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Posts Tagged ‘electronics’

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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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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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.

Next challenges:

  • 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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