Philco Predicta Danish Modern 1st working pictures

[dtvmcdonald]

You have to be very careful describing what these so-called "baluns" do.

I've looked at lots of sets with them and while the coils are such that they DO
allow making the usual baluns, they are often wired in strange ways. Very frequently they
do NOT provide a balanced antenna input. Sometimes its it is indeed 300 ohms
nominally balanced, but I've seen 300 ohms with one side grounded, other impedances
(including one with a 100 ohm connection to the tuner and 25 ohms out, and
in one memorable case in an RCA 9-T-249 75 Ohms to ground common mode ...
I.e. the two lines had the same phase!

No wonder so many old TVs had antenna problems.

I check this by feeding a suitable, balanced or unbalanced (some old tuners
of course had push-pull triode RF amps) signal in to the RF amp grid and looking at the
antenna terminals with a good 2-channel 350 or 500 MHz Tektronixs analog scope,
1 megohm input.