Allen MOS2 - 1105 installed

[jbird604]

I don't think I've said anything on this forum about the nice install we did last year with an Allen 1105 that came our way. I just created an album called "Installations" and posted a pic of that console in its current home.

Another member of this forum had been offering this organ for sale in a neighboring state. He and I had done some business in the past, and he contacted me with an offer. He needed a couple of the organs in his large collection serviced and was willing to trade us the Allen 1105 for making a service trip to his home. We agreed and set out on a trek to bring in the Allen.

We were able to repair his two nice instruments, a Conn 3m AGO theatre organ and an Allen MDS 3m theatre. We made arrangements to have the Allen 1105 brought to us, along with a big assortment of speakers and the large amp rack.

This organ, a 3m drawknob in the massive Allen "C" console was way too big to bring into our shop. We don't even have double doors and can barely get a "D" console in. We decided to install it in my associate's church in exchange for the church's existing Allen MADC 3100, a much more easily handled console, which I knew we'd have a better chance of re-selling anyway. This 3100 (which he and I installed right after he came to work with me, taking the place of an old T-12) had served his church well for over 3 years and is in great condition, but the big 1105 would be a major step up in spite of being a couple of years older.

We had to do some work on the organ to get it playing. As I recall it had a couple of bad chips on a Keyboard Array and one MOS board that was doing some odd stuff, perhaps a few other minor malfunctions. We got a crash course in MOS2 technology, figuring out what the various EPROMs did when swapped around among the different computer systems. We came up short one board for making all four MOS systems work, so we sacrificed the "C" computer, which was solely for the mixtures on the swell, great, and pedal. We discovered that transferring the EPROMs from the "C" system to the "A" and "B" system gave us excellent mixtures, so little was lost in not using the "C" system other than separate speakers for the mixtures, and we gained a source of spare parts for the rest of the organ.

We replaced most of the drivers in the speaker cabinets due to foam rot, then got it up and running pretty easily. It is now playing through eight Allen S-100 amps driving about 14 speaker cabinets. Two of the S-100 amps are driving subwoofers, augmenting both the "A" and the "B" system pedal channels, one of which contains the massive Contre Bourdon 32' and the other the Contrebass 32' and the 32' reed.

I had a big horn-type speaker (actually some sort of PA speaker) that we used for an En Chamade. When engaged, it parallels the choir channel that has the Trumpet and the alterables, making possible some huge reed effects, as it is many decibels more sensitive than the regular Allen cabinets.

We also discovered how to give it theatrical tremulants by moving some jumpers on keyboard array plugs, and changed the great manual's "chorus" tuning to a lush celeste, also by moving jumpers. Both the theatrical trems and the great celeste are connected to drawknobs or tilt tabs, and so are available to the organist at will. It has been quite interesting to discover how many features were built into the MOS2 system but never implemented by Allen in stock models.

Future plans include adding on the MOS2 MIDI adapter (if we can figure out a way to have it transpose the MIDI output) and possibly using the now-silent "C" computer for some solo stops or other creative application.

Sound?...... It's distinctively Allen, with the "growl" that you don't hear from most other organs. Not sure what it is, but it is something I hear in almost any Allen, even the big old analogs and in the latest Renaissance. It makes for an exciting listening experience, as much like pipes as anything you'll hear from a digital. The church is fairly live with a high vaulted wooden ceiling and minimal sound-absorbing material, enhancing the organ sound quite a bit.

To top it off, Matt and I visited a local retired pipe organ man and bought an assortment of large pipes that are now set up as display pipes in front of the Allen's speaker grilles. Seeing pipes makes one feel even more like you're hearing pipes!

[indianajo]

You used to be able to get distributors to duplicate PROMS for you. I used Hamilton Avnet and Pioneer back when. Perhaps you could save the organ with the bad PROMS by duplicating those in the working organ. Custom LSI is pretty much OEM or scrap, but PROMs are just bits out a pin, they don't matter on the internal technology much.

[circa1949]

Hhhmmmm. Jbird your narrative is making my head explode trying to understand how stops were controlled in MOS-2. In ADC at least, the backplane directly wires everything to everything else. After the key assigner, I'd assume that if you switched a PROM around they would respond according to their slot, not according to their ID. Does that make sense? There's no "universal bus" like a PCI backplane holding "universal codes" for the stops and notes that could trigger any arbitrary compatible tone-generator. (there is a serial connector from the console multiplexer to the key assigner, of course) How have you moved the PROM from one computer to another and not had the Mixture III activate when the Schalmei 8 was intended to activate?
Furthermore I thought the whole point of having a separate computer was not for audio distribution but because the "Brain power" (LOL) that it required to do mixture breaks took up a whole computer.
Maybe you are saying you just have a MOS2 computer that was entirely surplus to the organ that you've installed into the organ using the wiring harness intended for the bad computer? And I misread you?

[jbird604]

Not that complicated, circa. The mixtures on any MOS2 organ are custom, in other words they are coded on the EPROM that goes into the "mixture" socket on the MOS2 board rather than being read out from the hard-wired ROM on the board that stores most of the other waveforms. The "C" computer of an 1105 (as on the 705 and perhaps other MOS2 organs) is dedicated to mixtures, but it is wired to the stop control and keying systems just like the regular A and B computers are. In other words, the mixtures of the "C" computer are drawn by the same stop control line that draws the mixtures on a single computer MOS2 organ, or on the A & B computers of a big one like the 1105.

The "C" computer doesn't respond to any other stops drawn because there are no stop control lines tied to it from the rest of the stops. It isn't otherwise any different from the rest of the computer systems, except for the special Frequency Number chip on the KBA, which I'll mention again....

What we did was simply transfer the mixture EPROM from the C computer to either the A or B (forgot which) so that drawing the mixture stop now caused that computer to produce the mixtures that otherwise would have come from the C computer in the original scheme. The stop control system turns on the mixtures simultaneously on the A, B, and C systems when a mixture stop is drawn, so it doesn't matter to the system which computer is actually producing the stop.

The original design of the 1105 put the mixtures on their own computer along with a custom version of the Frequency Number chip that was capable of creating a variety of mixture break patterns. This was ostensibly because, as you said, the mixtures required the "brainpower" of a dedicated computer. I doubt that seriously. It was more of a marketing thing, but perhaps the mixtures were "better" coming from the C computer because (1) they had their own pair of audio channels which could be adjusted for bass, treble, and level independently of the rest of the organ -- you could even regulate the balance between the unison and the fifth components of the mixture because they are in different channels -- and (2) the player could choose between different breaks in the pattern. Other than that, no difference.

So, we still have the same mixtures but the mixtures come through the same audio channels as the rest of the stops instead of their own. There were actually EPROMs in the mixture sockets of all four computers, so we tried the various mixture EPROMs in different permutations on the A, B, and D computers and came up with something that sounded very nice. We happened to have two copies of the EPROM from the choir (D) computer because we at first mistakenly thought it was defective and ordered a new one from Allen (at a cost of about $150, I think... ouch!) We found that there were various mixture compositions on the various EPROMs and matched them up with the remaining A, B, and D computers to best effect.

I have learned a great deal about the functioning of the MOS2 system from this experience. It really is quite a nifty little system that can be configured to do just about anything you need an organ computer to do. Each stored sample is addressed by grounding a particular pin on one of the boards, which is normally what happens when a knob is pulled out, so you can change the tonality of any stop by moving its wire to another pin or even by putting in jumpers to cause a single knob to draw more than one sample at the same time. That is how they get the choir stops to sound a little different from the great and swell, even though they are using the exact same palette of stored waveforms. Add a chiff here or blend in another stop there, etc.

One of these days, we may get back to this project and possibly wire up some of the stops already being produced by the A or B systems to the C computer, though we'll have to purchase the standard Frequency Number chip for the KBA, since the one in the socket now has the mixture breaks built in and causes every stop to break back like a mixture! A spare knob in the great could, for example, draw a "Plenum III" stop which might be a combination of 8', 4,' and 2' stops sounding from the C computer through an independent audio system. Theoretically could add a great deal of power to the organ, could even be intentionally tuned off 440 a little to exaggerate the ensemble. Lots of different stuff could be done with that now-unused system if we get the time.

Still having fun! Hope everyone has a great weekend.

[circa1949]

John, as usual your technical explanations are excellently written.
It sounds as though the MOS2 system allowed more flexibility than the ADC system would have, to make these types of changes. Certainly on an ADC organ the stop and frequency selection words for one TG card do not go out to all TG cards...
I bet with the early 80s being the peak of the baroque revival, Allen was very keen to show how flexible their system could be with mixtures, hence the marketing-ese of having their own computer, if that is the reason as you suggest.

[jbird604]

I hadn't thought of the neo-Baroque connection, but you're probably right. One could get some really hot mixtures that way. Some of the churches we service where MOS2 organs are installed don't even use the mixtures (gospel-ish churches) and wouldn't know the difference if the C computer were completely removed. Had a minister of music once ask me to turn the mixture channels completely off because the organist was hard of hearing and couldn't tell that she was overusing the mixtures.

I think you're right about ADC TG cards. Moving a card (along with its EPROMs) from one slot to another will cause a given stop control to elicit the wrong tone. MOS organs are built quite differently, with every standard stop resident in the ROM of every computer board. (Custom stops such as mixtures are not in the ROM, as I said, but must be added via EPROMs.)

So any MOS2 board is capable of producing the entire set of some 36 voices that make up the standard two-manual plus pedal stoplist. The small single-computer organs use only one board to generate the entire stoplist, but larger organs used multiple boards. Early MOS systems always used MOS boards in pairs to create the illusion of ensemble and to create true celestes when the boards were tuned apart by the celeste tuning control. More elaborate MOS systems, especially in the MOS2 era, began to pick and choose which stops would be taken from which boards so that, for example, the Principal 8 could come from one board and the Octave 4 from another, enhancing their distinctness and slight pitch offsets. This concept was called "cascading" I believe, and became the basis for the ADC system of alternating the members of most choruses between channels for better ensemble and buildup effects.

Our 1105 has the A and B computers set up to basically mirror each other, most stops duplicated in both so a small amount of chorus tuning is always noticeable. Then the celeste tuning control can make a true celeste out of any stop that is drawn along with it. A truly Grand Celeste is heard when a tutti registration is drawn with celeste tuning. The "D" computer, in contrast, serves only the choir division (and provides a couple of pedal stops) and does not have a mirror system to celeste with. Therefore the choir has no celeste stops of its own, but the swell can be coupled down to it with a "natural" coupler (in Allen terminology) that preserves the celeste tuning of the swell if drawn. All choir stops are simply drawn from the swell palette in the ROM plus a few custom voices on the EPROMs. As I said earlier, some have additional chiff or other extra stuff mixed in via the stop control jumpers to make them sound a little distinct from the swell A and B.

Besides the mixtures, some of the 16' and all the 32' stops in a MOS2 system are on EPROMs instead of in the ROM. So, on the 1105 one of the systems has the 32' Contre Bourdon and the other system has the 32' principal voice. This is nice because we have separate subwoofers on each system and can vary the power and depth of the 32' stops independently.

I'm not particularly nostalgic about the MOS2 era and can appreciate the evolution of digital sound since then, but there are a lot of capabilities of the MOS2 system never fully exploited by Allen, so we keep on discovering nice things we can do with this organ.

[circa1949]

Wow, again, thanks for the informative write up. I learned several things about MOS era organs I didn't know in this post.
One thing: you imply the EPROM slots for extra voices were a part of the "Standard issue" MOS2 computer. Were such slots also found on MOS-1 era boards?

[myorgan]

Not that complicated, circa. The mixtures on any MOS2 organ are custom, in other words they are coded on the EPROM that goes into the "mixture" socket on the MOS2 board rather than being read out from the hard-wired ROM on the board that stores most of the other waveforms. The "C" computer of an 1105 (as on the 705 and perhaps other MOS2 organs) is dedicated to mixtures, but it is wired to the stop control and keying systems just like the regular A and B computers are. In other words, the mixtures of the "C" computer are drawn by the same stop control line that draws the mixtures on a single computer MOS2 organ, or on the A & B computers of a big one like the 1105.

John,

I know my mixtures on my 505 are on an ePROM as well on both computers (pictured below directly under the wire "jumpers"). I wonder if I could do something similar to what you did on my 2-computer organ? You made me curious about the jumpers being moved, though. My Pedal 4' Schalmei has never worked properly, and I wonder if the wire "jumpers" in the photo below could be the cause.

Thank you for your reports on your 1105 enhancements. In my search for a 3-manual instrument, I nearly purchased one, but the price was just too much.

Michael

[jbird604]

Michael,

What we did on the 1105 was basically set up the mixtures as they come normally on a 505, that is to say our mixtures are now on the A & B computers just as yours have always been. If you wanted to, you could have Allen burn you some new EPROM's with different mixture compositions. According to the MOS2 literature, any voice in the card catalog can be burned to an EPROM for the MOS2 board. They charged us $150 for each EPROM, as I recall, so it could get expensive experimenting with that. (I only ordered two EPROMs because I wrongly believed that two of ours were defective when it turned out in fact to be a problem with the MOS board.)

The 4' Schalmei is a voice that is on the "sub-octave" EPROM on the "A" MOS board. If it doesn't play at all, it would be because the EPROM is corrupt or has been altered for some reason, or because the stop control's reed switch is not making contact, or because the wiring between the stop itself and the Motherboard Array is broken somewhere. Those are the only things that come to mind anyway.

However, you may not actually have a malfunction at all. On some MOS2 organs the Schalmei 4' shares an EPROM slot with another stop in the pedal division and will therefore not play if that other stop is also drawn. I know that in some models it was a 32' stop that it shared with. Since the 4' is primarily used as a solo stop, the designers probably thought it would rarely be drawn at the same time as the 32' or whatever stop they paired it with.

Let me know specifically what doesn't work about it and maybe I can suggest a solution if there is one.

Good to hear from you, my friend.