Kevin’s Opto-Key build environment has transferred from Synthfool to This Old Synth. The components, software, circuit boards etc. arrived on Wednesday afternoon. Two days of work later I have partially completed 5 units. Time to get a components order submitted so I can complete them! I did purchase the very last of unit built by Kevin and have begun testing using it in my MiniMoog; exciting times!
My customer Aaron, was fed up with the poorly back-lit display on his black re-release SP-1200, and wanted to have the Noritake CU16025-UW6J display from Digi-Key fitted. They are expensive, and being glass, fragile; but they are bright and clear so a good upgrade.
Unfortunately the display is smaller than the aperture in the circuit board, and an attempt at notching out mountings was made to no avail. The display is also thicker than the original and spacers are needed to move it down. After several attempts I decided to make 3mm deep adapters on the 3D printer, and this gave me a stable mounting that I could squarely align the display on. You also have to make an adapter cable as the pins are reversed.
The next issue was that the display character area was smaller than the original, and it did not have a bezel. This looked bad, and the glass was exposed. Thankfully I had an 80×2 line lcd filter lens in my parts bin and could cut that to size. It covered the ugliness nicely, and protected the display. Aaron was happy! Final job was to open all switches and drum pads to clean them and restore playability.
This is a rare original ARP Soloist (before the ProSoloist or DGX). A lot or restoration work was done on it, but one issue remained; the slider shafts were all broken at the point where they exited the case, and the slider caps were long gone.
The sliders are the same type as fitted to the ARP 2600 and moog SonicSix, amongst others. This is a hard to find part, and here we have 4 sliders with 3 different values. Sometimes you can find replacement shafts for these on Ebay, but nothing was listed at this time. After much head scratching I decided to raise the existing sliders by 8mm, and needed some precise flat blocks to do that with. I came up with a design and after many attempts (that’s 3D printing for you!) I produced a nicely printed set.
The sliders were removed, opened, cleaned, re-greased, then set on the blocks which I had super-glued to the circuit board. There were locators in the blocks for the original slider mounting, so when the wiring was added using a heavy gauge wire extension, all was firm and true. I re-assembled the unit and added some Omni-2 style slider caps I had previously printed. All working and crackle free. Next move was to tune and scale it.
One item remains on the Soloist, the after-touch used conductive foam which was known to only last 8 months, and had all by disappeared on this 1975 keyboard. I would have liked to have got some modern strain/pressure sensors and tried to make a solution, but the owner decided to defer that to another time.
I’m generally against modifications. I have seen too many vintage synths spoiled by badly drilled holes that hold fragile switches, connected through an “umbilical cord” to undocumented perf-board based mods made decades ago, and then more wires soldered to the circuit boards. Working on the gear becomes difficult as these wires are holding the case to the cards, and of course wires fatigue and come away, and you have a challenge as to where they once went, and how the mod is supposed to work.
I do see the benefits of a good MIDI implementation, and I have come across a couple of MIDI kits where the designers have come up with good clean designs that can be unplugged for service. In this entry I talk about the Turbo CPU upgrade kit from MTG (Music Technologies Group). Grant runs the MTG and I have found him responsive, friendly, and helpful so recommend doing business with him.
In a different post I will talk about a nice MIDI solution for the Roland TR-808 from another vendor.
All of the instructions are available to download from MTG (http://www.musictechnologiesgroup.com/index.html), so you can see what is involved. I’m on the list of experienced installers and would be happy to implement this for you, but note that I am not a reseller so you have to purchase the kit from MTG. Turbo CPU a simple removal of the CPU and install of the Turbo CPU module.
I think most folks would be interested in the MIDI interface which for a solid installation does mean drilling the base of case to mount the MIDI interface card, and the back of the case for the MIDI connectors.
I prefer the look of the MIDI connectors squeezed between the lettering, and made some identification labels for the MIDI ports. Both customers for this kit wanted the MIDI interface, plus the “DIY” CV interface which adds a tiny MCP4728 chip to the v2.00 MIDI board (I was fortunate that the MCP4728 was pre-installed on one unit, but had to order it from Mouser for the other.
Fine soldering skills are required to install this chip as it is surface mounted, so ask Grant if you can have a card with the MCP4728 pre-installed when you order :-). The DIY CV interface gives you MIDI control over 4 CV wires and is a great addition that requires no trace cutting or case butchering, so I like it!
My customer won this working unit on Ebay, and agreed to pick it up from the seller. Between the end of the sale, and it being picked up the seller decided to get his tech to replace the batteries as they were leaking. Following this it would partly boot but hang, not good! Thankfully a heavy discount was negotiated to cover repairs, and in it came to my shop.
I have not worked on one before, and I was amazed to find it is based on the first IBM PC, ISA bus and all. This one had a lot of Forat mods, including the SRAM256 card which I have read is a favorite for folks to blow up by inserting backwards. I checked the batteries that had been fitted, and was only getting 2.4v, and one read shorted. I purchased new lithium batteries and a charger which confirmed that one was indeed shorted. New batteries were fitted. Powering on found the screen blank, and this was traced to the CPU being stuck in reset. The transistor between the batteries and this line was bad, and replacing it released the reset. Checking the CPU clock found it running less than 1/10 correct speed. The osc chip SN74LS04 and its socket were badly corroded by the battery acid; parts replaced and clock working correctly.
We were back to it hanging, but things were now solid as the battery/acid damage was out of the way. Address line A6 was found to be low amplitude, and the SRAM256 card was dragging it low. Removing the card put the 9000 into a mode where it was looking for the SRAM 256 card (Forat firmware and card), so it would not come up. From the schematic, -12v would go to A6 on the SRAM card if it were reversed (surprise!). I traced the shorted A6 line to a PAL on the SRAM256 card, and socketed the chip. Lifting the PALs leg enabled the card to be plugged back in. Bruce Forat was contacted for parts, but by the time he came back with a price I was well into reverse engineering the card fitted detection in the PAL using a logic analyzer and 8080 disassembler. Clearly the address decoding part of the PAL was still good as after zapping the code that looks for the card everything came up. The customer wanted to go this route as he wanted to see what else was bad on the 9000 (i.e. was it a write-off) before spending more on it. As the PAL is socketed he can eventually order the PAL from Bruce, and fit it along with the original firmware eprom.
Testing showed the floppy controller was bad, and a chip change fixed that (did the cable get reversed at some point?). Last thing was to replace the screen with a more modern high contrast led back-lit one. Customer sent me a YouTube video of it in action that night, he is delighted!
I’ve completed a few of these now, and must admit to a little trepidation when they come in as this synth was clearly made for education use (everything is squeezed into a plastic suitcase with no ventilation holes, or easy access to pre-sets for set-up etc.). Setting it up is difficult to say the least, and like others in blogs I have read, just got it to scale on both oscillators with minute adjustments. The original pre-sets are of very poor quality, so I replaced the majority of them; thy used very low ohm values and did not get a nice sweep on adjustment, so multi-turn replacements would be good, but space and access is an issue.
The main issue was that it did not scale, and sounded like hell. The root cause was the -15v regulator where the 723 regulator chip was holding it at -23v. Thankfully no other chips have died (yet) due to this. One of the rectifier diodes was also bad, so there was a lot of hum from that, but after replacement of these parts things settled down. The Mullard C280 “Tropical Fish” caps were falling apart, so they were all replaced, alog with the can caps. Next move was to service all of the pots/switches, re-bush the keyboard, and (struggle with the) set-up. Jim was delighted, which makes the effort worthwhile!
Back on my favorite brand, Moog. There is a steady stream of MiniMoog’s through here, which is my overall specialization, but I do not report on them as it is mostly the same issues that strike them. This is a Multimoog, which customer Jim scored along with a Sonic Six. This one had no output, and the key bushings were shot; the seller reported that it worked when he put it it away in the closet decades before!
It cleaned up nicely, and the bushings were replaced with each key cleaned by hand. The buss-bar plastic mounts had gone brittle, so I had to make some parts on the lathe, and rob a parts keyboard to resolve this. The lack of output was due to a dead CA3080E transconductance amp (common bug). Pots and switches were gradually worked with switch cleaner until they performed correctly. The buss-bars had to be removed in order to get the oxidization off them and the contacts. Unit was set up and played nicely. One surprise for me was that the after-touch was fully working, pity the design of this severely limits key travel.
This unit had been powered off for many years, and although it looked nice once dusted off, oxidization had done its worst on key contacts, made sliders crackly etc. The brass section was not working, and the pitch slider was broken. I could not find a source for sliders, and ended up grafting a metal shaft onto the pitch control. The sliders are of a closed and sealed type, but thankfully (and surprisingly to me) injecting switch cleaner into screw holes in the slider cases banished the crackles.
Investigating the brass issue revealed cracks in a circuit board from impact, with a poor repair attempt in the past. I wire jumpered all bad and suspect circuit board traces such that if the crack grew a little then it would still work. The final issue was key contacts, and in the end I had to remove the buss-bars in order to clean them which is a major strip-down effort. The contact springs were really bad and had to be repeatedly treated using isopropryl as a cleaner. The results are pretty good, but the only way to get it to 100% would be to find new springs. Customer just loves it!
This unit shipped in and per the customers description, had a dead keyboard and lots of slider issues. The sliders in here are rare, and the worst one was a dual slider type that could not be found anywhere. All sliders were found to be gummed up with old grease, dust, hair, and other debris. The unit had to be deep cleaned, and all sliders opened, cleaned, and lubricated.
The keyboard was giving a large negative CV out, so off the audible scale. The customer said it worked until he tried to clean the contacts due to a few bad keys. What he had done was to over aggressively clean it such that a lot of the CV contacts were dis-formed and resting on the gate bus-bar. These were straightened out, but the majority of keys were then intermittent and badly triggering. Closer investigation revealed that the contact wire that is welded onto the key spring was either missing, or broken at one end such that a consistent on and off point for the trigger and gate could not be achieved. Contact sites were rare and expensive, so it was a case of using the good trigger contacts from some of the contact assemblies to make complete good ones, then salvaging assemblies from my spares RS-09 (fewer keys) to fully populate the 101. This was a LOT of work, but the keyboard is fully functional now and the owner is delighted.
I have found a source of the correct original type of step switch used in the Roland TR-808, and put them in the Parts section of my main site. You have to be really careful on how you remove the switch actuator so as not to break it, and the ribbon cable to the panel is very fragile leading to intermittent/no step button operation. The originals get gummed up and dusty inside, and they cannot be cleaned. I stock 2 types, the original, and a sealed variant (which has a different action). If you replace a switch I would suggest doing them all.