R.Smyth wrote:

The unit on the antenna is just a small box, about the size of
a small candy bar. There is no power going to it. The only
wires are the twin lead at one end and the coax at the other.
(If it's an amplifier, it would need a power lead, no?)

Not necessarily. From what you say below, it sounds like the
plug-in unit would likely be feeding a DC voltage up the coax
to power the amplifier up at the antenna. Same principal the
cable company uses, actually. There is 60 VDC on the big
feeder cables to power the amps and line extenders. It gets
trapped out at the tap units where your house connects.

The other unit is a
box , maybe a little bigger than a cigarette box, which plugs
into the wall in the house. The coax from the antenna goes on
one connection and a coax cord to the TV set goes on the other.
There is a gain control on it and an FM signal trap. This would
seem to be the actual amplifier. It's a Radio Shack product.
I believe the top unit is defective because I may have broken
something in getting the old coax connector loose. I'm just
wondering what will happen if I merely replace the top unit with
a 250 to 75 ohm transformer. Thanks.

Maybe nothing, but likely improved if you broke the center
conductor loose in the coax connector. Yes, you can replace
it with a 75-300 Matching Transformer. Your downfeed cable
should already have an F connector on it, and the other end
you can srip the twin lead on both the antenna wire and the
"lugs" end of the 75-300 MT, twist wires and electrical tape
to secure. Soldering wouldn't hurt but isn't necessary.

Get rid of the plug in wall box down below, a female-female
F connector "Barrel" will tie those two cable ends together.

HTH.

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