Allen ADC: Intermittent static through speaker, expression pedal related?

[rjsilva]

I have heard that the Allen ADC5000 DK that I just installed is periodically having intermittent static through the speakers. (It wasn't when I was finishing setting it up for a couple of hours last night.) Apparently the static comes and goes although it is significant that the usage of the great expression pedal can cause it. It doesn't matter whether the expression pedal is moved to be open or closed.

Any thoughts as to what could be causing it?

[toodles]

Possibly an "iffy" connection that your re-installation fixed.

[rjsilva]

Toodles, do you think the expression pedal connection is irrelevant?

[myorgan]

RJ,

My immediate suspicion is there might be a pot on an amplifier, or a pot on a card in the cage that needs exercising. I chased down such an issue on one of my ADC organs, and it turned out to be in one of the antiphonal relays. If it is a relay, try switching the inputs from the main to the antiphonal, and change all the settings to include the antiphonal only. If that solves the issue, you will know where the problem is.

Please keep us posted.

Michael

[jbird604]

I'd only add that noise in any electronic organ can originate with any number of sources or components, so very detailed troubleshooting could be required before getting to the bottom of it. Some sources of noise in ADC and MDS organs that I can recall:

-- as noted above, POTS. POTS are everywhere, and they get ridiculously noisy with time. The job isn't done until you've exercised every pot throughout the organ and then returned it to the proper position. Make sure you know what each pot does before you move it though.

-- Three words: connections, connections, connections! ... Every point where current or signal has to pass from one metallic part to another, whether it's from RCA plug to jack, across the spade terminals on a fanning strip, from a screw to the threads, from the pins to the sleeves inside a Molex or AMP connector, from the cage sockets to the card-edge connectors on the back of each cage board, from the legs to the sockets of every IC -- every such junction is a potential source of noise, if the connection is not 100% firm and solid. An organ that is 35 years old could have dozens or even hundreds of iffy connection points, and the only way to be sure they are all good is to clean, lube, tighten them one by one. Solder joints can be bad too, but that is far less common. But if you see solder joints that don't look right, re-flow them.

-- Power supplies. The cage power supply on any ADC or MDS organ can become a source of noise when filter capacitors leak and cause corrosion of the copper traces of the pc board. When this kind of trouble is going on, you may be able to identify it by poking the components of the power supply with a wooden stick while the organ is turned on. If there are caps or traces going bad, the poking will probably provoke noise. Clean up the pc board, repair damage with jumpers, replace the caps.

-- ADC amp units can have internal problems that cause noise in the output. Most problems are corrected when you dismantle the amp unit, loosen, de-ox, lube, and firmly re-tighten all the connections inside, especially in the power supply area, and in the audio signal path. It helps as well to remove each module and tighten down any socketed or screwed-on components. Be sure to lube and firmly re-install the push-on connectors of each module and at the front panel I/O assembly.

-- Cage backplane pc board damage. The absolute worst thing that can happen to an ADC or MDS organ is for the battery on the USAV board to leak. When that juice leaks out, even a small amount of it, it can quickly migrate into the backplane board, and if it does, it will eventually eat through critical traces and bring the organ down. So very carefully inspect the socket into which the USAV board is plugged and use a strong light to look for any signs of corrosion in there. If you see any, there is a good chance that the battery leaked sometime in the past, and that damage has set in. I have no clear advice to offer if you find that kind of damage. On our MDS45 I had to totally dismantle the cage, use a chisel to remove the bad socket, plow up the damaged part of the board, and repair the destroyed traces with wire jumpers. It was an unimaginable mess! Allen wanted $8,000 to do it for me, so I just did it myself. It was not fun!

Now that I've given you all the most horrible possibilities, just check RCA cables and cage pots and you'll probably fix it right away!

[rjsilva]

I appreciate everyone's knowledgeable advice!

Honestly I wasn't too thorough about exercising the pots because in the back of the organ was a print out of one of jbird's posts from a few years ago which, of course, talks about exercising the pots :) And everything was is nice working order so I assumed whoever it was did what was in that post. I should have done it more thoroughly anyway.

I will report back once I know something.

[toodles]

The expression system uses a fixed incandescent lamp which shines on a CdS (Cadmium Suflide) photocell. The amount of light is controlled by a mechanical shutter which is what is moved by the pedal. The system is inherently resistant to noise, but anything is possible. If the variable resistor (mounted on the expression shoe) has a loose slider, it might induce some noise in the system--the resistor controls the amount of voltage to the lamp, but is adjusted only during setup--it's not moved for expression. A loose slider could make intermittent connection to the voltage and cause the lamp to flicker on and off--but it wouldn't be my first suspect. It's easy enough to check. Apart from that, a loose electrical connection to the CdS cell or lamp could induce noise.

These are just possibilities, not likelihoods.