Allen mdc-31 broken wire

[SteveM]

While looking at my mom's MDC-31 to try and see why it's not getting any sound, we found two pieces of wire just hanging there.

See the ends in the red circles.



Anyone know what this wire is for and if those two ends should be connected?

I'd rather not have to pay $200 for the technician to come out an look at it and tell me "oh, you have a broken wire!".

Steve

[myorgan]

Steve,

That certainly does look suspect. Do you have the ability to get a photo of where each opposite end of the wire connect? If we can see where they originate and the equipment they're attached to, that might help.

Michael

[SteveM]

I will be at the house tomorrow, so I can check.

Steve

[jbird604]

The MDC31 is not a model I'm familiar with from personal experience, though I have a service manual for it. It does appear that the two ends of that yellow wire were once connected. You certainly need to see where they lead though before just putting them back together. Someone may have clipped that wire for a good reason. Perhaps a dead or shorted out battery pack needed to be disconnected.

As with all older used organs, even an Allen that is dead has no resale value. And keep in mind that the MDC series were not Allen's best efforts anyway. There were a lot of compromises in those organs that make them less desirable than the MOS organs of the same time frame. They were a good buy at the time, because they were affordable and offered a lot of features for the money. But compared to later and more sophisticated digitals they are lacking in certain ways.

The heart of the organ is the MDC computer board, which handles all the interfacing of keys, stops, pedals, etc., as well as all the tone generation. If that board is bad, it will cost several hundred dollars to have it exchanged for a refurbished one at the factory. There is no other source for these, so you have to pay what they charge, IF that is the problem.

Thus, it might not be worth fixing, unless you just want to keep it going for sentimental reasons. Your parents hopefully got 20 or 30 years of pleasure from playing it, and even if they paid $8000 for it back then, that's just a dollar a day for all that fun! If it's reached the end of its life it was well worth the cost.

Of course, at least part of the time an older Allen can be dead simply because one of the fuses is blown on the power supply or amplifier. In either case that would make it audio dead. You should check all the voltages at the power supply terminal strip, and you should check the amplifiers by doing a "hum" test -- pull the RCA plugs off the last board before the amplfier and touch the pin with your fingertip. You should get a loud hum if the amps are ok.

In fact, you can check the entire audio path by simply pulling the three RCA cables off the main MDC board, one at a time, touching the center pin and listening for a hum in the speakers. If the audio is ok, the problem may well be in the MDC board, or in the stop or keyboard wiring.

If this organ were in absolutely perfect working order and perfect cosmetic condition, you still might have a hard time getting any money for it. Not too long ago, we had an Allen MOS-2 theater organ in the same "S" style console with princess pedals given to us because the owner had no offers even after trying to sell it for next to nothing for a year. That makes an MDC model seem worse than worthless. It would cost money to have it hauled away!

But don't despair if you want to get it playing and want to keep it. Any Allen tech worth his salt should be able to troubleshoot it in a half hour, and in the worst case, a bad MDC board, it might cost a thousand dollars to fix it. But there's a chance it's something far cheaper and simpler.

[SteveM]

Thanks for that thorough explanation.

Do you know if there is a schematic or service manual online?

If not, where would I be looking for the fuses?

I'm not at the house right now, so I can't see it - does the rear panel come off for easier access and if so, how?

Which board is the MDC board and which is the amplifier? I think I should be able to find the power supply pretty easily.

Steve

[jbird604]

Allen still, oddly enough, keeps the service literature on these models close to the chest. Not available except on the private tech site. And even there, it isn't a service manual as such, just a collection of various descriptions, the occasional schematic for a strictly analog component.

To answer some questions -- To remove the back of an Allen organ, raise the top lid. At the back you will see two or three rotatable latches that hold the back panel on place. Turn them to the horizontal position and the back will fall right out and you can see the entire interior of the console and all the components.

Look for fuses on the power supply itself. It's a black heavy metal unit in the floor of the console with a long fanning strip attached to a matching terminal strip. Fuses are probably in twist-off holders.

The power supply is marked to show the voltages that you should read from one terminal to another. You will need a decent digital volt meter to check this. There are at least some adjustments for the voltages, pots that can be turned with a slotted screwdriver. Be sure to adjust voltages with care, making tiny adjustments so as not to drastically over-voltage something.

If any voltage is way off, say more than a couple volts, there is probably something wrong inside the supply. But a missing voltage might just be due to a blown fuse, though there is possibly a reason why the fuse blew. You can try replacing fuses with the same amp rating and see if that brings it to life.

The MDC board is the large flat circuit board (maybe about 16" x 20" or so -- not sure of the exact size). It is the board with many chips and other parts, with three RCA jacks that output the three ranks of tones. It is static sensitive, so be sure to ground your hand before touching it. You can see if any chips are socketed, and if so, press them down gently to be sure they are firmly seated. Remove and re-mount any push-on connectors to restore good conductivity.

The amplifier is a type "D-40" and is probably mounted on the back side of the swing-out panel. You'll recognize it by the D-40 markings, and it will have two RCA cables on the inputs and some output wires on a terminal strip. If the amp is working, you will probably be able to hear at least a tiny bit of hum or hiss in the speakers. To test it, you can remove the two RCA plugs going into it and attach some other sound source, such as a CD player. Set the amp knobs low (3 or 4) before testing with an outside source.

[myorgan]

Look for fuses on the power supply itself. It's a black heavy metal unit in the floor of the console with a long fanning strip attached to a matching terminal strip. Fuses are probably in twist-off holders.

John,

Didn't Allen use mostly slo-blo fuses? I seem to remember that from my MOS-2. I wish I could remember what I did with the spares I had. They were in the card reader drawer until I put the organ in a church--now I can't remember where it is.

Michael

[jbird604]

Yes, slow-blow fuses on many power supplies. The docs are very sketchy on the tech site for the MDC models, but best I can tell there is just a single 1.5 amp slow blow fuse for the entire MDC power supply.

If it's not that fuse, then it could be one or more of the critical voltage modules within the supply, which don't have their own fuses, but are independently regulated, and thus can be dead or way off voltage.

If it's just an audio problem, it could be the amplifier. The D-40 amp has a push-button circuit breaker I think, instead of a fuse. I've seen several D-40 amps go kaflooey in a puff of smoke, and that worries me about this organ. If the D-40 goes up in smoke, that is actually the one part of this organ that Allen doesn't have a drop-in replacement for. Any decent two-channel amp will work, as the audio in this model happens to be ordinary line level, not the quirky MOS level. But the D-40 has its own built-in audio muting system, which might or might not be present in some commercial amp that one might sub in.

[SteveM]

Thanks. I will be at the house this weekend with my trusty Fluke meter and will report back.

Steve

[SteveM]

OK, it appears that there is a problem with the organ's power supply. Looks like the power supply number is something like 905-0013-2, based on the blurry picture I got of the system diagram.

Here you can see the voltages expected for each connection (all voltages are relative to ground, except the 12v on the right is relative to the 7.5v connection next to it).
The 20v on the left and the 7.5v on the right are both off and the trimpots do not change the voltages


I looked at the four separate boards in the power supply and the power connections going into each to check the transformer output voltages. All had voltages. Although I am not sure what they are supposed to be, the first two probably supply the 12v and 7.5v terminals, the 22.7 supplies the 20v and the 10.63 supplies the 5v and maybe one of the 7.5v.


I can't make heads or tails of the circuit in place, as I can't see much of it. There appear to be diodes on the board, likely the rectifier, there are the usual filter caps, there are two of what appear to be transistors in metal cans on the board and each board has a wire coming off it that plugs into a Motorola 852-JE3053 power transistor. Nothing looks burned. Bad filter caps might give me bumpy voltage, but that shouldn't give me low voltage, would it?

If the power transistor provides the regulation, I could swap the plugs from one to another to see, for example, if the 20v power comes up to normal.

I didn't try checking for hum on the connections at this point as JBird suggested, because if the power supply is bad, then any other tests will be unreliable.

Steve

[jbird604]

Use a diode tester and check all the rectifier diodes in the system. Also check the capacitors if you can. The other components can fail of course, but not as likely as diodes and capacitors.

It is entirely possible to make up a power supply to replace that entire unit using modern switching supplies, though you'd have to do some creative bridging and stuff to come out with the prescribed voltages. But it should be repairable, and may well be just a couple of the rectifier diodes.

[SteveM]

Use a diode tester and check all the rectifier diodes in the system.

I have a fluke 73 and it has a diode test mode. Can I test it in the circuit?

Also check the capacitors if you can.

Would I just be checking for DC continuity thru the capacitor to see if there is a short?

If I remember my electronics, caps will pass AC but not DC, which is why they are used as filters in the power supply.

Steve

[John Vanderlee]

The parts that you call transistors are most likely voltage regulators. I have found them to fail. It's a relatively cheap part. The generic part number should be on the case. If in doubt, replace.

John

[John Vanderlee]

One more thought: Twice I have found power supplies having low or no output, which was due to a load problem. Many circuit boards have tantalum capacitors which kind of look like an orange jelly-bean. They have a habit of shorting out thus loading the supply down. If you are brave: turn the organ OFF first, and measure with an ohm meter the wires leaving that particular supply, anything that reads just a few ohms tells you the problem is down stream on one of the circuit boards. Next unplug and replug all boards where you see these caps until the ohms reading jumps up, now you have the offending board and the bad cap will be on there - check it with the ohm meter. After you remove the cap and replace the board you often find the organ works just fine without it, but it's best to replace it.