Pro one oscillator board finished

Just finished revision 1 of the Pro one oscillator board! It looks quite good and fits on a 10 x 8 cm PCB. It contains the CV-mixers and two oscillators. Next up is the audio mixer, envelope follower, reference voltage generator and noise generator.

SY77 floppy drive repair/replacement

I picked up a Yamaha SY77 last week. Except for a minor scratch (5 x 2 mm) it is in mint condition. However, the floppy drive is not working, something quite common with these I think. I will open it up tonight to see if it is only the drive belt. The synth can be used without the floppy, so it doesn't worry me that it is not working, but it would be nice to have it back in fully working condition.

The display backlight is a bit dim too, I have found a replacement on Ebay that I may buy.

All in all a very nice machine, one of the last in the line that started with the DX7.

To open the synth, put it upside down on something soft. Unscrew all the screws you can see on the underside - 20 in all on my SY77. Carefully lift the bottom plate and push it towards the front of the synth as soon as it clears the edge of the plastic parts.

You now have access to the floppy drive. The drive is located under the gray steel bracket towards the back of the right side (when upside down) of the synth. The bracket is held in place by three screws.

The floppy is attached to the main circuit board via a rather long ribbon cable, so removing it is very easy. It is not possible to detach the ribbon cable from the floppy without unscrewing the lid, so remove it from the circuit board side.

 The floppy is screwed to the bracket with four black screws. Remove them and you will have access to the drive belt.

After removing the bracket, it was very easy to spot the problem - the drive belt had snapped. Replacements are available online for around $17, so this should be an easy fix :)

Sequential Circuits Pro One - Octave selection rotary switches

The octave selection rotary switches on the pro one works a little differently than most other.

They are in fact four position, DP switches, meaning that they have no common pin, the rotary action short circuits two and two poles.

The way the selector works, is that it selects 0, 1, 2 or 3 connections to the 1V reference voltage. In the various positions of the switch it means that:

• Pos 0 : pin 10 is connected to pin 2, and pin 5 is connected to 6, connecting the octave summer to R161, which in turn is connected to ground.
• Pos 1: pin 1-2 and pin 6-7 are connected, connecting the octave summer to the reference voltage through R162 (100k)
• Pos 2: pin 2-3 and pin 7-8 are connected, connecting the octave summer to the reference voltage through R163 and R164 (100k * 2)
• Pos 3: pin 3-4 and pin 8-9 are connected, connecting the octave summer to the reference voltage through R162, R163 and R164 (100k * 3)

Mouser.com sells a C&K switch with the number R20407RN02Q. I don't know if the kind of shaft is the correct length (it is .500", for a total height of about 1.05"), but the switching mechanism is the correct one. The same switch is sold by digi-key.com too.

NB: The switch has a 0.25" (6.35 mm) shaft. Other pots may use 6mm shafts.

New general synth stuff blog

As I've finished the Introspectiv 9090 (909 clone), the so909.blogspot.com blog will not be updated any more. It's also about time I moved to a more general blog, as the latest posts are not related to the 909.

Thus, here is "A to Synth", my new company blog. I have a few nice things coming up shortly:

• the full description of the PG-200 (JX-3P programmer) protocol, including data format and events for each pot/switch 
• a parts list for the Sequential Circuits Pro-one 
• redrawn schematics for the Pro-one in Eagle format All time permitting though, as these are only part time projects.
• vector versions of several well known synthesizer companies' logos, as well as synth logos.