Troubleshooting Low Volume on Allen MOS-1

[tilaloz]

I just bought an Allen Organ 282-C S/N AC6666 for my daughter to use as a practice organ at home. I don't know anything about organs but I'm not afraid to read a schematic or use a multimeter.

We tested the organ before we bought it. My daughter was generally pleased but she admitted later it didn't seem to produce enough volume even when the expression pedal was full open (full on? fully up? fully down?). Since we got it home (~100 mile drive), it seems to have developed a few intermittent issues (described below) or maybe it had intermittent issues all along and we didn't test it long enough. However, the consistent issue is that it does not produce enough volume. It seems like with the size of speakers and power of the organ, it should be able to be really loud. It is certainly loud enough for my daughter to practice, I just wonder if it is a clue of a bigger problem given some of the other intermittent issues. The organ sounds really clear just not loud.

The organ has two external speakers both Allen but they don't appear to be the original speakers because they aren't the same color (HC 8F Assy 906-0253-1 SCHEM 081-4083).

Intermittent Issues:

• After hooking up in my house, I got some crackle in the speakers when playing. I checked connections and redid a couple of crimped ends on speaker wire. After that I got some crackle but then it hasn't happened again.

• Later, I also had problem with one of the channels not producing any sound. I swapped the speaker cables to determine if I had a dead speaker or dead amp. Everything started working again and I haven't had that problem again.

• Occasionally, especially while I was troubleshooting the crackle problems, the organ goes super quiet. Even with the expression pedal full on, the organ is barely audible with every stop open. The sound is clear and correct, just super quiet. I keep forgetting to listen if both channels are producing sound when this problem happens. (Is there any difference in the sound that comes out of the two channels? Are organs "stereo"?)

• For awhile, the Swell to Pedal wasn't allowing any sound to come from the Pedals. It is working now.

I understand from reading this post here here that there are some things I could try but there was a lot of uncertainty on which organ that poster had and I'm sure of which organ I have.

It seems reasonable that come cards and cables could use reseating after the drive. I also don't know how long ago the organ was serviced maybe that is the right answer. This forum seemed so eager to help and if there are few easy things to try first, I am willing (even if ignorant) to try them first.

[jbird604]

Many if not all your troubles could be due to the need for ordinary routine maintenance. This organ is from the 1970's, so lots of reasons why there could be faulty interconnects throughout the system, and that is what causes most malfunctions in those models.

The volume control pots on the amplifiers are always a possible source of intermittent volume or low volume. Try exercising them both. You might even go so far as to take the amps out and squirt some WD-40 or other cleaner into each pot to lube and clean it up.

Also, everywhere there are RCA plugs and jacks they need to be cleaned up. Pull each plug out of the jack and use a Q-tip with a bit of Vaseline on it to wipe a thin film onto the center pin of the plug and onto the shoulder of the jack. This will simultaneously remove corrosion and allow easier insertion for a firm fit. Insert each plug with a gentle twist so as to insure good contact.

Another step you can take if you are careful -- remove each board from its socket, one at a time, and wipe each card edge connector (the rows of gold-plated long, thin surfaces that mate with the innards of the multi-pin plugs) with Vaseline, removing nearly all of it. This will remove corrosion, as on the RCA plugs, and make reinsertion easy. Make sure each one is firmly re-seated.

If you see any socketed chips on any boards, gently press on them to be sure they are firmly seated. In severe cases I have had to remove socketed chips and clean the legs with a Q-tip/Vaseline, but that is not normally necessary.

The intermittent Swell/Pedal coupler could well be due to a poor contact at the stop tab. Raise the top and look at the backs of the tabs. You'll see that there is a simple tongue-and-leaf arrangement, and the central tongue coming into contact with one or more leafs is what turns each stop on. If the area looks dirty, clean the metal surfaces with a Q-tip dipped in WD-40 or other safe electrical cleaner. Sometimes one of the leafs will get bent so that it doesn't make good contact with the tongue when the tab goes down. You can gently nudge the leaf back into position if you see one that is not firmly contacting when the tab is depressed.

Key contacts likewise may need cleaning, but do NOT use any spray cleaner on them or any liquid cleaner. I use the $100 dollar bill method -- slide a clean fresh $100 bill between the upper and lower contact surfaces of each key and, while holding the key down, gently wiggle the bill so it "wipes" the contacts and polishes the surfaces. (If you don't have a $100 bill, a $1 bill works just as well ;-)

BTW, the HC-8 speakers are probably not original to that organ. It would've been shipped most likely with HC-10 or HC-12 speakers, which are much larger and heavier. But the HC-8 ought to be sufficient for home use, though they won't put out as hefty a bass.

Yes, the two channels put out different sounds. One group of stops called the "flutes" comes from the Flute channel, as do the Pedal stops. The rest of the stops, called the "mains" -- including strings, diapasons, and reeds (except 16') -- come from the Main channel. So you should balance the two channels with the volume knobs so that you get similar volume levels from the 8' Prinzipal and the 8' Hohlflote on the Great manual. There is also a "Bass Boost" miniature pot on the DAC board or on an adjoining relay board that can be used to adjust the overall amount of bass in each channel.

That should get you going. If not, you might have a defective component, but that is not terribly likely on a MOS organ.

Good luck!

[Admin]

One thing to add to jbird604's sound advice is that the technology used in these models is very sensitive to static discharge. This time of year, with its heated, drier air, this is a real concern. You could potentially do serious damage if you zap something in the organ. Make sure you touch a metal chassis to discharge yourself before removing boards, IC's and adjusting controls.

Also, since this a two channel organ, I'm wondering whether both channels go quiet simultaneously. It would be puzzling if they did.

Finally, rather than WD-40 and Vaseline, I'd spring for a can of DeOxit, pricey, but the gold standard of contact cleaners.

[tilaloz]

Thanks for the quick replies! I haven't tried any of the steps yet because I'm waiting for the DeOxit to show up (couldn't find it in town).

Some of the descriptions of the problems I was having were wrong because I wasn't paying attention to which stops I (or my kids) were using . The Flutes/Pedal channel is definitely the super quiet channel. The main channel seems to consistently work.

[tilaloz]

Many if not all your troubles could be due to the need for ordinary routine maintenance. This organ is from the 1970's, so lots of reasons why there could be faulty interconnects throughout the system, and that is what causes most malfunctions in those models.

The volume control pots on the amplifiers are always a possible source of intermittent volume or low volume. Try exercising them both. You might even go so far as to take the amps out and squirt some WD-40 or other cleaner into each pot to lube and clean it up.

I didn't remove the amps but carefully sprayed some DeOxit into the pot with the a rag to catch the excess.

Also, everywhere there are RCA plugs and jacks they need to be cleaned up. Pull each plug out of the jack and use a Q-tip with a bit of Vaseline on it to wipe a thin film onto the center pin of the plug and onto the shoulder of the jack. This will simultaneously remove corrosion and allow easier insertion for a firm fit. Insert each plug with a gentle twist so as to insure good contact.

I completed this step with the 12 RCA connections I found. We also had to vacuum out a fair bit of cat hair.

Another step you can take if you are careful -- remove each board from its socket, one at a time, and wipe each card edge connector (the rows of gold-plated long, thin surfaces that mate with the innards of the multi-pin plugs) with Vaseline, removing nearly all of it. This will remove corrosion, as on the RCA plugs, and make reinsertion easy. Make sure each one is firmly re-seated.

If you see any socketed chips on any boards, gently press on them to be sure they are firmly seated. In severe cases I have had to remove socketed chips and clean the legs with a Q-tip/Vaseline, but that is not normally necessary.

I wasn't brave enough for this step. Especially when I saw several wire-wrapped connections. Everything looked well seated.

The intermittent Swell/Pedal coupler could well be due to a poor contact at the stop tab. Raise the top and look at the backs of the tabs. You'll see that there is a simple tongue-and-leaf arrangement, and the central tongue coming into contact with one or more leafs is what turns each stop on. If the area looks dirty, clean the metal surfaces with a Q-tip dipped in WD-40 or other safe electrical cleaner. Sometimes one of the leafs will get bent so that it doesn't make good contact with the tongue when the tab goes down. You can gently nudge the leaf back into position if you see one that is not firmly contacting when the tab is depressed.

There was quite a bit of built up crud on the Swell/Pedal coupler and a few of the other stops. I cleaned them all with DeOxit and tweaked a few back in place.

Key contacts likewise may need cleaning, but do NOT use any spray cleaner on them or any liquid cleaner. I use the $100 dollar bill method -- slide a clean fresh $100 bill between the upper and lower contact surfaces of each key and, while holding the key down, gently wiggle the bill so it "wipes" the contacts and polishes the surfaces. (If you don't have a $100 bill, a $1 bill works just as well ;-)

I did this method too. All the keys still work so I didn't bust anything.

BTW, the HC-8 speakers are probably not original to that organ. It would've been shipped most likely with HC-10 or HC-12 speakers, which are much larger and heavier. But the HC-8 ought to be sufficient for home use, though they won't put out as hefty a bass.

Yes, the two channels put out different sounds. One group of stops called the "flutes" comes from the Flute channel, as do the Pedal stops. The rest of the stops, called the "mains" -- including strings, diapasons, and reeds (except 16') -- come from the Main channel. So you should balance the two channels with the volume knobs so that you get similar volume levels from the 8' Prinzipal and the 8' Hohlflote on the Great manual. There is also a "Bass Boost" miniature pot on the DAC board or on an adjoining relay board that can be used to adjust the overall amount of bass in each channel.

Thanks for this tip. This is probably common knowledge for most organ people but this was key to figure out what was wrong. I believe I still had a bad crimped connection on the speaker wire to the Flute channel. I have never had such cheap crimped ends. I soldered them all on after that problem.

I retuned the pots to match volume and turned them up a bit; they were all the way low when I started. The organ produces plenty of volume now. My daughter can make things in the house rattle. She's a pretty happy camper.

That should get you going. If not, you might have a defective component, but that is not terribly likely on a MOS organ.

Good luck!

Thanks again for the help!

- - - Updated - - -



One more quick question... I noticed this wirewound resistor is set near one extreme of its range. I noticed it was hooked to the expression pedal but couldn't guess its purpose.

[myorgan]

I completed this step with the 12 RCA connections I found. We also had to vacuum out a fair bit of cat hair.

'Nuff said!;-) Cats seeking a hiding place are the bane of an organ's existence. Someone correct me if I'm wrong, but it wouldn't hurt to build a box to put inside the console to keep cats from getting in–that is, if you have cats. Otherwise, you'll be good.

I believe I still had a bad crimped connection on the speaker wire to the Flute channel. I have never had such cheap crimped ends. I soldered them all on after that problem.

Your solution works, unless you ever need to move the console separate from the speakers. I move my organs all the time, and usually lose a crimped end or two when the stage crew gets to them. I keep several of them in my kit, but they are being made cheaper and cheaper as time goes on. Once I even lost a speaker when a trumpet player jammed the speaker cord between his stand and the risers.X-(

One more quick question... I noticed this wirewound resistor is set near one extreme of its range. I noticed it was hooked to the expression pedal but couldn't guess its purpose.

I can't say I know exactly what this is for, as I do not have it on any of my organs, but Allen usually used it in antiphonal speaker relays to help set the volume of the antiphonal speakers to a greater or lesser extent. My guess would be either the starting volume of the expression pedal (loud or soft). Feel free to experiment with it, as it won't do any harm to adjust it.

Somewhere in your post you mentioned something about not re-seating the boards. John wasn't talking about the wire-wrapped pins. When referring to the boards, he was referring to the green or white boards that have connections seated in sockets in the organ. If you've ever added logic boards to expand a PC's capabilities, you will know what we're referring to. When moving an organ, those can sometimes jostle loose, but it sounds like everything's working fine, so I'd leave well enough alone for the time being.

Congratulations on getting everything up and running for your daughter. One more organist among the masses!

Michael

[jbird604]

That wirewound resistor is the adjustment for the brightness of the expression lamp. In your photo, look at the small round metal cover with a black wire coming out the middle of it, going to the resistor. This holds the expression lamp, a little 12 volt type 57X bulb (a type of automotive dashboard lamp that is no longer in use in autos). You can see the bulb by removing the unit from the side of the expression pedal.

The lamp provides a light source for the expression system (AKA "volume" control pedal). The lamp produces a set amount of light, and the expression pedal has a tapered slot inside that lets more or less light pass through as the pedal is pushed up and down. There is a light-sensitive device ("light dependent resistor") in a closed cavity on the side of the expression pedal opposite the lamp.

The light dependent resistor (LDR) changes electrical characteristics as light shines on it. The organ's audio system uses this to shunt away more or less of the audio signal at the input to the amplifier, and thus control the volume as the player moves that pedal up and down.

The wirewound resistor serves to adjust the brightness of the lamp, which is normally adjusted only when the lamp gets replaced. The lamp needs to be bright enough to do its job, that is to reduce the volume as required by the player, but not so bright as to produce a very drastic and rapid reduction in volume when the pedal is moved only slightly, as it would do if the lamp were an extremely bright one.

If you were to loosen the set screw and move the slider around you could probably tell that this affects the "taper" of the expression curve. If your daughter were to complain that she didn't have enough range of control with the pedal, you could try brightening the lamp, or if she said the pedal was too abrupt in its action, you could dim it a bit.