(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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If one removes the hardboard at the back of the Lowrey 44 and takes out the two screws at either end of the big metal tray (the one with the pedal board, the tone generator and the QC board on – visible at the top in this post), then that whole tray pivots outwards around its bottom edge and comes to lie flat. This gives you access to the rest of the organ.
Pay no attention to the man behind the disconcerting metal flap…
The visible bits of the “rest of the organ” turn out to be: the backs of the keyboards, the disconcertingly gorgonoid backs of the tab stop assemblies, and two circuit boards on the back of the flap. One of these circuit boards is labelled “C. F. B” and looks exactly like what the photographs in the service manual claims is the “Rhythm Board”. The other board, on the other hand, is labelled “Rhythm Board” and looks like nothing on earth.
Something tells me these aren’t ROHS compliant.
- Trying to work out what contacts on the backs of the keyboards correspond to which keys, starting with the top keyboard. I know that the top seven keys are wired separately (“solo”, according to the service manual) for some mystery reason. I briefly attacked the things with an ohm-meter and failed to really get anywhere – so the next step is to attack once again the service manual and try to work out how the contacts line up with the schematics.
- Trying to get the screws out of the pedalboard. This is quite a major challenge given that the things seem to have cemented themselves in place. I think I may need a bigger screwdriver.
- Trying to work out how to lift out the whole keyboard/top assembly – it looks like I can, but the multitude of screws are disconcerting.
- Getting a set of two manuals from a Church organ half-way across the UK on a train. This could be fun…
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