Best RF Connection to old TV

[vol.2]

I have an old late 70s Sony TV which only has and Antenna input on the rear.

It has a switch to go between the pole antenna and an "external antenna."

I have been using an RF modulator to hook up to the back of the TV, but the TV only has screw terminals for the external antenna, not a Coaxial input like later TVs I own.

I have a few questions/issues about this hookup

1) My RF modulator has a coaxial output and the TV does not. I have been using a "75 ohm to 300 ohm adapter" on the end of a coaxial cable to break out the coax to spade terminals and then screw that onto the TV

I have no idea if this is correct, I just know that it needs to be wires or some kind of terminal connectors to even hookup to the back of the TV

Is this correct? In other words, is the Sony TV expecting 300 ohms at the antenna terminal, and therefor it's correct that I'm using this matching transformer for that purpose?

2) I am getting some occasional interference in the RF signal. My instinct is that there are so many devices now that put RF into the airwaves that I'm getting stray waves from wireless connections and smart phones and stuff like that. I live in a city, so this seems like a plausible answer.

Is there some way to mitigate external interference to my TV from the RF modulator?

I am somewhat experienced in repairing old TVs, but I lack RF experience. I've already been in this thing and had to replace all the caps and many other parts just to get it working correctly again, so I'm totally fine going inside and working on it.

My thought was that I could maybe bypass the external antenna connector and go straight to the VHF tuner with an RCA style coax cable (which it seems to use) directly into the tuners. If I did this, what considerations should I have in regards to impedance matching? Would this help at all?

Thanks

[old_tv_nut]

1) Yes, the 75-300 ohm transformer is the correct thing.

2) The signal from your modulator should be strong enough to overcome RF radiated interference, even though the 300 ohm connection is not as well shielded as a coax cable. So, this could require some investigation to see how the interference is getting in.

Regarding going via coax directly to the tuner, it maybe doable, but you need to provide a schematic and/or pictures of what's in the set that connects the 300 ohm screw terminals to the tuner coax input.

Questions: Does the set have separate 300 ohm screw terminals for VHF and UHF? Are you sure you are connecting to the right ones?
Are you sure the tuner has a coax input, or could you be looking at the tuner's IF output cable?

[vol.2]

Thanks

Originally Posted by old_tv_nut

2) The signal from your modulator should be strong enough to overcome RF radiated interference, even though the 300 ohm connection is not as well shielded as a coax cable. So, this could require some investigation to see how the interference is getting in.

Okay. Well I don't get the interference all the time. I usually see it during the day and evening, but if I'm using it late at night it seems to go way. My thinking is this is the time when less people are using devices that might cause interference.

Regarding going via coax directly to the tuner, it maybe doable, but you need to provide a schematic and/or pictures of what's in the set that connects the 300 ohm screw terminals to the tuner coax input.

This is the coax that goes from the VHF tuner to the antenna block. I ohmed it out to the inside of the block, and it connects to the antenna/external switch through a capacitor.




This shows the location on the antenna block where that coaxial cable (from the VHF tuner) goes to:

Questions: Does the set have separate 300 ohm screw terminals for VHF and UHF? Are you sure you are connecting to the right ones?

Yes it does and yes I am. I know how to do all that and I wasn't making any mistake about it.

Here's the antenna block and the VHF tuner:

[Electronic M]

Usually when there's a coax from the antenna board to the tuner it's 75ohms (there are exceptions).
There's probably* no harm in trying to unplug the antenna board RCA cable from the tuner and connecting an RCA to F-type coax adapter.

*Some TVs are hot chassis and there could be 120V on the shield of the tuner and it's antenna... Before connecting the 2 I would measure the AC and DC voltage between them and if either is over 10V I'd avoid connecting them directly.

[vol.2]

Originally Posted by Electronic M

Usually when there's a coax from the antenna board to the tuner it's 75ohms (there are exceptions).
There's probably* no harm in trying to unplug the antenna board RCA cable from the tuner and connecting an RCA to F-type coax adapter.

*Some TVs are hot chassis and there could be 120V on the shield of the tuner and it's antenna... Before connecting the 2 I would measure the AC and DC voltage between them and if either is over 10V I'd avoid connecting them directly.

Thanks

Good point. I actually pretty sure it's a hot chassis and shouldn't do that. I think that's why the antenna block isolates the RF through CR101 and CR102

Is there maybe some way around it, like copying the decoupling capacitor on my own?




This is the power plug input. Pretty sure that's hot.

[Electronic M]

It looks like it could be hot. You could copy the capacitor scheme.

If this uses one of those 1/8" mono headphone connectors for the stock user RF connection you can buy a F-type coax to 1/8" headphone jack style antenna connector and connect the coax to the factory user antenna hookup without the balun, and that would likely block interference.

[vol.2]

Originally Posted by Electronic M

It looks like it could be hot. You could copy the capacitor scheme.

If this uses one of those 1/8" mono headphone connectors for the stock user RF connection you can buy a F-type coax to 1/8" headphone jack style antenna connector and connect the coax to the factory user antenna hookup without the balun, and that would likely block interference.

It doesn't have one of those 1/8" jack antenna inputs. It's only the screw terminals on the back unfortunately.

I guess I could try to figure out how to replicate the capacitor scheme, but I'd have to make sure I kept the impedance at 75ohms, and I need to figure out what those resistors across the decoupling capacitors are. There is no part number for them at all. I guess it could just be a board trace or something? Maybe it's 300 Ohms?

Ah yes, the service manual calls them "component combination" CR101 and CR102, and describes them as "antenna isolation"