Rca 9-t-79

[old_tv_nut]

Originally Posted by Kevin Kuehn

No I don't think it had anything to do with CRT type. They made several models, I believe all in 1950 (including some with all glass CRT's), that all shared that same flyback type. And apparently licensed it to other manufactures, for instance I have a 1951 Emerson with the same circuit.

Were other sets using ferrite cores or iron cores at that time?
If ferrites with the desired properties were not available then, an air core may have been used because the available ferrite or iron (powdered iron?) cores were too lossy.

Also, I don't know if the resonant frequencies of the primary and secondary were tuned at that time the same way they were later: 3rd harmonic tuning or 5th harmonic tuning makes the secondary ring at an odd multiple of the primary flyback pulse, which produces a high voltage pulse with a flatter top instead of a pure sine wave. This makes the high voltage supply droop less with increasing current, because the rectifier is seeing a more constant average voltage over time with a varying current.

[MadMan]

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)?

[MadMan]

Fixed my crt tester (I think), picture tube tested dead. I replaced 2 cans' worth of filter capacitors and fired it up.

It's kinda blurry, have the focus magnet at max. Could be the yoke is not all the way on. (Have a feeling someone's going to say tube is bad, I mean, probably, but it lights up, so...) CRT appears to have a spot burn. It's not terribly bright, but I'd think watching in a lit room would be marginally possible. Couldn't hear the HV or vertical at first and I was kind of disheartened. Maxed the horizontal hold and then I could hear it. Vertical is really quiet though. Played with the ion trap for about 10 minutes - thinking I wasn't gonna get any light on the tube - before finding out that it likes hanging out all the way back on the plug cap... which is unfortunate because it's completely broken. I tried flipping it but it's still as far back (single magnet).

So far, not bad for all original wax paper bombs and half the tubes being bad or marginal.

[Tom9589]

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?

[Electronic M]

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

[Kevin Kuehn]

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]

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

[MadMan]

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]

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..