Allen 2MP (1990s ?) Need help with model for Manuals - Service and Owners

[kiaya611]

Hi, I just got this Allen 2MP organ and I was hoping someone might be able to help me get a model and perhaps date so I can try to find a manual for it. I have ALLOT of pictures, but I won't bore you with all of them...just a few showing the organ and the places I found the SN and another number which keeps popping up as well...It seems to be an MOS-1 according to some people, so I was hoping I might be able to find a service manual for it. Thank you in advance for your help. Steven

[myorgan]

Steven,

Look for a label on the back crossmember where the top is hinged. Your console, however, is a contemporary console, so I don't know exactly where that label would be. You will be looking for a label like the one pictured at the bottom of my post.

The AO number is probably the finish code, and the AC number is probably the serial number.

I hope that helps get you started until the pros weigh in.

Michael

[kiaya611]

Thank you very much Michael. I will look some more for a plate like that.

[jbird604]

Definitely MOS, as the spaceship-looking console was only used for those models and the very earliest of the ADC (which this cannot be, for other reasons). So it's MOS, and since it has the folding top rather than a rolltop lid, it's MOS-1.

It has speakers in the console, so it's of the 100 series. It has the simple 5-piston preset (if it were ADC it would have at least 7 pistons), and it has a card reader. All these characteristics -- it's MOS-1, self-contained, simple presets, and card reader present -- make it a model 122, I believe, though it could be a 102 with a few upgrades.

Of course all the early MOS organs from 1971 through about 1980 are nearly identical anyway, and the same service literature applies to them all.

The System 122 (or whatever this one actually happens to be) is a single-computer version, meaning there will be just one MOS board, one DAC, two amps wired to separate speaker systems, and one each of the various peripheral boards. A basic MOS power supply group putting out -27 volts, plus and minus 5 volts, the card reader voltage, and a 12 volts unregulated, probably on two assemblies in the floor of the console.

Unfortunately, Allen still refuses to release the service literature of these now 45-year-old organs to the public. But in truth there was not a huge amount of information in there anyway. Just diagrams showing how it's all wired together and such. Few schematics of critical boards, as they generally wanted techs to return defective boards to the factory rather than attempt in-field repairs.

You should open it up, clean it out thoroughly but carefully, because very careful not to disturb any wiring, even stuff that seems to be just loose in there. Since it is self-contained, there are no connections going out to speakers or other outside equipment, and if you're lucky, no extraneous wiring that has been added.

Once you have it really clean inside, you should try to remove each and every connector and clean the connecting points. I wipe on the thinnest possible layer of Vaseline on the card-edge connectors to ease insertion and protect against future corrosion. Some people use other types of protectants.

Carefully, and taking precautions against static charges, remove the circuit boards one at a time, and clean their card edge connections too before applying the Vaseline and re-inserting firmly but gently. Follow the audio signals from DAC to amps, making sure you clean the RCA plugs and jacks at each point.

Finally, with it all back together, use a meter to carefully set the -27 volts and the two 5 volt supplies precisely. It should now run well.

The stop tabs have simple open air switches which may need to be cleaned using a Q-tip dipped in alcohol. Be careful not to bend the little switches. If you do, line them up so that they make good contact when the tab is pushed down.

Key contacts are also open-air, and may need to be vacuumed clean. If any of them stutter or refuse to sound, try slipping a clean dollar bill between the contact points and moving it around gently while pressing down on the note. This should clean any accumulated corrosion off the points.

Set the volume levels at the amps (knobs or stubs of volume pots near the input jacks). Set the bass boost on the DAC board or on the small assembly next to it, to give a solid bass output, but not too much at higher volumes.

If the expression pedal won't make changes in volume, the little lamp inside is burned out. Pop off the lamp holder from the side and insert a new type 57 automotive lamp. (the original has a black dot painted on the apex, which you can try to do or not)

[kiaya611]

I finally found the plate and it is a 122-c. (1970s) Here is the link to all of the pictures I have taken of it. Maybe it will help with what they all do and how to adjust the trem and other things. Also, I want to connect my Microverb4 to all of the speakers (internal and external/Antiphonal..as they are calling it). I am just not sure where the best place to ut it into the circuit to get both channels and the inner and outer speakers. Here is the link: https://www.dropbox.com/sh/4evifrhqf...09Dxd9rca?dl=0 Hopefully this will help since I can't seem to find the Service Manual for this organ. I hear that Allen is pretty tight with their service manuals...even one's this old. Thanks, Steven

[jbird604]

That' a nice-looking specimen, and pretty darn clean inside. All the advice I gave above still applies, though, as this is good maintenance and should be done every few years as a preventive measure.

You have all the standard MOS-1 boards. Tuning is accomplished with the coil on the CLOCK board, trem is adjusted by the TREM wheel on the KBA. Only the Trem Rate is adjustable, the depth is preset. The KBA also has pots for "speech articulation" which is a subtle "yip" or pitch shift that you can hear on a sustained high note while you play staccato notes in the lower range of the same keyboard. A peculiar concept, but it does in fact add something to the "realism" of the sound. Bass pots on the DAC board allow you to regulate the amount of bass in each channel, as I pointed out above.

You have the standard set of two amplifiers (S-50 type, which are a bit unusual to see in MOS-1, but quite acceptable). And someone has installed a "Univrel" antiphonal switching relay board. Presumably, you can switch your internal and external speakers separately with some tabs on the front.

Much has been written on this forum about connecting a modern reverb unit to MOS organs. You need to have to reverb unit in series between the DAC and the amplifiers, and you must set it for a relatively "dry" field, which is to say, you can't have it "all wet" or else you won't hear any of the direct sound.

COMPLICATION: MOS organs do not operated with standard "line level" audio in the signal paths, so inserting the reverb unit into the audio path will disable the expression function (which is connected directly to the input of the amplifiers). To get around this, there must be an isolating resistor in the audio line coming out of the reverb unit on the way to the amp input. The value normally specified is 22K ohms, and you need to break each of the signal wires and solder in a resistor of that value. Do not break the shield/ground connection though, or at least be sure to re-connect it.

Wiring the reverb into the system this way will give you reverb in both the internal and external speakers. If you wanted to have reverb only in your external speakers, you would have to get an extra pair of amps to drive the external speakers.