Loud buzzing from Allen MDS-1

[crapmaster]

I have a new-to-me MDS 1 that was working fine until yesterday. It's making a very loud continuous buzzing noise. I've turned every stop tab and piston on and off and none of them altered the sound. I also left it unplugged for a while. It still plays, but the sound can barely be heard over the buzz. The volume can be altered with the swell pedal but is quite loud even when the pedal is at its quietest. I've been using Hauptwerk on it, and that still works fine. My cat went inside the organ a few days ago (through the swell pedal opening) - not sure what she might have done in there. I used it quite a bit after that and it was fine until last night. I'm not mechanically inclined, but wonder if there might be anything else I could check. Thanks for any help...

ETA - Now I think I picked the wrong topic, but I don't see a way to delete the post. If there is a way to do that, please let me know.

Post so moved. Michael, Moderator

[jbird604]

Since the cat went inside, at the very least you should take the back off and check to see if any of the plugs have been dislodged. The cat may have pushed on a wire somewhere and caused it to loosen a plug in a socket or something.

A lot of hum can be caused by a lost ground connection somewhere in the system, so use a flashlight if necessary and carefully inspect every piece of wire or cable, every RCA plug, every Molex connector, every screw-down terminal block. You must first ascertain that nothing is loose in there.

As far as the electronics, the most common cause of hum that is not merely mechanical would be a failed filter capacitor in a power supply or amp. Since you say the noise decreased to some degree with the expression pedal, it might be coming from somewhere besides the amp itself. There are two items in the audio chain besides the amp -- the cage itself and the audio processor board. Either one is as likely as the other, but you should first at least do some checking to rule out the amp.

You can try first just disconnecting the RCA cables where they enter the amps and see if the hum goes away. If it doesn't, then your trouble actually is the amp, which might mean a problem with the amp power supply. However, my Allen at home developed a loud amplifier hum a few weeks ago I fixed it by simply opening up the amp cage and tightening up all the screw-on and press-on connectors inside. You should try that first, if the noise doesn't stop when the RCA cables are pulled off the amp.

If it's not the amp, it could be the audio processor. You can find out by disconnecting the cage inputs from the audio processor. Trace the two RCA cables that run from the cage to the processor and pull them loose at the processor board. If the hum stops, then the trouble must be in the cage. If it does not stop, then it must be in the processor board.

Repairs to either could be beyond your ability, but you can certainly at least try doing the "routine maintenance" that we talk about so much on here, regarding nearly all Allen organs. There are a number of RCA plugs/jacks, push-on connectors of various kinds, and screw terminals holding lugs through which ground and signal must pass. Also, there are sockets involved with some of the chips and some of the connections within the cage. Every one of these is a possible source of noise, and should be cleaned and lightly lubed with Vaseline or other favored electronic lube. Don't overlook the "exercising" of all the mini-pots that control voicing in the W-4 cage.

If your organ happens to have Trem generators these are also a common source of trouble, You can try disconnecting them in the same manner as the cage and processor. If a trem generator is bad, it can be fully bypassed by using a simple male to male RCA coupler to link the cables currently connected to the input and output of the trem.

Another possibility is the power supply in the tone generator (W-4) cage. The old-style power supply was prone to some failures, so Allen has now started shipping only a new switching-type power supply for the MDS cage. You may need that changed, which is also not an amateur repair.

Anyway, you know your own limits, so try to do everything you are comfortable with and capable of. But if it goes beyond your skill levels, you may have to find a tech.

[crapmaster]

Thank you. I can try a few of these suggestions, I think. There is a shop near me that works on Allens, but I wasn't very happy with my one prior experience with them.

Not sure if this is relevant, but I noticed that pressing the Reverb tab greatly increases the volume of the noise.

Would there be an easy way to disable the inner workings of it and only use it with Hauptwerk? The only reason I'm not doing consistently using Hauptwerk now is because I don't have a computer that I can keep connected to it all the time. It might be cheaper to buy a laptop than have that repair shop fix it - and I'd been planning to eventually get a more permanent Hauptwerk setup anyhow. I live in an apartment and only play with headphones on, and I've been happy with the results from my mid-range laptop. Please forgive me if I seem like an idiot for even asking that question!

[jbird604]

Not sure if this is relevant, but I noticed that pressing the Reverb tab greatly increases the volume of the noise.

Maybe the trouble is a bad ADR-4 reverb board. If you look at that board you'll see it has RCA cables attached to a single input jack and a pair of output jacks. You can simply disconnect them all three and see if the hum goes away. If so, you've found the bad part.