Losing video
 

Wish I could embed my video (link below). Anyway, I've started having trouble with my '56 packard bell black and white. It'll typically work just fine for a while and then the picture's contrast will gradually decrease followed by losing v-hold and finally no picture. I can typically flip through the channels and get it to show again, though the longer it's on the shorter the period of time it keeps working after doing so.

I thought the tuner contacts might be oxidized so I cleaned them (3x5 card fed into tuner to clean both sets of contacts) and lubed them with contact cleaner/lube. That doesn't seem to have helped so I'm now doubting that the problem is the contacts.

What should I suspect? I re-capped the chassis and replaced the tubes 2 or 3 years ago but I'm tempted to try swapping in some of the old tubes (which were surely not all bad) in the video section to see if it might be one of the tubes. But while I'm somewhat experienced working on 70's-90's arcade monitors, I'm not that knowledgeable about working on tube based stuff. So I'm guessing one of you guys can point me in the right direction at least. :)

Here's a video demonstrating the issue once it gets to the point where it's hard to maintain a picture for more than a few seconds. In this video as the picture goes out I'm repeatedly turning the tuner knob a fraction of the way to the next channel and then back in order to get the picture back.

https://youtu.be/rcOMdW2Il5w

Try swapping the I.F tubes one at a time and see if that helps. If it doesn't try the tuner tubes. Not sure if your set uses one but check if you have a bad video detector diode. I couldn't tell from your video, but does the sound also go dead? If the sound keeps working then check your video output tube. If tube swapping doesn't help then there could be component problems. The model number would also help.
Ed

Maybe an AGC problem?

Another thing to watch for with loss of signal symptoms is AGC issues.

Voltage checks are your friend here monitor AGC, tuner and IF voltages. If the AGC voltage is changing so as to reduce IF gain (instead of increase it) in step with signal loss then that is your problem.

If it gets to the point where it wants to stay dead, and you have a B&K analyst or the like then tuner and or IF injection may show a bad stage.

[EDIT] Wayne beat me to AGC.

Thanks all. I didn't get a chance to get back to my TV today but I will definitely follow the leads given here. Much appreciated. I'll report back when I get further into this.

P.S. It's a model 21DK1 with 98D1 chassis. Some pics and other details are in my intro thread from shortly after I got the TV: http://www.videokarma.org/showthread.php?t=262622

Reporting back on the cause. It was the mixer tube in the tuner circuit. Another symptom I wasn't really noticing was that when turning on the TV I'd have raster with no picture for about 15 seconds and then it would fade in. Once I swapped in a new mixer tube, start up characteristics went back to what they used to be: raster with an unsynchronized picture (not sure if that's normal or another tube that's weak, but once warmed up I have no sync problems). So nice to be able to enjoy my TV again!

May want to try the 11 second test: after the set is good, warm, and stable (10+ min of operation) turn it off for ~11 seconds then back on and see what aspects take longest to recover. If all else is good, but sync takes awhile to come back swap the sync separator tube and try the test again. If weak video try video tubes. If all signal try tuner and IF one by one....Etc.

I'm by no means unhappy with it as is but I will give that a try. At a minimum it'll be a good learning experience. Thank you!

The 11 second test points to the sync separator tube, which I don't have an extra of. Ordered a handful of tubes last night to give me a nearly complete set of spares so I'll swap that in when it arrives.