Opto-Key Project is here, and the builds started

Opto-KeyKevin’s Opto-Key build environment personal injury claim transferred from Synthfool to This Old Synth. The components, software, circuit boards etc. arrived on Wednesday afternoon. Two days of work later I have Opto-Keypartially 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!

CHD MIDI Upgrade for the Roland TR-808

CHD TR-808 MIDI ModWhen writing about the MTG TurboCPU upgrade I promised to put a blog entry up for a really nice and non-intrusive MIDI mod for the Roland TR-808, and here it is. The thing I really like about this implementation is the degree to which they went to make in non-intrusive (i.e. drilling holes!).


CHD TR-808 MIDI ModIt comes complete with a printed manual of step-by-step instructions, and supporting CD.

For the MIDI connector the existing Din-Sync connector is removed, along with its In/Out switch. A new connector that includes MIDI In/Out is fitted (a special adapter cable is supplied. The switch is replaced by a 3 way one, In/MIDI/Out.

CHD TR-808 MIDI ModThe “umbilical cord” for triggers and power is socketed at the controller end, so for service it can be unplugged and the TR-808 will still work (so this is not a CPU upgrade/replacement like TurboCPU).


CHD TR-808 MIDI ModOnly one hole needs to be drilled, and it is for a MIDI active Led and sits in the Start/Stop switch, under the yellow label. Awkwardness here is that they specify a metric drill, and for accuracy a CHD TR-808 MIDI Modwoodworking one is probably best. I’m sure you could get by with a slightly larger US drill and use a band of shrink-wrap tubing or some-such to tighten the fit. Of course you could ignore this step and just know that center switch is MIDI on!

Be sure to keep the parts removed such that the TR-808 can be put back to standard if need be. Here is a link to the CHD TR-808 MIDI kit. I may well contact CHD and offer to distribute this in the USA.

E-Mu SP-1200 Noritake Display

SP-1200 Noritake DisplayMy 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.


SP-1200 Noritake DisplayUnfortunately 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 SP-1200 Noritake Displayneeded 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.

SP-1200 Noritake DisplayThe next issue was that the display character area was smaller than the original, and it did not have a bezel. This looked bad, andSP-1200 Noritake Display 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.

ARP Soloist meets 3D Printing

This is a ARP Soloisist Slider Fixrare 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. ARP Soloisist Slider FixSometimes you can find replacement shafts for these on Ebay, but nothing was listed at this time. After much head scratching I decidedARP Soloisist Slider Fix 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.

ARP Soloisist Slider FixThe sliders were removed, opened, cleaned, re-greased, then set on the blocks which I had super-glued to the ARP Soloisist Slider Fixcircuit 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.

This Old Synth acquires the exclusive rights to Synthfools Opto-key for MiniMoog project

As a tech I’m increasingly frustrated with the chore of trying to restore the (oxidized) key contacts on the MiniMoog to full operation, and unless it is frequently played the issue of “squirelly keys” comes back. I have been installing Opto-key for my customers which removes the key cleaning cycle, AND adds simple note-on/off  MIDI.

I was concerned and saddened to hear that Kevin Lightner at Synthfool could not continue the Opto-key project  as it addresses the biggest reliability/playability issue with the MiniMoog, and have negotiated with him to transfer ownership here.

Once the on-hand materials such as circuit cards arrive I will commence building the product to Kevin’s spec and make it available for sale.

As I’m also a software developer with experience in embedded systems I plan to enhance and support Opto-key moving forward. I’m really excited by this!



MTG Pro One Turbo CPU

Turbo CPU

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.


CV WiringI 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.

Installed MIDI interface cardAll 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.

Drilling holes for MIDII 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 squeezed between logosMIDI 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.

Alternative MIDI connector mountingFine 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!

MIDI interface with plug connected CVFor my implementation of DIY CV I installed a connector at the MIDI interface card for the 4 wires so it could be disconnected for service.

Linn 9000

Linn 9000 1My 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 Linn 9000 3is 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 Linn 9000 6shorted. 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 Linn 9000 4dragging 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 Linn 9000 7enabled 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.

Linn 9000 2Testing 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!

Moog SonicSix

SonicSix1I’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 SonicSix2plastic 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 SonicSix3oscillators 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.

SonicSix4The 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 ofSonicSix5 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!

Moog Multimoog

MultiMoog1Back 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 MultiMoog2mostly 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!

MultiMoog3It 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 MultiMoog3transconductance 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.

Tax Time is Quiet Time

Rack1The end of March was really quiet, so I guessed folks were focusedRack2 on doing/paying their taxes, rather than getting their gear repaired. I came across a nice professional work-station and rack that were available due to a lab move, so went and grabbed them to update the shop. Nice eh!