Rca 9-t-79 - Page 2 - Videokarma.org TV - Video

MadMan · #1 12-01-2019, 08:16 PM

I put the whole shebang on my workbench and brought it up on a variac. I got filaments but nothing else. I tested power in and out of the 5U4. Power in, nothing out. It tested poorly on my Hickok earlier. I pulled it out and the getter flash looks light gray instead of normal, which I'm pretty sure it looked normal before. Either this tube went to air slowly or maybe it was cooked. Which makes sense, because after I changed it, the set pulled 300 watts, I heard the HV try to come on a couple times, and two of the three big can capacitors got really hot. So I'm going to order some caps.

In related news, the crt tests near-zero on my tester. However it tests the known-good Admiral's tube at just above one. I've figured out that all the resistors in my tester have drifted high, and doesn't make the 300v it should be making. I'll be getting resistors for that too.

Also, there are two capacitors to ground from line and neutral on the plug. Do those need to be special capacitors to handle line voltage (all the time)?

Tom9589 · #2 12-07-2019, 06:51 AM

I replaced a CRT once that required the ion trap to be placed very far back. I tried all that you did, but it still wanted to be all the way back touching the base. The set worked well for years until it was replaced. I've always wondered what was different about this CRT. Was the electron gun not installed properly? Wrong electron gun assembly?

Kevin Kuehn · #3 12-07-2019, 12:10 PM

If the center spot on the CRT is only visible when the tube is lit, it's not a burn, but rather vaporized metal has been deposited on the back side of the phosphor. The deposited material is causing that area to shield the electron beam from lighting the phosphor. That's what happens if the set gets operated for a long time with the Ion trap improperly adjusted. That CRT seems to have fair brightness, although if it won't focus well it's possibly also the result of it being operated with the Ion trap improperly set. Once the electron gun aperture material becomes vaporized there's no way to fix it short of replacing the gun, otherwise you live with it as-is. If your high voltage isn't up to spec that could also be affecting the focus. I'd say you've got very good life signs considering the all original caps.

jr_tech · #4 12-07-2019, 12:47 PM

Check the voltage on G-2... if it is too low, electrons in the electron gun will be easier to deflect with a magnetic field, requiring a weaker or oddly located ion trap.

just a WAG,
jr

Electronic M · #5 12-07-2019, 11:52 AM

Could be your ion traps magnetism has weakened since new... I'd try another trap.

MadMan · #6 12-08-2019, 11:26 PM

I thought I'd try my hand at actual diagnosis instead of using the shotgun approach. Main problem was no snow / no video coming through. Doing some voltage checks and found everything WAY out of line. Eventually traced it back to a main power resistor R125 on Sams, 4k 10w. It's mounted on top of the chassis behind the tuner. Wide open. My luck that I have 20 new 5k resistors and not one 4k. I put a 5k in there for the hell of it, it made no appreciable difference in the operation of the set and got VERY hot very fast. So now I'm thinking like there's something else shorted that burnt up the old resistor. Probably a capacitor lol. Also there's a resistor R131 8k 10w, that I cannot find for the life of me. Sams shows on the alternate chassis it may not exist, but it should on mine. However, Sam's schematic is preferential to the tv/radio model which is missing the end of the audio section and some other stuff.

Thoughts?

Also, digikey does not stock any 10w 4k, would 25w be ok?

As an aside, a week ago I found a Zenith console stereo in the trash. Someone ate the record player and trashed the cabinet, so I stole the tubes. The first one I pulled was a 6AU6, and I was happy cuz I needed one for this tv, but the rest I pulled were clearly more modern numbers. But with the help of a tube substitution book, apparently I can use 5 (for this tv) out of the 8 or so that I took. Yay!

>>SCHEMATIC<<

Electronic M · #7 12-09-2019, 09:08 AM

Most sets old enough to have primarily cardboard tube paper caps troubleshooting before shotgunning those caps leads you to just shotgun the caps more slowly/methodically and pull the hand full of resistors and misc parts you'd troubleshoot and change post recap into the process....

The best sets to do the kind of troubleshooting techs did when new are the 60s sets that still have most of their original caps good..