MDS-41-S Amplifier Hum

[CarillonBells]

I've not been around as much lately, in part due to taking lessons, practice, and becoming more involved in our local theatre organ group. (Add regular life and dealing with a chronic illness to that and time seems to practically disappear).

At any rate, I've been dealing with a festering problem where I get some intermittent crackling in one of the channels of my MDS-41-S. Could be worse, so I have mostly let it go. For what it is worth, it looks like the amps have been worked on in the past. However, I was finishing out a hymn today and hit the Tutti to round things out. No clue whether that contributed, but afterward I now have a persistent hum in one channel that doesn't change in volume when moving either expression shoe.

At first I wondered if there was a failing crossover component on one of the speakers that might be causing the crackling, but now I fear that one of the amps may have a bigger problem. I do hear some hum from the console before the mute relay kicks to unmute the speakers, at which point I hear hum from the single channel. So not sure if it is creeping in from somewhere or if it is isolated to the amp and the gentle power supply hum was likely there before this issue arose.

I'm not thrilled at the necessity of servicing an amp, but would appreciate the "forum notes" on the common failure points for them or if there is a normal rebuild procedure for the modules. I suppose I shouldn't be surprised given this model could date to as early as 1994. Capacitors do start to show their age, so maybe some preventative maintenance is called for here. I'd rather rip everything apart and future-proof things than have to keep moving the console and pulling the back off. So, on that note, I'd be curious if crossover component failure is at all commonplace and if I should maybe just dig into things there too while I'm at it!

As always, the knowledge here is much appreciated.

Regards,
Corey

[jbird604]

I have had several ADC amps develop serious hum. Most of the time I've fixed it by simple maintenance. Simple but not necessarily "quick and easy."

I trust that you have already done the "routine" stuff we do inside Allen ADC consoles, such as cleaning all the RCA plugs and jacks throughout the audio paths of all channels, exercising all the pots in the cage, re-seating any plug-in boards or assemblies wherever they are found.

You probably know well that a common cause of hum is poor grounding, and this can occur anywhere some assembly is strapped to the organ ground system with a wire and screw with star washer. Check the AC distribution box and if there is a stud with a bunch of ground wires attached and held down by a nut and washer, loosen all that and squirt a bit of WD-40 into it before firmly tightening it back down.

Since your hum is not expressible, it almost surely originates outside the cage, but it never hurts to make sure that all the cards in the cage are firmly seated. Now to the amps...

Remove the amp chassis from the console or rack, remove the cover, then look at the power supply section in the left rear of the unit. The two large heavy-duty capacitors have both wires and a large metal heatsink attached to the bolts on top of the capacitor bodies. Loosen the bolts a few turns each. Also loosen every other connection point in the power supply section including the spot where the ground wire is connected to the chassis with a "star" washer, the screws and/or nuts holding the wires to the diodes and other parts. Now wash all the places where metal has to conduct electricity with your favorite electrical cleaner. WD-40 may be as good as anything, or you may prefer something more exotic. It doesn't take much, you just want to wash away any corrosion or other gunk that may have collected at these inter-connect points. Then tighten it all down securely. Take care not to twist or kink any wires, and don't let wires rotate around as you tighten and wind up touching each other or something else.

Look through the system and identify every other spot where current or signals pass across a connection point. Each amp module has a couple of push-on connectors, and you should pull each one loose, lube it with something that makes you feel good about it (I usually just use Vaseline, but then I'm the Vaseline King of the forum, you know). Firmly re-connect the push-on devices with some sliding motion to clear away any corrosion.

Don't forget the front panel area, where there may be push-on connectors or some wiring connected by screw terminals. (The ADC amp went through several revisions, and different methods were used to wire the input/gain control section to the modules). Just look for any spot where electrical contact might be less than perfect. Spray a tiny bit of WD-40 into each of the volume pots and sweep the pot through its range many times to be sure it's clean. Clean up the RCA jacks on the front as well as the screw terminals where the mute voltage and speaker wires will go.

Now, if you're really ambitious, take each module out by removing the several screws in the bottom. With the module out of the chassis, you can do all kinds of stuff to make sure connections are clean and snug -- loosen the large power transistors from their heatsink and pull them slightly loose before tightening them back down. If you see anything else that is socketed, give it the same treatment.

Once you put it all back together, it SHOULD at least be quieter. If this doesn't solve the hum problem, at least you have done a needed maintenance procedure.

Ok, if you do all this stuff and it still hums, you may have a defective amp module, as much as I hate to think about that. You can of course replace an Allen ADC amp with any standard line level amplifier, though amps need muting in order to prevent loud thumps at turn-on and turn-off. I've found that many modern amps, such as the iNuke series from Behringer and certain Carver amps, among others, happen to have built-in muting that keeps the amp silenced for a second or two after power-up and silencing it instantly when the AC power is removed. Finding amps that self-mute this way saves you a lot of grief when replacing ADC amps.

[CarillonBells]

Hi John,

Things were cleaned up when I got the organ about a year and a half ago. I'll run through everything again just to see what is going on. I should have mentioned the audio seems clear aside from the hum when playing, so as you suggested it could definitely be a ground on a physical connection that isn't making good contact or another loose connection. I did fix the capture power supply where the capacitor terminals were loose, so maybe there's something in this power supply as well. I'll report back one I get a chance to take a peek!

[jbird604]

Yes, just check to make sure all the RCA plugs are still seated firmly at the cage and other points, then open up the ADC amp chassis to check the connections inside. Hope that will clear up the hum. If you have a capacitance tester, you could verify the large capacitors, but then again if it's only got hum on one channel, it's more likely to be the individual amp module itself or the push-on connectors on that module.

[CarillonBells]

Figured I should follow up, this is the weirdest thing. The organ has five channels (two amps) - a dual channel and a triple channel. I finally took off the back hoping I could access enough without moving the console, and that was thankfully correct. I had been playing and the buzz was present, so my first course of action after spinning the volume pots had no effect, was to press the circuit breaker on the dual channel amp just to see if resetting it would change the noise. The buzz went away and I figured it would return with some amount of playing, since the amp had effectively been power cycled, but so far it has not - through multiple power cycles and practice sessions with spirited use of volume!

It has been much more enjoyable to practice without the distraction!

[jbird604]

That's peculiar, Corey. Since the hum was obviously related to that one amp chassis, my guess is that there is some kind of faulty connection inside there, and the act of pushing on the circuit breaker just happened to wiggle the affected component enough to make two metal parts rub together a little and knock off a tiny bit of corrosion and re-establish the connection. I'm pretty sure that will be a temporary fix, but it could last for months or years. No need to tear into it until the hum returns!

If (and when) it comes back, open that amp chassis up and go through the process of cleaning, lubing, tightening up all the connectors in the unit, including the connectors on the individual amp modules. Remove each module from the frame, poke around on anything that is socketed, then re-install the modules carefully and securely.

At least you isolated the problem to a particular amp unit, so you are halfway there.