service manual

[jwl5151]

I have an Allen MOS-1 632-3 there is a complete service manual available on line. My question is , is there how much of a difference is there between the MOS-1 and MOS-2 electronically .

[myorgan]

I have an Allen MOS-1 632-3 there is a complete service manual available on line. My question is , is there how much of a difference is there between the MOS-1 and MOS-2 electronically .

jwl5151,

Wait until the experts weigh in, but there is enough of a difference, much of what is discussed in the MOS-2 manual (somewhat standardized) is not especially relevant to MOS-1 organs. However, some things supersede the model difference. For example, the DAC-3, and presumably the DAC-4 cards will work in a MOS-1 organ, but didn't come out until the MOS-2 era. The MOS-1 organs used certain amplifiers, while MOS-2 organs used other variants, but I think that information would be manual-specific, or addressed in a separate audio service manual completely. I know the ADC organs have a separate manual for ADC Audio.

I hope this gives the information you need.

Michael

[jbird604]

MOS-2 is, in the words of Allen, merely a "refinement" or "repackaging" of the original MOS technology. While some of the boards are interchangeable, for the most part they re-designed all the components. The basic operating voltages are changed too. Even the card reader is different, though it reads the same cards. So it looks quite different in many ways, but Allen maintained that the tone generating technology was not fundamentally different.

One major change is that just about all the "optional" features available on MOS-1 were made standard on MOS-2. All MOS-2 models came with a card reader, for example, and the previously optional features called "frequency separation" and "delay" were incorporated directly into the new MOS board itself. The new "second church" voicing that was found only on "dash three" versions of MOS-1 organs became the standard stop list for MOS-2 church models. The 8' and 4' members of flue choruses were then placed in opposite audio channels for better ensemble. Functions that previously required a satellite board were integrated into one of the primary system boards. Chiff was greatly improved, as was percussion and sustain, though MOS-2 still doesn't hold a candle to ADC on those.

Also, the "code" system of the MOS boards was greatly simplified. Whereas in MOS-1 there are 3-digit numeric codes plus alphabetic prefixes and suffixes to specific the stop list and features installed on a board, with MOS-2, there are only two types -- one with the classical stops installed, another with a set of theater stops. The variations in mixture types, 16' and 32' stop selections, and certain other tonal features were accomplished by installing customized EPROMs into the two sockets on a MOS-2 board. So they no longer needed all those different "codes" of MOS boards, as they could fully customize any board with EPROMs.

The old double-stack and triple-stack key contacts required on certain MOS-1 models were no longer needed to maintain isolation among the computer systems in a multi-computer model. A new system of key multiplexing called "SDDS-MUX" was employed to transmit keying data to all computer systems simultaneously, and that system handled all the coupler functions as well. This simplified wiring within the consoles considerably.

So MOS-2 was a nice improvement and enhancement of MOS, but not really ground-breaking in the way that ADC was just a few years later. Given all the free "extras" and incremental advances, I'd take a MOS-2 model over a MOS-1 given the choice. But those who find certain MOS tonal qualities (the MOS "buzz") so annoying will not find MOS-2 to be much better.

The results at the high end of the model line can be pretty astounding though. We refurbished a MOS-2 system 1105 a few years back and installed it in a nice local church. Whenever we go to service it I am genuinely impressed. I think it's one of the best-sounding digital organs in this area, despite being 36 years old and several technological steps behind the current models. In a way, MOS-2 simply brought together all the best work that Allen had done up to that point. Lots of good stuff going for it.

[myorgan]

In a way, MOS-2 simply brought together all the best work that Allen had done up to that point. Lots of good stuff going for it.

Bottom-line it for us, John. Would a MOS-2 service manual be of much benefit for someone who has a MOS-1 organ?

Michael

[jbird604]

No. If you have a MOS-1 organ and your MOS-1 manual has been kept up to date with all the revisions and service letters and so on, there won't be anything you need in a MOS-2 manual. The MOS-1 manual as issued to dealers in 1971 had to be updated each year or so, as models were revised, certain new features were added, such as adjustable random motion, and when the capture system changed over from sequential to DM, then DM-2, then DM-3. The power supplies went through several revisions, as obsolete components were replaced and circuits were modified for stability and reliability. Amplifiers changed over from the T-50 to the S-100. Trem generators were upgraded to the Trem III. New speaker systems were introduced along the way.

The MOS-1 manual was issued in two volumes. The first volume covers all the basics of MOS, all the boards and assemblies that are common to all MOS organs. The second volume describes all the multiple computer models and has descriptions and some schematics of certain boards and assemblies that are used only in the multiple computer models and custom organs.

So if a person has the complete set of two volumes, along with all the revisions, additions, etc., then that is all that can be had on MOS-1 organs. The MOS-2 manual only covers the new boards and power supplies and such that are common to all MOS-2 organs, along with the SDDS-MUX keying system. All in one volume, it also describes the multi-computer MOS-2 models. Charts and tables detail how the various models are customized with jumpers and wiring changes and such, how the tremulant, celeste tuning, frequency separation, and other features can be adjusted over a wide range. There are tables listing the correct EPROMs for each MOS board in all the models. So if you own a MOS-2 organ, you will find a wealth of information in the manual.