19-01-2023, 01:10 AM
And some pics to show a few of the flavours of circuitry. It is often said of organs that they consist of a very simple circuit repeated a ridiculous number of times. For instruments of this generation that is not true overall, but it certainly is in part.
* The tibia filters are a good example, dozens on one PCB, identical except for component values.
* The divider keyers, located on a block of four stacked boards at lower manual level, arguably form the 'core' of the organ where many of the frequencies and raw waveforms are generated and switched. I will explain these in detail when we get to fault-finding them, but they contain an interesting hybrid of logic and analogue. If they look terribly inaccessible, they are from here, but the whole subchassis pulls out once you have the back off, and the four boards unfold.
* Another board with a mix of logic and analogue is the accompaniment generator, one of the automation features for the non-professional organist to play pre-programmed basslines and rhythms. Functionality is shared between this board and the one to its left. There are three mask-programmed MOS LSI ICs here, but also tone synthesis circuits that they control.
* The audio routing and complex voice filter board is a bit of a jungle. When you push the first valve down, the music goes around and around and comes out in half a dozen places. There is probably a bad 4016 on here that will need finding. If anything is going to make me cave to getting a service manual, it might be this one, as I probably don't have the patience to trace all the comings and goings of the signal routing logic.
* The stop logic board is simply that - logic. No DC voltage-controlled keying, no waveforms, just switching.
You will have noticed that the wiring is all wire-wrapped and in places forms quite a tangle. Some boards are rather awkward to access for soldering until you release nearby looms to set them free. But, the general lack of edge-connectors and similar offers a reliability advantage over some competitors' organs. Certain models such as late solid-state Hammonds are notorious for intermittent connector problems; these days you can pretty much guarantee that if you move a working organ, it won't be working when it arrives. The few wire-to-wire connectors in the Wurlitzers are good quality Molex and trouble-free.
* The tibia filters are a good example, dozens on one PCB, identical except for component values.
* The divider keyers, located on a block of four stacked boards at lower manual level, arguably form the 'core' of the organ where many of the frequencies and raw waveforms are generated and switched. I will explain these in detail when we get to fault-finding them, but they contain an interesting hybrid of logic and analogue. If they look terribly inaccessible, they are from here, but the whole subchassis pulls out once you have the back off, and the four boards unfold.
* Another board with a mix of logic and analogue is the accompaniment generator, one of the automation features for the non-professional organist to play pre-programmed basslines and rhythms. Functionality is shared between this board and the one to its left. There are three mask-programmed MOS LSI ICs here, but also tone synthesis circuits that they control.
* The audio routing and complex voice filter board is a bit of a jungle. When you push the first valve down, the music goes around and around and comes out in half a dozen places. There is probably a bad 4016 on here that will need finding. If anything is going to make me cave to getting a service manual, it might be this one, as I probably don't have the patience to trace all the comings and goings of the signal routing logic.
* The stop logic board is simply that - logic. No DC voltage-controlled keying, no waveforms, just switching.
You will have noticed that the wiring is all wire-wrapped and in places forms quite a tangle. Some boards are rather awkward to access for soldering until you release nearby looms to set them free. But, the general lack of edge-connectors and similar offers a reliability advantage over some competitors' organs. Certain models such as late solid-state Hammonds are notorious for intermittent connector problems; these days you can pretty much guarantee that if you move a working organ, it won't be working when it arrives. The few wire-to-wire connectors in the Wurlitzers are good quality Molex and trouble-free.
