(This is a nerdy update at least as much for my own memory as for anything else.)
To try to get around some of the issues I’ve raised in the last post, I’m going to try a new ‘relayboard’ using Opto-Isolators. An opto-isolator is effectively an LED and a light sensor (a phototransistor) in a sealed, dark box.
It should be possible to wire one of these up like this (ish):
I’m looking at a Toshiba quad-opto-islator chip that will switch up to 55v at up to 150ish mA (datasheet) – and given that the existing diodes in this beast seem to be rated at 100mA, it probably won’t go toasty. The voltage on the organ side is somewhere less than 23 volts, according to the service manual (schematic 5; page 18 of part 2; the left hand side – point BB is the pink common wire to all the keys on the upper keyboard)
So it should be fine to switch it through the transistor side of the opto-isolator.
Still need, though, to find the value of the resistor that protects the LED: according to the datasheet, the LED will eat 1.15 volts at up to 50 mA (0.05A). The power supply is at 12v. For safety’s sake (and to avoid warmth), we’ll stick to pushing 20mA through the LED, giving a resistor value of:
So a 620 ohm resistor should do the job. Next step: prototyping this…
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Cruft of ages, unglued for me…
I have sanity-checked yesterday’s results!
The non-solo keys need to be wired to point BB, which I tracked down on the circuit diagram. This turns out to be accurate on the rhythm/percussion board – and the wire is, indeed, pink. Going from the left of the board there are two groups of two big orange capacitros. To the left of the second group of two capacitors are three spiky contact things. The uppermost of these has the pink wire and is point BB.
To test this, I rigged up a trailing wire between this contact and the note contacts (also identified from the PCB diagrams) and checked that everything worked. It did.
Above the first group of two capacitors is another set of three contacts. Point BA is the middle one of these: according to the service manual, the first one is the sustain control line, and the third one is at ground. It has a turquoise wire attached to it at the moment. This, jumpered to the seven solo contacts, gives the required results. So this is where the second half of the relayboard needs to be jumpered to.
- Have successfully de-glued the plastic plugs and sockets that connect the pedals and the amplifier to the top of the organ.
- Have utterly failed to undo any of the structural screws.
- In other news: I need a bigger screwdriver.
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