Recently had a Korg MS20 to look at that was exhibiting some very odd behaviour. Generally the tuning was a bit off and there was some residual LFO modulation of the VCOs even when all modulation controls were at minimum. Odd as the measured keyboard voltage looked OK. Took the synth apart (the MS20 is not that easy as all the pots have to be un-done just to get at the board) and it was found that the faults changed if the internal looms were moved about – even stranger.

Usual suspects checked (power supply voltages etc.) and everything seemed OK. Time to check with an oscilloscope and bingo – several volts of parasitic oscillation at a frequency well above audio all over the ground plane. This was going to be tough as there are a large number of culprits (op-amps) on the main board. Started at the main regulator and hunted for the IC where the maximum oscillation and tracked it down to the buffer for the keyboard divider.

Changed IC and – no change!

Hmmm, that’s odd. Back to circuit diagram and no supply decoupling around offending IC (actually very few anywhere) so time to strap a couple of 22n capacitors close to the IC across the rails and hooray – no more oscillation.

Calibration of keyboard voltages went back to exactly the right values, oscillators were re-calibrated and were accurate within a cent across the entire keyboard, re-calibrated everything back to the original Korg spec and the whole synth went back to sounding really good.

MS20 should have done Glastonbury by the time you read this – another satisfied customer…

Recently been fixing an ARP sequencer which arrived in a non-working state. After some time with an oscilloscope and a couple of replacement parts normality was restored.

So while testing I pondered the artistic use of  such a beast, surely with a maximum of 16 notes it must be very limiting. However I came up with a way of using it that seemed to give endless variations but with the chance to determine how things go so it is not a totally random sequence.

Here goes with the explanation, hope you follow it…

First set up a number of notes that have a reasonable musical relationship but make sure that you program in more than one occurence of the notes you want to hear most often. Say 3 of one, 2 of another, 2 of another and one of a note you only want occasionally.

You then run the sequencer in random order mode and you will get a constantly changing stream of notes whose probability of occuring is determined by the number of duplicates you have put in, more duplicates ensures that the note is heard more often. The final sneaky bit is to use the skip function and that allows adjustment of the probability of the occurence of a note being played on the fly.

To complete this I used the other 8 trigger bus switches to determine if a gate was sent allowing notes to be made into rests.

If the 8 seed notes are also put in a meaningful order you can flip back to sequential mode giving a repeating motif that you can latch on to within the more semi-random stuff.

Try doing that on a computer!

This has inspired me to start work on a more modern version of a hardware sequencer, watch this space…