dead notes on an Allen 202

[moller3x]

Recently acquired a MOS1 Allen to use as a home practice instrument. Unfortunately, none of the Gs or D#s will play on any of the stops. Is there anything simple I can check or repair on my own? Any advice is appreciated.

[indianajo]

It has been 48 hours and none of the Allen experts have jumped in.
If your organ has IC's, square packages with 14 or more pins, your best source of parts and schematics would be your Allen authorized serviceman for your district. This is an expensive option, and probably a different organ would be more appropriate. One that has public domain schematic diagrams and industry number marked parts.
If you google search "organforum.com:Allen Mos 1" you can see some of the extensive threads that have been posted on MOS1. Some people have sucessfully repaired these.
See post #2 of this thread https://www.organforum.com/forums/sh...sues-long-term for a list of makes we amateurs find repairable. 32 pedal organs tend to be Baldwin, Rogers, The Hammond RT2,RT3, and 1930-s E, the Wurlitzer 4700 and up.

[Admin]

Do you have the problem with both manuals? How about the pedal?

[don60]

If the problem spans both manuals, it would probably be caused by a fault in the Keyboard Array (KBA). I will have a look at my MOS1 manual tonight to see what the troubleshooting section has to say. The KBA is repairable in the field--Allen actually encouraged it back in the day.

Meanwhile you could look for obvious signs of wires that have been disconnected or broken. The keyboards used a multiplex arrangement, so one wire off could create a repetitive pattern of dead notes.

[don60]

G and D# are on different matrix keying busses, so a single bad line would not cause both notes to be dead. However, G and C# do share a bus, as do G# and D (just in case a letter or a sharp sign got switched when you wrote your post). Let me know for sure and I will try to help.

Also let me know whether you have the early "discrete" KBA or the later one that uses integrated circuits throughout.

Allen recommends as a first check that you rotate the transposer knob and see whether the dead notes follow the pitch (i.e., move up and down the keyboard) or the physical keys (i.e., remain fixed regardless of the transposer setting).

Don

[moller3x]

Yes the problem affects both manuals and the pedal.

[moller3x]

Don,

Thanks for offering to help. The notes not playing are in fact C# and G. Using the transposer does not have any effect on the physical keys. Swell, Great, and Pedal all have the same dead notes.

[jbird604]

I once serviced an Allen with the same pattern of dead notes. A cat had gotten up inside the console and broken a wire at the left end of the swell manual, as I recall. That one wire was the multiplex line for all C# and G notes throughout the organ. If you find a wire has been broken at that point, strip it back a bit and re-solder to the place it came from, which should be obvious. If it isn't a broken wire and you are indeed missing every C# and G on both manuals and pedals, it could be a diode or transistor on the KBA board, as Don has suggested. You might also be sure the detachable plugs on the KBA are securely pushed onto the board. Take them off (with the organ powered down) and re-install with a slight wiggle and firm press. Look at the wires coming off the KBA to be sure one is not broken at that point.

[don60]

Don,

Thanks for offering to help. The notes not playing are in fact C# and G. Using the transposer does not have any effect on the physical keys. Swell, Great, and Pedal all have the same dead notes.

As John just mentioned, C# and G ARE on a common bus and will be dead throughout all divisions if a fault exists on that bus. I have been boning up on my MOS1 troubleshooting; the next step is to check the action of the transposer on different positions (obviously not using a C# or G note). If notes are missing or of incorrect pitch with the transposer set to -2 or +4, then the problem is in a small block of circuitry near the output of the KBA. Please let me know the results of this test.

John, do you know a standard part number for the NAND gate chips used in the KBA?

Don

[jbird604]

Can't find that info right now, Don. Will try to look it up when I'm at the shop later today.

[moller3x]

I tried the transposer test using a playing C note and there were missing pitches when I tried the -2 and +4 settings.

[jbird604]

Sounds like you're gonna need a new KBA board unless you're a skilled electronics repairman. If the board in the organ is the old original one, Allen will have to sell you a new one rather than repair it. If I had it here, I'd probably attempt repair, since it's standard off the shelf parts. However, truth be told, it's often less expensive and more satisfactory in the end to just send it to Allen or have a tech order you one.

Before spending hundreds of dollars for an Allen exchange or new board, though, I'd sure pay for a service call and let an Allen tech swap his test board into the organ to be sure that the board is indeed the problem.

[don60]

I tried the transposer test using a playing C note and there were missing pitches when I tried the -2 and +4 settings.

With all G and C# notes dead and transposer settings -2 and + 4 faulty, Allen says that the problem is likely to be output transistor Q10 (a 2N5373 PNP type). If you or someone you know can replace a transistor, you might be able to save hundreds of dollars on a board repair.

Don

[moller3x]

ive done soldering work on a pipe organ console and little bit on electronics. of course, I have no idea which is output transistor Q10 on the KBA, but sure appreciate any advice. I thinking sinking several hundred more dollars into this little MOS-1 organ with non-programmable pistons would be a mistake. A cheap DYI fix might save it fromt he landfill.

[indianajo]

Allens are so *****ed mysterious, everybody sends you to the witch doctor for another $250 shot of information and a ten cent part. I'm paying $.09 for MPSA56 TO92 PNP transistors from Newark.com, you could replace them every one for $2 ? See this thread, this guy is parting out an Allen, see if any boards match up: organforum.com/forums/showthread.php?18303-Parting-out-an-Allen-system-505-B&p=274318#post274318
You can get a base check on transistors with a DVM ohms scale, see Thomas Floyd Electronic Devices, the Electron Flow Version an obsolete community college text I found at Goodwill for $2. Lots of practical tests for parts that were all differential equations when I went to college.
As far as the mysterious nand gates, it is either 5 v Power supply and ttl(74xx) or LSttl(74lsxx), or 9-12v power supply and cmos, either 4xxx RCA or 74Cxx from TI. If I knew the pinout, the power supply voltage, and the dates on the caps or transformers, I could figure out what the IC was, but as far I'm concerned you can throw all the electronics away and use the gold contacts on the keys to build a Hauptwerk simulator using a PC. Then if something blows up, it is a $59 part from 10000 mail order suppliers.