Mid-Continent MD-91 (LET) clock mystery trivia

[Vance Harral]

This isn't necessarily a Mooney-specific question, but posting here because I think it's factory original, and I'm guessing others here with the same clock have answers.  It involves a clock identified as a "Mid-Continent MD-91 (LET)"

Short version: why does this clock have two power connections?

Long version:

If you search for this clock on the internet, you get plenty of pictures, auctions on e-Bay, etc.  But no installation/maintenance manual that I can find, not even at Mid-Continent's factory site.  Below is a picture of the back of the unit, from one of those online surplus sites.  Notice the unit has two connectors on the back.

As I said, I think this is factory-original equipment, though I'm not 100% sure.  Thing is, the factory schematics for our airplane show a clock with only two terminals, not four:

The factory schematics show one of these terminals going to ground, and the other to the battery, through a 5-amp fuse.  That makes sense: this is an old-school electric clock which needs power, and if it's going to keep accurate time, it needs power all the time, which it gets from the ship's main battery.

If I trace the wires from our clock, one pair has a connection to ground, and a wire going down the side of the fuselage - I presume to the battery.  So far so good.

The other pair has a connection to the same ground, and a wire that goes through a 1-amp fuse to a bus bar that is energized when the master switch is turned on.  At first I thought this was some form of redundant power, but that seems unreasonably complex for such a simple instrument.  I'm now thinking it's just a light.  But I haven't flown at night in a while, and I honestly can't remember if this instrument is internally lit or not (even if it is, the bulb may be burned out).

Does anyone have a manual for this clock, or just know what that second power connection is for?  If it's a light, anyone care to speculate why it's on the master switch bus in our airplane, rather than the lighting bus?

 

 

[PT20J]

That looks like a light bulb in the upper right corner of your picture. Bet it's just on all the time.

I'd all Mid-Continent and see if they can send info.

[carusoam]

Looks like if you twist the connector… you get access to the bulb that is attached to it… 

Like Skip mentioned… it is directly connected to the dimmer circuit of your instrument panel…

So… two sources of power… one gets dimmed, the other doesn’t…

Best regards,

-a-