ADC Amplifier hum fixed -- FYI

[jbird604]

Several forum members own Allen ADC/MADC organs and even later organs equipped with the ubiquitous ADC amplifier cage and modules. I had a problem today with the ADC amp in my Renaissance R-230 which was surprisingly simple (though time-consuming) to fix. This might be encouraging or helpful to someone else:

Note that the ADC amp in my R-230 (which was built in 2003) is much older than the organ. It came out of the MDS-45 at church, and bears a build date of 1992, when the MDS was built. (There's a reason why I traded amps between the R-230 and the MDS-45, which I related in a post long ago.) So it's a 26 year old amp, and probably long overdue for some maintenance.

Symptom -- 60 Hz hum. I first thought a couple months ago that the organ had just a tad more hum in the audio than normal, but my organ is backed up to a wall in a tight little nook and has my Conn "pipe" speaker sitting on top. So I didn't want to pull it out as long as the hum remained tolerable.

Today, I left the organ on after playing, and was working at my desk nearby when I noticed the hum getting louder. My wife eventually walked by and remarked about it, and she's not normally very concerned about the organ ;-)

Suddenly it got much louder, but I knew it wasn't loud enough to be an open filter cap, so I just let it be for another minute. Strangely enough then, it WENT AWAY for a little while! But then after a few minutes it came back roaring. At that point, I decided to do something.

Opening up the back, I started troubleshooting, and soon discovered that turning the amp knobs all the way down did not affect the hum, nor did pulling the RCA plugs out of the input jacks. So I knew it must be the amp itself. Hard as it was on this old creaky body, I unhooked the amp and removed the screws holding it down and took it out.

I placed it on a table, took off the wire cage, and looked at it. Nothing looked amiss. Since the hum was quite loud but intermittent, I figured the first thing to check was connections. Since it came through all the channels, it pretty much had to be the common power supply. Though everything looked snug, I got the nut driver and the big flat screwdriver and loosened up all the screws and other hardware holding the various bundles of wire lugs and such to the two large filter capacitors that are the power source for all modules.

Didn't go so far as to actually remove anything, but it was all loose at that point. I wiggled things around a bit and then started tightening it all back down securely. I made sure the big screws on top of the filter capacitors were tightened securely. I tightly snugged up the various 1/4" nuts holding some lugs and star washers and such that connect the common buss of the power supply to the chassis through a resistor and a capacitor.

Then I double-checked all the push-on connectors on each of the four amp modules, as well as the push-on connectors on the front panel board that holds the volume knobs and input jacks. Each one got pulled part way off then pushed smartly back into place.

Finally, I turned it on its end and firmly tightened all the screws holding the four modules in place on the frame.

All this didn't take much longer than it's taking me to type it out. Not a huge job, but of course it was getting into the console and removing the heavy cage that was hardest on this old man who normally "farms out" all the physical work on a service call to my much younger associate tech!

Did it work? Well, I put it all back together, making sure to get every cable snugged up. I had already put some Vaseline on the connectors a year ago, so they were still clean and smooth. Then I turned it on, and .......

SILENCE! It was so quiet I wondered if I had actually killed the amp! But it worked. And it worked beautifully. Quieter than it has been since I got it. I can't tell you how it will be doing in a week or a year, but right now it seems to be 100% back in order.

So, if you have ADC amplifier hum, roll up your sleeves and get to work!

[myorgan]

John,

Thank you for sharing your adventure. Was this one of the amplifiers from the MDS-45 or MDS-85? It's nice to hear the problem can be solved so simply–even though it might be time-consuming.

I'll have to try this once my garage is finished. Thanks again.

Michael

[jbird604]

This was the amp that was originally shipped inside the MDS-45 at church. When I got the R-230 it had only a dual amp in it and was configured for two channels. After I reconfigured it for four channels, I needed a four-channel amp, so I took the quad out of the organ at church and replaced it with a dual and a triple. Killed two birds with one stone -- got the 4-channel amp I needed on my R-230, and added an extra channel on the MDS-45, which I use for the new subwoofer I installed.

I suspect that ADC amps of all ages probably need routine maintenance. You may recall that when I got the R-230, which is only 15 years old, the amps had gone intermittent because of corrosion inside the volume control pots. They cleaned up easily, but this was actually one of the reasons the church had "given up" on getting this organ fixed! Nobody who had worked on it had recognized that the amp was the problem, instead replacing cage, power supply, and other parts.

It's likely that any organ with these amps will eventually have some audio problems caused by either dirty volume pots, poor connections where the push-on plugs go to each module, or in the power supply, as was the case with mine.

[ahlborn]

John, a nice story with a happy ending.
Even in hi-fi audio devices, it is recommended to disconnect, clean and re-connect all the jack/plugs once a year to prevent bad contacts.
Probably in your case it was some bad ground contact in the chassis. Generally, just spray a small amount of de-oxidant and reassemble the chassis to stay calm for many years.

[jbird604]

I'm feeling pretty confident that my simple amp fix is going to be permanent. I've had the organ on for several hours today and it is still as quiet as can be. Now that I've done this, I realize that it had been getting hummier and hummier for months, but I was trying to ignore it ;-)

Makes me wonder just what was actually causing that hum. It sounded for all the world like one of the filter capacitors had lost most of its capacitance, about half as loud as the huge hum you get when a filter capacitor opens up completely. So if the capacitor didn't actually fail, there must have been a good deal of resistance that had developed between one of the terminals and the metal plate that serves as a common buss in that supply. In spite of it seeming to be firmly screwed in place, there must have been some serious but invisible corrosion going on.

[ahlborn]

John, do the large capacitors have solder terminals? Or do they have terminals with nut and bolt?

[jbird604]

The filter caps are the "computer-grade" style with big screw terminals on top. Wish I'd taken a picture inside the amp. Maybe I'll snap one at the shop, as I have one opened up on the bench right now.

All connections are firmly bolted to one of the screw terminals or to the metal plate that serves as the common buss of the supply. Wires leading away from the supply are of course soldered to their lugs. It all looks to be extremely solid and reliable, but somewhere in there it developed enough resistance to make it hum. I'm guessing that one of the big capacitors may have lost its solid connection to the metal plate, though the screws felt tight when I checked them.

[AllenAnalog]

Thanks for this useful tech info about ADC amps. I've not had any reason to look inside mine... yet.

While you are taking pics of the opened one at the shop I'd like to ask you a question about the modular construction of these amps. If you have a spare 2-channel amp, would it be possible to transplant one or two amplifier modules into another 2-channel unit to make it a 3 or 4 channel amplifier? Is the power supply the same no matter how many amp modules are installed in a given chassis? Are the wiring harnesses and other infrastructure amenable to doing a field upgrade like this if you have reasonable electronics bench skills?

I am aware of the limitations of using 3 or 4 channel amps because of the common power supply but if one only had 8' and higher pitched stops on those channels I gather there would be no issues with running out of power.

[jbird604]

Larry, all ADC amp units have the same power supply in there. Allen used to advertise a quad amp as having "100 watts per channel," but they had to backtrack a bit and admit that when you have all four slots filled you don't actually have "four 100 watt amps in one cage." They issued a service letter at some point that stated how much actual RMS power is available from each module in each configuration -- single, dual, triple, and quad. I think it's more like 60 watts per channel when all four are in use.

But even at my church, where the MDS-45 came with only a quad amp to handle all four channels, including one with a 32' stop, I never noticed that it lacked for power. So there shouldn't normally be any problem with running out of juice no matter how many modules are in there. It might be risky of course to expect all four channels to carry 32' stops.

As to adding more modules than came in a given unit -- I tried it once but ultimately gave up. For one thing, the older ADC cages had a hand-wired input jack and volume control for each channel, but later ones have a printed circuit board that is pre-punched for all four channels but only populated for the number of modules used. I don't remember which kind I was trying to modify, but it might be easier with one than the other. Worth a try, I suppose.

You'd have to transplant the connecting cables and such along with the amp module itself, as the cage only has the cabling required for the number of modules it was shipped with. And if you are transplanting modules from one of the older cages to a newer cage (with the pc board) you may find that the cables are slightly different and require some tinkering.

Anyway, as I said, I tried doing it and failed. I think that was last year when I was trying to build up a quad unit for my R-230 to make it a four-channel organ. When I found it impossible or too hard, that's when I decided to take the quad out of the MDS at church and give that organ a dual plus a triple. (There was PLENTY of room for an extra amp cage in the MDS, but the inside of the R-230 is packed and there isn't room for a second amp cage.)

You may have better luck than I did, so give it a try if you like. Be careful and take it slow and you may make it work. No real technical reasons why it can't. Each module is fully self-contained and depends on the cage only for its power supply and for the input/output connections.

[myorgan]

Thanks for this useful tech info about ADC amps. I've not had any reason to look inside mine... yet.

AllenAnalog,

if you want a sneak peek inside the ADC amplifier, check out this post: https://www.organforum.com/forums/sh...l=1#post422618.

Hope it helps.

Michael

[jbird604]

Here are some pics I snapped at the shop of an old rusty ADC amp module that came in with some organ. Looks like it was installed outdoors! This is NOT MY AMP from my R230 (just to make that clear -- I'd be ashamed if it were)! But the chassis looks more or less the same on the inside.

The pics show the power supply section from two angles. You can see the two "computer-grade" filter caps, and you can see that one terminal of each cap is bolted directly to the metal plate that serves as a common buss for the supply. To that metal plate are also attached the bundle of black wires that go to the common power terminals on modules and on the input board too. The yellow wire is probably the center tap of the transformer, which would be grounded in this type of power supply.

And you can see that there is a bundle of red wires attached to the outer terminal of one cap, and a corresponding bundle of blue wires on the other cap's outer terminal. These are of course the B+ and B- operating voltages going to the amp modules.

Also attached to the metal common ground plate are a lug that goes to a resistor and a capacitor that go on to be bolted to the metal chassis itself, providing the connection from the amp common to earth ground (not a zero ohms connection, but a "floating" connection of some sort). A pair of large blue power resistors, one from the + side and one from the - side, also bolt to the common plate, presumably to bleed off the capacitors when the AC input is removed to prevent a shock hazard for the tech working on it.

The other pic shows the back of the little pc board that holds the volume pots and the input jacks. You can see that only two of the four sections of it are populated. It should be possible to add the missing parts and wire up a wire harness to turn this dual into a triple or quad amp, but last time I tried it I gave up because it turned out to be harder than it looks. YMMV.