Electronic organs - circuits, experiences and interests

[Lucien Nunes]

The other thing is the number of contacts for the keys. The more complex the organ, the greater the number of contacts.

An interesting point raised by @joebog1 but there are some twists. Historically, yes, with additive-synthesis organs like tonewheel Hammonds, each harmonic that you want to make available in the tonal palette needs its own contact. Hammonds have nine busbars behind the manuals, and close 9 contacts onto them when you press a key. Compton did the same with ten busbars directly operated by the keys in their cheaper models, but this makes it difficult to couple manuals together which is a requirement for serious organs, plus it can make the keys clunky if you try to add too many contacts, and/or they don't all close exactly together if you press the key slowly.

To improve on this, Compton and other makers of electronic church organs used keying relays with however many poles were needed for the harmonic palette. Then, the key itself only needs to make one contact for the relay coils of each division it can control. For example, the Great manual keys might make separate contacts for its own relay, the Swell relay via the Sw-Gt coupler, and for each of the sub- and super-octave couplers. The same obtains on the electric action of pipe organs - the number of contacts made per key depends on how many divisions it controls, at how many octaves. Downstream at the key relays, there are as many poles as there are harmonic components to mix. Compton key relays usually have 10 or 21 poles for tone signals, plus sometimes have a few extra poles for special functions. The biggest of the Makin electrostatic organs took this to extremes, with generator systems with up to 63-pole key relays.

The Wurlitzer 950 has only one contact per key using a gold-plated wire and busbar (see pic). Touch depth and weight are adjustable by a screw and a spring above. Although there is only the one contact, there are multiple different ways that is used to control the organ, so it starts to get a bit convoluted. The discrete one-wire-per-key switching first passes through a bank of analogue switches on the long board behind the bass end of the lower manual, which route the appropriate manuals to the keyer inputs. One bank of router outputs leads to the 'organ voices' divider keyer assembly, which has one logic-level input per key. Each 147247 keyer IC only handles three footages, so the keying inputs are commoned across multiple ICs. Another output drives the DC-keyed sustain voices, where it is fed through an LPF etc. to form the envelope from which diode-keying controls the signal amplitude.

Then there are a couple of sections, 'effects' and gadgets that use either sequential or X/Y multiplexed keyscan instead of discrete switched lines. Some of this has to do with IC pin count; The solo 'Orbit' synth and the pedal tone generator both have 40-pin LSIs with an X/Y matrix input to allow one IC to scan all the key contacts. Therefore a few discrete-wire to X/Y convertors are needed, two are visible in post #9 pic 3, they are the 2nd and 4th PCBs on the lower hinged-up chassis with rows of transistors. A separate non-multiplexed but partially matrixed read of the lower manual is needed for the accompaniment generator LSI, which also controls the arpeggiator which actually drives the key lines.

Then there is a gadget called 'Chord Magic', a kind of intelligent coupler from the lower manual to the upper which 'thickens' and enriches the voicing without interfering with any melody line. It works by scanning the lower manual to identify what chords (if any) you are playing; it simultaneously scans the upper manual to find what you are playing there and adds a suitable inversion of the identified lower-manual chord beneath the melody on the upper manual voices. Because the Chord Magic IC is both reading and driving the upper manual, to prevent it reading its own chords back in it has to momentarily tri-state its outputs every few milliseconds to test the key contact states. The drive glitch is short enough that it doesn't make it through the debounce to the actual keyers. As with the synth LSI it uses multiplexing to overcome pin count limits. Looking at post #9 pic 5, the small board under the bass end of the hinged-up upper manual muxes the key lines to the pair of 40-pin packages on the corresponding board at the treble end.

So overall there is rather a mish-mash of different ways of reading the deceptively simple single-pole key contacts.

Wurlitzer gold wire key contact shown below. Pics to follow of Compton key contacts and relays.