connecting the rest to it. Ah, by the way - i converted the PSU to 240V also, you should do this today in 230V times, devices get more into overheating when set to 220V than into any trouble when being set to 240V.

With the parts i ordered, i also ordered a small selection of suitable knobs after some pre-selection, because the one installed (in the close-up of the knobs, it was the upper left one) really looked misplaced there. I have chosen the one i liked best and the owner of the synth also liks this one best.

Nearly done. The last thing was a full adjustment of everything, there might be a bug in the adjustment guide in the VCF LFO gain area, so i left this value where it was - most of the adjustment was still okay anyway. After loading the factory sounds and wiping off the whole synth with a soft cloth, it's done.

I think i have it some days around to play a bit with it.
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CEM5508 / PD508 replacement - mission accomplished

Finally - the project completed. While i posted the steps inbetween privately, this is the public post as a follow-up to to tell you how this worked out at the end.

With the theory described in the first post, i implemented the control logic using a small STM32 development board, optimized the firmware and checked if everything works as expected. The timing is not as tight as with the original CEM5508, but on one side, it's more than good enough for the microWAVE, and seen from an external point of view, it even meets the signal timing from the datasheet - just the sampling period moved some nanoseconds internally.

Next was completing the prototype to replace the half-dead CEM5508 in the microWAVE and check if there are any sonic oddities. I tested many of the sounds in the unit and voices 1+2 with the substitute sounded identical to the remaining voices still using the original CEM5508. So this is the way to go.

The circuit was not that difficult, it's straightforward with the goal to use only cheap standard components and not special stuff like the SMP08, which costs 10 times as much as the classic 4051/TL074 combination with C0G capacitors (PP would even better, but you don't get them in SMT) i use for the S&H stages. For the switchable impedance, i didn't even look for something simpler, even if there was an 8-channel analog switch with addressable latch out there (i'm sure Maxim has one, they have everything :), it would add another expensive special chip that might be unobtainable in the future. So i use 2x4066, HCT259 and 8 resistors for this. The control logic is implemented using a STM32 because it's fast enough, cheaper and easier to handle than a CPLD.

So the PCB layout was the biggest effort, as usual. With those holes for the trimpots and capacitors, it was a bit complicated and took 4 or 5 days to complete. I might not have needed the holes for the capacitors, but they are close enough to the board that i would need to keep away components from those areas anyway, so it doesn't hurt, i can use the capacitors to fix the board in place with hot glue later. Sure, rising the board by another 2mm with another socket inbetween would also work, but that would decrease the stability of the whole thing even more.

The first PCB run failed, i missed my own documentation and botched the exposure time, but it was good to re-check if everything fits as intended, i optimized the layout a little bit after i found some critical spots.

Yesterday, i made the final PCB. The process from film to etched PCB took just half an hour, as usual, i gave it a tin bath for an hour then and started soldering. Around 4 hours of soldering (plus some breaks inbetweek), everything was put together and electrically checked, no shorts, i just had one broken trace, so the overall quality of this PCB is very good, the process meanwhile is close to perfect. To avoid that this stuff rots away with the years (or maybe decades?), i coated it with a plastic spray, which also gives it a very nice finish.

Today was the big day - i adapted the firmware to the final layout, which is necessary because i assigned channel numbers and GPIO lines in a way that simplifies the PCB layout, so the firmware needs to compensate this, which is not a performance penalty, everything is basen on LUTs. Performed a last dry test to not damage the microWAVE if there is a bug, and flashed the firmware. Also no big deal, no further problems. 26mA drawn from the +5V digital power supply, the analog supply is also not too heavily loaded with all those components.

Then that great moment - board installed in the microWAVE, the cable for power and reset soldered to the CPU, finger on the power switch, adrenaline level goes up to extremely unhealthy levels.

Flip the switch - nothing.

Okay, now let's connect the mains plug to a power outlet.

Next try - the usual popping sound on power up, no strange sounds. Hit some keys - IT WORKS!. I can't describe this feeling when you work on something like this for weeks, power it up for the first time and it just works. This mixture of excitement and panic... I still suffer from adrenaline overdose, it's so awesome, even if it's just a logical consequence of the work performed before :)

So this project is finally completed after several months i have the machine around already. There is another big project in the background and this challenge blocked it all the time, so i'm happy this finally worked out as expected and i can give the friend back his microWAVE, maybe the only one on the planet that will never again have problems with dying CEM5508s.

I recorded a bunch of sounds with the CEMs before, the half-dead one always worked fine when cold, so this is a valid reference, i can now compare those sounds with the voltage control prosthesis in place, will play with some parameters to check if the switchable impedance stuff works for envelopes as expected, but for now, everything sounds as it should. I also need to stress test the whole thing, closing the enclosure, bury the unit under some blankets to make sure it has the required long-time stability.

This design would also work with the microWAVE Rev. 2 analog board, but it would need a new PCB layout, which is most of the work. In theory, it would also work for the WAVE, but the space limitations in that machine are a real problem for this large amount of stuff you need for the substitute. But at least there is a way to save those microWAVEs, which get ripped apart to get the CEM5508s to repair WAVEs. And the solution for around 20€ component cost (without PCB, assembly and development costs) replaces 4 CEM5508s, which sometimes appear for 50-60€ each in the web. And they are selling used or NOS chips, that will very likely fail in the near future, so the substitute is the only long-term solution for this problem.

Let's hope the other CEMs and the ASIC in the machine a more durable...
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