aaa

Firstly, what is your input source, direct cable or DVD hookup or via analog modulator (mini tv transmitter).

Do you have the local-distance switch set to distant or local or distant?

Have you fine tuned the channel selector???

Are there any local interference producing sources close by the set?

What is the condition of any lead in wire or cable to the back of the set?

If you are using a local modulator and receiving signal via the TVs local antenna, what's the condition of the lead in wires and the antenna?

If you're set had UHF, have you accidentally connected your input to the UHF terminals or the correct VHF terminals??

George

aaa

Do you have any other source that you could connect for a comparison? Digital converter box, VHS tape player, etc.

Is the problem the same if you switch both the modulator and the TV to a different channel?

Interference can come in many forms: computer power supply, light dimmers, fluorescents, etc. I even discovered some new LED floodlights that create interference.

Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html

You might want to check the Balun coil on the tuner (that is where the antenna leads go into the tuner). these often fail on "Hot" chassis sets because, for some reason (lighting strike, Line surge, e.t.c), breaks their isolation from the chassis, leaving one (or both) antenna leads at chassis potential. a "blown" balun will cause the problem that you are seeing on your screen.

aaa

I've Seen this before. you might want to check the balun coil on the tuner (that is where the antenna connects to the tuner). this is a common problem on series string sets because the insulation used in the balun coil breaks down, causing chassis / Antenna isolation problems.

This isn't a series string or hot chassis set. It has a power transformer like the 17" tabletop models.

Even with the tuner grounded to the chassis?

Yes, The Balun isolates the antenna input from the chassis as well as act as a matching transformer to match a 300 ohm lead to the tuners actual input impedence.

This isn't a series string or hot chassis set. It has a power transformer like the 17" tabletop models.

Does not matter, it also is used to match the tuners input to the 300 ohm lead. regardless of whether the set is a power transformer type or Hot chassis type, the balun still provides some isolation for safety reasons.

aaa

Both coils are the balum(matching transformer). There are two coils on each plastic form. Check cont. through each one. Also check the lead from antenna terms. to the tuner. Also check 0.22mfd cap on tuner, if leaky can also cause this. All the best, Tom

aaa

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.

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.

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.

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