Tuner By Pass

[sylvania]

Anyone ever by pass the channel tuner send video and audio into tv chassis or is it more trouble than its worth?

[N2IXK]

Originally Posted by sylvania

Anyone ever by pass the channel tuner send video and audio into tv chassis or is it more trouble than its worth?

Have installed switchable direct line inputs into a couple of RCA 721 sets. Very simple to do, and gives a higher resolution image without going through the tuner and IF strips.

Installation on that particular set is very simple, requiring only a coupling cap into the first video amp grid, and an audio input into the top side of the volume control pot.

Installation on other sets may not be as simple, particularly hot chassis sets.

[sylvania]

Thanks I will check it out see if I can do it.

[Electronic M]

The 2 things that have to jibe (assuming a transformer powered set) are video polarity and amplitude at possible injection points relative to the composite video standard. If either is wrong/not close it could be fixed with a user designed video amp or attenuator, but that can get complicated....

[N2IXK]

Originally Posted by Electronic M

The 2 things that have to jibe (assuming a transformer powered set) are video polarity and amplitude at possible injection points relative to the composite video standard. If either is wrong/not close it could be fixed with a user designed video amp or attenuator, but that can get complicated....

Yep. Fortunately the RCA 721 chassis (and most other early RCA sets) make this really easy, with the first video grid needing negative sync, and ~1vpp signal level, exactly in line with a standard composite video signal level from modern gear.

If you need to invert the signal, or adjust the amplitude, you can always build a suitable preamp using either a period-correct tube, or a modern video op-amp like an AD811.

Don't forget that a composite video input is supposed to present a 75 ohm load to the video source. This usually takes the form of a simple 75 ohm resistor from the input of a high impedance stage directly to ground.

[sylvania]

Thanks for the information, I should of mentioned this was a Sylvania chassis.

[Phil Nelson]

This article describes how I added switchable A/V input to an Admiral 24C15 TV:

http://antiqueradio.org/A-V_AdapterForVintageTVs.htm

As others noted, not every TV is the same.

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