Technical 1947 Ford Ammeter Question

A friend has a'47 Ford V-8 that is really in great shape....except that the ammeter is causing some problems.
When the car is shut off, the ammeter stays in the middle of the gauge;
If you leave the battery connected, the battery will run down.
If you disconnect the battery at the post, the ammeter swings to the left.

After about 10 or 15 minutes, if you reconnect the battery, all is good.

Voltage regulator, maybe? The one that's on it now has a blue cover and is not the original.

And, the horn doesn't work. IDK if the two are related.
Also, how do you get the horn ring off ?

 

OK, found a discussion on the Horn Ring; so, we'll try the "push in center and use a rubber mallet" method

 

Likely the voltage regulator. Possibly sticking contacts, or a bad relay.
Removing the yellow Batt. wire from the regulator should verify it.

 

What happens when the yellow wire is removed?
Is this when shut off?

 

If removing the yellow Batt. wire from the regulator causes the ammeter to return to zero (with the ignition shut off),
the problem is most likely in the regulator.
Hope that makes sense.

 

Check for a stuck cutout relay (what it sounds like); the battery will backfeed into the generator windings and smoke it. Mechanical voltage regulator points require periodic cleaning. Eventually.