Now first and foremost, I know absolutely nothing about circuitry - joined an computer science chatroom and they couldn't help, I have cross referenced the symbols to the internet to try and understand myself - and I still fail to understand. I must resort to asking here, so sorry for polluting this area with a question which will only benefit me, rather than benefitting the community.

that said:

I am trying to restore a 1970s synth, manufactured in italy, with my father. Nethier of us know any circuitry. We just want to repair the switch board, that is all. I BELIEVE from my research, that the switches are DPST latch switches each has 2 sets of 15 pins to save you counting.

I have ONE question. Which circuitry diagram matches the photo of the switch board circuit? Once we know which page we need, then we can get to work and use google for everything else! Thanks in advance

(sorry for the abnormally large photos, I have downsized them twice already! This will stop you needing to learn forwards though) (the soldering was not our handy-work)

SBC-> switchboard photo

v BACK v backboard photo page 1 page 1

page 2 page 2

page 3 page 3

page 4 page 4

page 5 page 5


  • \$\begingroup\$ Do you have a photo of the back of that board? Also, do those switches have labels (e.g. on the case)? \$\endgroup\$ – Alex Hajnal May 7 at 18:17
  • \$\begingroup\$ I've added the photo of the back, and there are no labels on the switches :) \$\endgroup\$ – Codewell May 7 at 18:33
  • \$\begingroup\$ First, none of the schematics you posted are for the switch board. Next, those look like a common modular switch type popular in the 70's and 80's. If so, then each one is 10 pole double throw. Please post a photo of the front panel around the switches, or anything else that identifies their functions, so we can see which one does what function. \$\endgroup\$ – AnalogKid May 7 at 18:46
  • \$\begingroup\$ Also, I don't think these switches are "bailing" (press one, and all others are released). I see the alternate-action toggle mechanism, a short diagonal bar, under the springs in the first photo. \$\endgroup\$ – AnalogKid May 7 at 18:51
  • \$\begingroup\$ The 'mechanism stuff' is between the brown area and the springs. You are describing a reflection of the plastic there. There is a flat sliding bar with diagonal 'teeth' beneath this area and perpendicular to the switch, this allows only one button to be 'in' at once. You can see on the bottom one missing the button - there is a tiny bit of metal there exposed, when we push the button in, it pushes this metal in too, this interacts with the 'slider bar'... I've uploaded more photos. That is a shame to hear that none of the schematics pages match the switchboard! \$\endgroup\$ – Codewell May 7 at 19:29

Looks like that board is the "Presets switchboard". The long rectangular boxes are multi-pole switches. I'm quite sure they're radio buttons (only one active at a time). Depending on which is pressed the components on the other circuit diagrams will get different values. Exactly which values are set by which button are shown in the table on page 1. For example, the first three columns show how each button affects the value/state of R1, R2, and S10 in the V.C.A. Envelope Generator circuit (on page 3). If, e.g., the "Piano" button is pressed then R1 gets the value 33kΩ, R2 is 0Ω (i.e. directly connected), and S10 is on (connected).

I've taken another look at things and it looks like the top switch (by the dual connectors) is the "off" switch; it contains S1—S8. The remaining multi-pole switches control each preset (as shown in the table). I think the bottom one on the picture (by the single connector) is "piano" and the second from the top is "synthy". There is no circuit diagram provided for that board; it would probably be a good idea to draw one before proceeding further. One other aside, I think R4 is always 100kΩ (the notes are a bit hard to read). (Further edit: the additional photos you added match the above)

I think the most challenging part of the repair will be sourcing replacements for the damaged switches. That kind of part is very hard to find. If I were in your situation I'd probably just design a replacement board using 8 momentary push buttons, resistors, a regulator and a bunch of 74-series ICs (latches, etc.) for logic, relays for the audio signals, and maybe some LEDs or bulbs for indication. Alternatively, and easier for a beginner, you could use an SP8T rotary switch, some resistors, and some relays. Lots of ways to approach it...

  • \$\begingroup\$ This is incredibly helpful, thank you \$\endgroup\$ – Codewell May 8 at 12:30

Those large push-on/push-off switches appear to be "presets" for the various voices (synth, harpsi, flute, etc). Their contacts appear as S1, S2, S3 through S8 on the schematic pages. So they aren't limited to one page. (I see them appearing on every page except page 2).

Looking at the photo of the back of the board, it looks like those switches are selector switches -- only one at a time is engaged. The wiring shows them tied together to create a kind of logic explained in the table on Page 1. Good luck, looks like fun!


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