Need help with Allen organ repair-no tech available in my area

[Frank Kuhny]

Hello,

I have an Allen MOS-1 organ (serial number AC-4274) and live in Tallahassee. I am a doctoral student at Florida State studying piano performance with Dr. Ian Hobson and secondary organ with Dr. Ian Quinn. I play organ for the Cathedral of St. Thomas More. I practice on my organ daily and really need it to function.

I have called Allen for service, but there is no technician in my area. I am on a waiting list for someone to come from about four hours away, but I have been waiting for several months and am being told it could be much longer.

As for working with electrical devices, I have a Bachelor of Science in Electrical Engineering from Notre Dame (as well as a Bachelor of Arts in Piano Performance). I do understand circuit boards and have a soldering iron. I am also a piano technician, having studied with Mr. John Minor at the University of Illinois and Mr. Jeffrey Cappelli in Chicago. I am confident that I could make a good attempt at the repair with a little guidance.

The first symptom that I had was that the swell pedal stopped working in the fall. The main issue though began in mid-December. In the middle of playing the organ, it began to make a very loud static noise constantly. Now, the static is only tolerable after having opened the back and having disconnected the input cable for the “Main” stops leading into the amplifier. It would appear the static is coming from that input as the static and “Main(/principal)” are no longer amplified. The flutes stops do not appear to be associated with the static. I was able to examine the organ and view its circuit boards, and nothing appears obviously damaged. When I called Allen, a technician had me try to clean the contacts on each of the AV cables, but no change resulted in the static output from the speakers.

Any suggestions would be much appreciated. I will do my best to answer questions right away, but please excuse a delay of a few days since I work long days teaching and accompanying.

Thank you in advance for any help!

Frank Kuhny, Tallahassee

[jbird604]

Frank,

Welcome to the forum. I hope we can help you. Your situation is becoming all too common, with organ service techs harder to find all the time. It's not uncommon any more for us to get calls from people four or five hours away desperate for service, and it sometimes takes a while to work such a trip into the schedule. Sounds like you have the background to do a lot of your own work. I actually started out the same way, as a musician who got into piano tuning and rebuilding, then found a big need for organ techs so I slowly transitioned to that field.

With intermittent noise or static, hum, distortion, or other audio issues, the first thing to check is always the RCA cabling throughout. Your discovery that the noise is in the Main channel only is valuable. I trust that you have cleaned and snugged up the RCA cables all the way from the DAC to the amps on both channels. I am a proponent of using Vaseline to clean and lubricate the pin and shoulder surfaces of RCA plugs/jacks to remove corrosion and ease insertion. Some people use other chemicals.

You could also try removing only the Expression cable going into that amp, and then only the Mute cable to see if either of those happens to be the source of the trouble. I guess you've surely already "exercised" the volume control pot on that amp. Also the bass boost pot on either the DAC board or the attenuator board next to the DAC, depending on the version of the organ you have.

Don't fail to check the voltages on the MOS power supply unit. The -27 volts is particularly critical, and if it isn't pretty close to spec the system can generate noise or distortion. Also check the voltages with the AC scale of your meter, and take note if any DC voltage has more than a few thousandths of a volt of AC riding on it.

A final bit of routine maintenance that you can do involves the removal of each circuit board from its socket, removing any plugs on the sides. With the board out of the console, wipe over the card-edge surface where the plugs and socket attach with a bit of Vaseline, then use a clean paper towel to wipe it all away. This process removes oxidation and leaves behind a tiny bit of lubricant to make the board easier to insert into the sockets. While you're at it, if you see any socketed chips on any boards, at least loosen them a bit and press them back in firmly. In extreme cases, I have pulled all the socketed chips and cleaned up the legs with a sharp blade and/or application of Vaseline.

Beyond these simple routines, you can do deeper cleaning, such as cleaning each stop tab's delicate leaf switch with a Q-tip dipped in alcohol or WD-40 or some other solvent. But you'd probably already know if the noise is coming from one particular stop.

As to the swell pedal not working, check the lamp in the holder on the side of the shoe. If the lamp goes out, expression is stuck at fully on. It's a common #57 lamp that can be found at some auto parts stores, though it is supposed to have a little dot of heat-resistant paint on the end to keep the direct output of the filament from hitting the LDR cells too harshly and producing expression that is too rapid or extreme. You can vary the brightness of the lamp using the little adjustable wire-wound resistor just below the expression pedal with a wire that goes straight to the lamp.

It can be hard to find a noise in the system, but doing all this routine maintenance is the first step. After that, poking around on each individual board, touching and wiggling each wire and cable and connection inside the organ, etc. -- that is about all you can do, short of replacing individual boards. Audio trouble will nearly always relate to either the DAC board or the MOS board itself, so if you are forced to start swapping out boards, those are the ones to try.

Keep us posted on your progress. If you will need boards, perhaps someone here can help you find the correct ones. Note that all MOS boards are not the same, with some designed for specific models of MOS organ. Also, all DAC boards are not the same, some being a later version that requires an extra power supply voltage and other minor mods to the console.

Good luck!

[Frank Kuhny]

Hi John,

Thank you so much for your detailed response. I will set aside a few hours this week to go through everything you have described and follow your suggestions.

I will get back and update later about the progress of the situation. Thank you again for all of your guidance. I appreciate it very much!

Best,

Frank

[samibe]

John, is there a chance some components might be failing on one of the amplifiers?

Frank, if you switch the inputs to the amp, does the static move with the mains?

[myorgan]

Frank,

Welcome to the Forum! I hope you continue to contribute here long after your problem is solved.

Through John's guidance, I've gained enough knowledge to keep all my Allens performance ready. He's the best, and his patient guidance will serve you well. His advice above is exactly on-point.

Again, welcome to the Forum.

Michael