Monday, 10 October 2011

KR-55 clock-in mod

The Korg KR-55 (or Rhythm 55) is a great sounding drum machine. Originally released in 1979, it followed on from the MiniPops series of preset rhythm machines and was in direct competition with Rolands CR range. While it is not programmable, it is pretty flexible with lots of variations and some excellent fill-ins. There are no individual outputs but at least it gives some control over the voices with seperate(ish) volume pots. You can hear the KR-55 on many late 70s, early 80s tracks - Mute Records guvnor Daniel Miller made great use of it's trigger output, driving his ARP sequencer and of course the excellent snare drum can be heard throughout Depeche Modes Speak & Spell album.
It's one major drawback is that it has no sync or clock input and this makes it a bugger to integrate into your set-up...'s a pair of KR-55s waiting for the external clock-in mod.
It's a fairly straightforward process, lifting one resistor and using a switched jack, but there's not a lot of room on the rear panel to mount the new jack socket...
care needs to be taken when fitting the jack socket so as not to foul the top pcb when the machine is re-assembled.
The KR-55 runs at 8ppqn so a clock divider is required if you want to sync it to midi or sync24. The clock output on the Kenton midi to cv converters is ideal for this (divide by 3).

Here is some further information on the Clock Input mod for those interested…
There are two large circuit boards inside the KR-55, the upper pcb is the voice board and the lower one is the logic board and this is where you need to work…
The upper voice board needs to be removed, there are five screws securing it, remove them and carefully lift the board over… You may also need to carefully unplug the connecting cables to give you more room. You can now unscrew the PCB pillars to remove the lower logic board...

Locate R48 on the logic board (it's just below IC3, see pic below)… this is where to access the internal clock. Lift (de-solder) the right-hand side of R48… connect a wire from the new mini jack "tip" to the lifted leg. Connect a wire from the mini jack "contact" to the pcb pad the leg was lifted from. Connect a wire from the mini jack "sleeve" to any 0v point (gnd).

Now, when a jack is inserted to the new socket, it disconnects the internal clock from R48 and inserts the external clock signal instead… Removing the jack restores the internal clock. You can, of course, use a separate switch to select between internal or external clock but I chose the switching mini jack as there really is very little space inside the rear panel.
The clock input should be 5v positive pulses. Do not use gate or trigger signals that are more than 5 volts without some form of attenuation.
When using an external clock you will need to press the Start button on the KR-55 before sending the clock signal - this puts it in "ready" mode.

Disclaimer: Only attempt this modification if you are confident to do so and understand what you are doing. I accept no responsibility if you damage your machine.

Monday, 26 September 2011

VCO 2 4 Free

This came to me recently along with some Doepfer modules... "it doesn't work, any good to you?" - well, yeah, I said! It's an interesting one, the VCO2 from Analogue Solutions... apparently discontinued... it's a dual VCO with some nice features - portamento on both oscillators, sync and cross mod, osc 1 has a wide range that allows it to function as an LFO, and independent or linked CV pitch control.
So, onto the lab bench to find out what's what... didn't work, no surprises there, but two hours & several new parts later it burst into life... highly likely it had been 'plugged in the wrong way round'.
It sounds nice & raw, similar to Rogue or Prodigy VCOs. Oscillator 1 has saw, tri & square waves ... oscillator 2 has saw and variable pulse waves and can be synced and cross-modulated with osc 1 creating some really dynamic sounds. It's fairly stable and the tracking is not bad... what you'd expect from a 3046 based VCO I guess.

Prod Mods

A nice Moog Prodigy in for a service & minor surgery - it's one of the later models with interface jacks on the back panel... these make it quite a flexible machine. The user wanted the S-trig converted to a standard positive going gate signal, so a 1/4" jack socket replaced the Cinch Jones connector and a small circuit added inside... no cosmetic alterations and completely reversible if needed.
A 'silent' VCO was brought back to life with a CA3046 transplant, and after a general clean and calibration the Prod sounds great. 

Friday, 27 May 2011

The Sequentix Cirklon Sequencer

Colin Frasers' incredible Cirklon sequencer - a machine that integrates beautifully with all gear, old and new... midi, usb and analogue cv/gate connectivity. The range of functionality with this thing is truly amazing...

Colins' efforts in designing this machine and his attention to detail really deserves huge credit... just take a look at this...

full details at

Sunday, 22 May 2011

TR-808 start/stop switch - pt.1

So, another TR-808 with a worn out start/stop switch. These original alps switches are almost impossible to find - Technology Transplant may have a few... then again they may not. Options... repair original or find a workable replacement :-) Repairing the original switch is tricky but possible and has been documented elsewhere on the web...
Ok, if you can't find a new one...
Roland used a lot of generic parts in all their late 70s early 80s gear and many switches, pots, sliders etc are common to many machines... but these 808 switches are a bastard for an exact replacement. Nearest are switches used in the MC-4 microcomposer, the 'cross' where the cap fits needs slight alteration but it's mechanically compatible otherwise - but using an MC-4 as a parts donor is a capital offence...

Donor CSQ?
ah, the CSQ-100/600 sequencers, made at the same time as the 808, right? same plastic ends, right? same Start/Stop switches, right? Wrong... but can they be made to work?
It just so happens I have a damaged CSQ-100 pcb here... lots of useful switches on it... be continued

Monday, 16 May 2011

two and a half Transcendent 2000's

This week I'm working on some Transcendent 2000s. It's an interesting synthesizer designed here in the U.K by Tim Orr in the 1970's and published as a self-build project in the magazine Electronics Today International in July/August of 1978. They were quite popular in the U.K and a kit of parts was available through Powertran electronics until around 1984. In fact, the Transcendent 2000 was a 'first synth' for a lot of British musicians (myself included)... reason? ... it was pretty cheap... if you couldn't afford a Micromoog or an ARP Axxe or even a MiniKorg 700 the Transcendent was really the only option. It was also fairly simple to build - if you knew the hot end of a soldering iron from the cold and could follow some basic instructions... famously, Bernard Sumner filled his sleepness nights by building one back in his Joy Division days.
I like the Transcendent 2000, it's full of cheap 1970's technology so, yes, it's a bit noisy and weird but it sounds dark and raw, the filter is unpredictable and really needs taming when it self-oscillates (more on this at a later stage...). The waveshaping in the VCO is unusual for a basic monosynth - but then again a lot of things about the Transcendent are unusual.

I have two complete machines here for service plus another main PCB - all with various problems. Both keyboard assemblies need re-alignment of the contact wires, all three have iffy pots and switches and various other age-related conditions...

Original magazine article and build notes

Transcendent 2000 main PCB


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