Flute stops not under expression

[paul2169]

Hi - just discovered that none of the flute stops are under expression anymore on my Allen Digital MOS-1 203C 1978 model. I'm wondering what the problem is and how to fix it. Thank you, Paul

[beel m]

I'm sure our Allen techs will respond. In the mean time, take the "expression" plug from the MAIN amp and plug it into the FLUTE amp. If you got your flute expression back, it's in the expression pedal unit or the cable to it. If not, put it back then swap the flute and main inputs to the amplifiers and see if the problem is now in the main system.
I'm assuming you cleaned all the plugs and connectors on the amps with Deoxit or, one expert recommends, a very thin coating of Vaseline (then wipe it off)
Let us know what you find. It may be as simple as a burned-out lamp in the photocell expression unit.

[paul2169]

The flute expression was restored when I switched the plug from the main amp to the flute amp. Checked all the cables. So I'm hoping it's a simple fix. Paul

[tucsondave]

That narrows it down to the photocell or wiring to the cell for the flute channel.
td

[paul2169]

Bulb is lit so looks like a photocell replacement. Where do I get a replacement and how do I replace. Looks like soldering is involved. Paul

[beel m]

Paul, while you're waiting for the Allen experts to chime in, look it up in the Archives. I seem to remember this very issue has been covered- maybe multiple times. Good luck.

[John Vanderlee]

Before you do that I would put an ohm meter on the shielded cable from the photocell to the amp and see if the resistance varies as you move the pedal. I 'm not sure of the ohms range but probably somewhere between 1 K and 10K. If it varies the cell is OK.

[Admin]

It would be best to do this as close to the physical location of the cell as possible to eliminate the possibility that this problem is due to a cable or contact problem. The dark resistance should be about a megaohm or greater and about 3-5k when lit.

Everything you need to know about the Allen photocells is in this thread:
https://organforum.com/forums/forum/...otocell-values

[myorgan]

Paul,

If the bulb is lit, has it lost its black dot on the bulb the organ uses to regulate the light? It could be it flaked off. Worth a check before replacing the photocell if you don't have to.

Michael

[Admin]

The loss of the black dot on the bulb would not cause a loss of expression. The light falling on the photocell is controlled by a mechanical shutter which completely blocks it when the shoe is fully open. The black dot is to mask the bulbs filament to defuse the light and prevent hot spots that would cause sudden shifts in volume as the shutter is opened.

[paul2169]

Good morning,

Thank you all for your responses and suggestions. Here's what I know:

1) bulb is lit with black dot intact
2) plugs and connectors cleaned with Deoxit
3) took expression plug from the MAIN amp and plugged into FLUTE amp and flute expressions restored.
4) additionally, I took expression plug from the FLUTE amp and plugged into MAIN amp and lost flute expression.
5) I don't own an OHM meter so will need to borrow one to confirm that it's a contact or cable problem.
6) I've read the ARCHIVE articles and still unclear about the specific photocell replacement part for my model (there are also broken parts links in the ARCHIVE).

In the event I need to replace the photocell, a part number, etc. and instructions would be appreciated.

Paul

[Admin]

The Ferroxcube and Clairex photocells mentioned in the thread above are discontinued items, but you may be able find them, for a price, from internet sources that specialize in obsolete parts.

Of course you could always contact Allen for a replacement part, but you'd probably have to go through an Allen dealer or Tech.

I wound up using the cell below as replacements. I ordered a bunch and hand selected them to match the resistance characteristics called in the patent document. The only problem with these cells is that their response time is bit sluggish compared to the originals.

[tucsondave]

The replacement part for the original LDR's is Hamamatsu P380. I don't know where to find them other than Allen Organ Co.
td

[Admin]

The spec sheet for the P380 shows a light resistance of 4 - 13k and a minimum dark resistance of 20 M with a rise time of 35 mS. The PDV-P8001 above is the closest to those characteristics that I could find at the time (10 years ago!). The PDV-P8001 has a minimum dark resistance of 200K which is quite a bit less than 20M spec'd for the P380; however, I recall that all of the P8001 I received had dark resistances > 1 M which should be adequate.

[John Vanderlee]

I don't generally like to recommend a particular store, but Harbor Freight sells a digital multimeter for a few dollars. Sometimes it's even one of their Freebees!
I keep several for different places.

[Pipeorganbuilder]

I must agree with John about the Harbor Freight meter. They are less than ten dollars and offer features that will help you out on your project. Not the quality of my Fluke meter but you don't need a $500 meter to check the voltage or resistance in your Allen organ. Hope you have one of their stores in your area. Best of luck with your repair.

Michael

[paul2169]

The photocell was replaced this morning which resolved the flute expression problem. Thank you all for your expertise and help! Paul

[Pipeorganbuilder]

Congrats on your repair. What photocell did you use?

Michael

[paul2169]

I ordered the photocells here!


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