Injecting composite video into a Hot Chassis set?

[Jon1967us]

The TV I'm troubleshooting, 1957 portable Trav-ler is a nice little set, with a good CRT, is worthy of restoration but has some video sync issues. I'm attempting to feed a composite signal from my BK 1249A into either the video amp grid or the 1st IF grid, as a way of process of elimination.

I haven't done this before (I've only restored a couple sets) and was wondering if

A. It's safe or possible to inject a signal onto a hot chassis set, feeding the signal from the generator onto the grid of the Video Amp, and ground to chassis. I remember reading something about this and suspect this isn't a wise idea.

B. If not, is there an alternative way to bypass the tuner, and inject signal somehow?


I assume we're connected to the "Video" output with "IF" selected.



And injecting into pin 7?

[zeno]

The usual place would be just after the video detector diode. The
generator should have a 1V P-P output preferably adjustable.
Use an isolation transformer. Changing to a safe & effective way
is overly complex. Some sets did it with a photo optical isolators
like some 1980's NAP sets. Also you need an isolation transformer
anyways. Dont confuse isolation with a variac ! Some come together
in one package many dont.

73 Zeno
LFOD !

[Jon1967us]

I failed to mention that I do have the set on a Sencore Powerite, so it looks like I won't fry my test equipment...

[Jon1967us]

I failed to mention that I do have the set on a Sencore Powerite, so it looks like I won't fry my test equipment...

[zeno]

A Power Rite, good selection. Everyone should get one right
after a good meter especially on solid state sets.

[AlanInSitges]

If you're using the powerite you don't need to worry about whether it's a hot chassis or not. You're good to go.

You'll want to start at the end of the signal chain and work your way back to the beginning, checking for results along the way.

First step would be to connect your composite output to point B in the schematic, leave the level control set at the 1V reference position. If you don't see good results there you can try adjusting manually to see if you get a stable picture - the Sams says it's a 5V signal there, so you may need to go that high. If you still have trouble you know the problem is after point B.

Next you'll use the IF output jack to connect to whatever the input and output of the various IF stages are. Sams normally has a test point labeled with a diamond but if not you can usually do grid and secondary of the IF transformers for testing. Set the yellow button to IF, the others don't matter.