Korg MonoPoly Arpeggiator Trigger

The Korg MonoPoly is a great synthesizer for those who love to experiment with sound. It's capable of some unique tricks and is quite unlike any other monophonic synth of the era. One of the best things about the MonoPoly is it's arpeggiator - and the ability to cycle through each of it's 4 VCOs - some excellent weaving and evolving rhythms can be worked out of it. BUT... the one thing that has always bugged me about the arpeggiator is it's sloppy timing, or should I say REALLY sloppy timing.
When fed with an external trigger signal - S-Trig - there is a small but very noticeable delay. You may not notice it when you're simply playing the arp from the internal clock but when triggered, for example from a drum machine, the delay is apparent.
Now, when I first noticed it many years ago, I initially thought that I had a slightly dodgy MonoPoly but no - every other one I've worked with over the years has the same annoying delay. It can also be noted that the Korg Polysix shows the exact same behaviour with it's own arpeggiator - they both share a similar design.
Obviously there is a lot of talk on the www about what type of trigger you need to use to clock the arpeggiator on  both the MonoPoly and Polysix... some using a spikey audio signal like a rimshot, some using a dedicated clock output from a Kenton etc, a lot puzzling over V-Trig or S-Trig conversion... but no-one really talking about this small but annoying delay.
To really dig into this I decided to do some tests with a Korg KR-55 - it has a dedicated trigger output which can be set to various divisions and it's an S-Trig signal... Korg's very own stock standard trigger system so should work perfectly.
The MonoPoly manual tells us that the arpeggiator expects a +V to GND trigger signal - which we all know as S-Trig... so, put the trigger out from the Korg KR-55 into the arp trig in and all should be OK, yes?
No. Well, yes and no... yes it triggers the arp fine, but not quite in time - nearly in time, maybe near enough for some, but not near enough for many others - including me.
What's really interesting is if you put your S-Trig triggers into the Gate "trig in" on the MonoPoly - polarity switch set to S-Trig of course - and trigger the envelopes, it's perfectly in time, absolutely bang in time!

A quick look at the block diagram in the service manual reveals that the 'Arp trig in' and normal gate 'trig in' are routed differently - the arp trigger goes via the main key assigner CPU - this is where it decides which VCO to use and, of course, is how we get those nice changing patterns of cycling oscillators - whereas the gate trigger goes directly to the Trig Detect and to the EGs...

Some detective work is going to be needed to find out where the delay is introduced - by the key assigner CPU or the following circuitry? I don't know yet... I do know that there are some very clever guys out there who have a far better understanding of how the key assigner CPU and associated circuitry works and they have done some extensive diagnosis of the timings and voice assignment process - certainly for the Polysix, maybe also for the Monopoly?

So anyone with experience of these machines and an interest in this type of technical question is very welcome to share their thoughts and any ideas on how to potentially solve this timing problem...

Some Evidence...

Logic screen-grab showing MonoPoly arpeggiator delay

A test file was recorded in Logic - left channel (upper) is the MonoPoly arpeggiator set on a short, percussive sound triggered by the KR-55 trigger out set to 1/4 notes. The right channel (lower) is the KR-55 playing a bass drum on 1/4 notes and a hi-hat offbeat for reference. Note the MonoPoly delay... around 11ms.

Logic screen-grab showing MonoPoly "gate" trigger

Next, a file was recorded with the same KR-55 beat triggering the MonoPoly via the normal gate 'trig in'. Again, the upper left channel is the MonoPoly and the lower right channel is the KR-55. Here the MonoPoly is in near-perfect time.

Short video showing arp trigger delay

Of course, if you're recording arpeggio patterns in a DAW it's an easy job to just pull them back in time but triggering the arp in real time from a drum machine etc is so rewarding and musically inspiring that this has become something that I'd really love to work out, if possible... so if you have any thoughts I'd welcome hearing from you...

Further Korg KR-55 notes...

I've had a few KR-55s in with the same fault over recent years... they're generally very reliable old machines but I've seen several that would power up but not run. The fault on all of them was a failed transistor on the logic board... the large 2SD288 (TO220) which serves as the 5v voltage regulator... here it is on the board...

...and here it is out of the board...

I'm not sure why this part has failed on a few machines, possibly, because of the weird angle it sits at, it may come into contact with the top pcb if it's not bent down sufficiently and create a short -  anyway, it might help if you've had similar problems with your KR-55... it's a very simple job to replace it.

Korg KR-55 Clock Input update...

I've added some info on the external clock input mod for those who expressed an interest… see the original KR-55 Clock In Mod post for details on how to carry it out...

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... Here is a way to add an external clock input to your KR-55 for less than a Pound, Dollar or Euro...

...here'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.



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...

...it 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. 

jaydsr