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Archive for the ‘Electronicals’ Category

Pruning the electronic forest

The version 2 relayboard is on the right; it uses the solid-state relays mentioned in my last post to control keys. It works. I’m making another one now; after that I will have enough to MIDI-ify the whole upper manual.

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(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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I’m still alive, and so is the project!

Relayboard no. 1 is finally complete; this board, to remind any imaginary readers I may have, will electrically replace about half the keys on the top keyboard.

I wonder which species of bird is nesting on my sofa

For the non-electricians: a relay consists of an electromagnet and a little switch in a box, so that when the electromagnet is turned on, then so is the switch; and when the electromagnet is turned off, the switch is turned off.  This is nice here, because I don’t trust the electronics inside the organ to be predictable – so I can control the “keys” (the “switch” side of the relay) via another circuit without actually passing any electricity between them.  So far, so good.

I have now tested the control side of the board, and it more or less works; next up, testing the switch side of the board.

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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.

Other stuff:

  • 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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This post does not contain any pretty photos.  Those in search of organ erotica will need to come back next time.

In the grand scheme to decouple the manuals of the Lowrey Genie 44 from the back end, I need to build something to sit, electrically, where the keyboards “were”, to emulate the keyboards.  I’ve decided to use relays for this purpose, given that I don’t entirely trust the electrical environment inside the organ, and I’d rather not spend ages soldering transistors down just to generate that lovely burning semiconductor smell that we all know so well the first time I turn the thing on.

Each key on the top keyboard is effectively a switch; and as such I’ve obtained a moderate sized bag of single pole single throw relays from RS which claim to be able to take up to 30VDC through the switch bit, and have a 12V coil voltage.

This is quite convenient, given that the MIDI Boutique decoder I’ve got my eye on works on 12VDC – so I can avoid proliferation of power wiring.

How should the relay boards work?

Now that I have the correct service manual (the TG44-I Service Manual from the Lowrey Forum website – you’ll need both PDFs) for the organ, I’m closer to an answer, at least for the upper keyboards.

The upper keyboard is divided, as I said before, into two groups of keys.  The upper 37 form the “upper keyswitches”.  There is a common bus that runs along these, which is connected to the terminal for each key switch.  That’s a horrible sentence – this diagram may help.

For the 37 “upper keyswitches”, the bus line comes out of the murk of the Auto Wow circuitry (this is at the top left of schematic 2 in the service manual, on page 14 of part 1).  This is labelled as point “BB”.

Finding point BB involved a significant treasure hunt; I’ll save you the effort if you’re following along at home and point out that point BB is at the top left of page 5 of part 2 of the service manual, on the PCB layout for the Pedal and Percussion Board.  This should be the terminal where the disconcertingly pink feed cable I found last post should emerge.

Where do the other ends of these wires go?  The answer to this one can be found on page 2 of part 2 of the service manual.  The contacts are scattered around the Tone Generator and Keying Board in what seems to be a fairly ad-hoc order in some places.

So, the first part of the relayboard arrangement for the upper manual needs to sit between the pedal/percussion board and the tone generator board.

The remaining 7 keys on the upper keyboard, the lowest 7, are the “solo keys”.  These are monophonic; only one can be played at once.  It turns out that my previous supposition that they shared a bus with the non-monophonic keys was – to put it bluntly – wrong.  Page 18 of part 1 of the service manual clearly shows that the bus for these keys emerges from point BA:

Turning once again to part 2 of the service manual to consult the PCB layouts, point BA turns out to be in the upper right of page 4 – that is to say, right next to point BB on the pedal/rhythm board.

Where are the other ends of these?  There is less distance to look this time – in fact, the keyswitch contacts for the seven solo keys are on the left hand side of the very same page 4.

This means that these 7 need their own isolated area of a relayboard, and are strung between one side and the other of the pedal/percussion board.

The lower manual is a thornier area that, as yet, I will not touch.

Conclusions:

  • Upper manual relayboard needs to be in two parts.
  • One takes its bus from point BA in the middle of the rhythm/percussion board and jumpers it to individual keyswitch contacts on the tone generator board.
  • The other takes its bus from point BB in the middle of the rhythm/percussion board and jumpers it to individual solo keyswitch contacts on the side of the rhythm/percussion board.

Next steps:

  • Attach a piece of wire to BA on the rhythm/percussion board and see if I can play the keyswitch contacts for the non-solo keys.
  • Attach a piece of wire to BB on the rhythm/percussion board and see if I can play the keyswitch contacts for the solo keys.
  • Try to work out how to attach wires to the things…

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First Parts Post!

This is mostly here as a braindump for me, but feel free to poke if useful.

  • Something to turn user events into MIDI: Highly Liquid’s MIDI CPU.  This seems to be more flexible (programmable by SysEx is going to be indispensable at least while prototyping) and cheaper than other offerings, and looks like it’s fairly highly spoken-of.
    • Probably looking at one of these per manual, plus one for the tabs.
    • I’ll need to fab up a keymatrix somehow.  I’ve got some ideas about that, but watch this space.
  • Something to turn MIDI events into ‘fake’ keypresses: MIDI Boutique’s euphoniously named mdec48dd-plooks plausible.
    • Again, one of these per manual, which will probably get expensive.  Exactly what kinds of relays and so forth I’ll need to wire this back up to the organ will depend on what’s going on electrically inside.
  • Something for couplers: I’ve been eyeing up doing hardware coupling between manuals/logical devices. Again, MIDI Boutique may come to the rescue here: the mr4 may be persuadable.

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