View Full Version : Direct video injection on a Predicta holiday


madlabs
12-01-2016, 11:25 AM
Hi All,

So, I want to try direct video injection on my Predicta Holiday, primarily because I want to see how off my IF is. And if it looks way better then I'll probably feed it a signal that way. I'd prefer to receive the signal through the antenna as intended, but I don't have the tools or know how to align the IF. This being a hot chassis set I know this is a little riskier than on many sets but if I always run the set off an isolation transformer I'm hoping I'll be ok. I picked up a $15 DVD player with composite output at the thrift store, so if it explodes in flames I won't cry.

So, looking at the information at Phil's site, it looks to me that I would inject the signal right after C20 on my set. Is it possible to inject the signal just before C20? I ask because that would make the connection point very easy to access. Also, should the output from the IF be disconnected before attempting to inject the signal? Any other things I should look out for or know?

Thanks!

Electronic M
12-01-2016, 12:55 PM
I'd need to see the schematic to be able to comment on the exact connection...

Generally speaking you want to run the TV and only the TV on the isolation transformer*, Ground the cable shield to chassis, and connect the center of the coax to a point after the detector with low or no DC voltage relative to chassis ground tube grids are good. If all points have DC voltage then use cap .1 uF or larger (you can go as high as you like) in series with the coax center and injection point. Choosing an injection point can be easy or impossible....You see each tube (video and IF) that the signal goes through inverts the video so if you inject at the grid of the first video amp you could get negative video (black and white reversed) , and injecting at the plate could give you positive video or vice-versa....You have to choose an injection point that gives you positive video, then you have to choose one with a comparable designed signal level....If you inject your signal at a point that wants more signal you may not get enough contrast, if you inject at a point that wants too little signal you may get too much contrast. Some sets don't an injection point that is the right polarity AND is easily able to accept composite video signal levels....When that happens you have to make a additional video amp circuit or go back to RF.

It is advisable to disable the IF system for injection (unless you get no audio or video noise with no RF signal input)....On a series string set the best way may be to find a dud last video IF tube with a good heater and snip off or bend the plate pin (if the detector diode is easy to get at hat would work too)...If using audio injection you may want to do that at the audio detector too.

* I've seen isolation transformers loose isolation from primary to secondary shorts so it is wise to check for voltage between a leg of the primary and a leg of the secondary with the transformer powered (should be 0 volts).

tom.j.fla
12-01-2016, 01:11 PM
C20 is a good place to inject your signal. Be mindfull that the contrast control will not work in direct injection mode as it is a gain control for the I.F. and R.F. amps. Some other means would be needed to control the contrast in that mode. All the best,Tom.J from Fla

madlabs
12-01-2016, 01:28 PM
I'm not smart enough to snip the section of the schematic, but here is a link to the Sam's: http://www.earlytelevision.org/pdf/Philco_G4242-Predicta-Holiday-Sams-439-1.pdf


C20 is a good place to inject your signal. Be mindfull that the contrast control will not work in direct injection mode as it is a gain control for the I.F. and R.F. amps. Some other means would be needed to control the contrast in that mode. All the best,Tom.J from Fla

Uh oh. Any suggestions as to how to accomplish this? This is my first television and my ignorance is profound.

Thanks folks!

Electronic M
12-01-2016, 02:30 PM
Get a pot and use it to reduce the composite line in amplitude.

madlabs
12-03-2016, 10:54 AM
OK, I'll try it out next week and many thanks for the help. I'll report back and let you all know if it worked or blew up.

Thanks!