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.

[old_coot88]

Assuming it's a wafer type tuner, occasionally one of the rotors is not turning with the shaft. Visually check that all the wafers are secure on the the shaft with no slop. In the case of snowy pic, the RF wafer would be the main suspect.

[robert1]

Originally Posted by 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.

That is why i mentioned the balun coil in a earlier post. i actually blew out a balun on a late fifties RCA just by merely connecting the CATV cable with a 75 / 300 ohm matching transformer to the antenna leads. the matching transformer was apparently bad because there was a low impedence between the cable sheath & the 300 ohm connectors. that is not supposed to be, but i guess that is what i got for using a cheap matching transformer without checking it first.

[oldtvsandtoy]

Ok just tested my Sony DVD with Radio Shack RF Modulator modulator on my admiral 24a12. Picture is nice and clear. Guess its time to try the tuner