Predicta Holiday - First power up attempt - Page 4 - Videokarma.org TV - Video

Electronic M · #46 09-08-2016, 01:12 PM

Looks good!
The 300ohm balun is advisable to use....If you lack one you could try hooking the center lead of the 75ohm coax up to either 300ohm lead and see if you have signal.

If there is a Radio Shack near you, they carry an over-built Balun/TV antenna matching transformer for ~$5 that should work nicely.

madlabs · #47 09-08-2016, 02:17 PM

Located a balun. It is 20 miles away, one way. Luckily the XYL is going to town anyway. So that problem is solved.

However, I am wondering, should the RF modulator be powered from the same isolation transformer of the set? And if so, I'm guessing I should make sure the neutral is connected the same way on both? And what about the DVD player?

Tom, I am going to get a Blonder-Tongue modulator, but the rat shack one was free and fell in my lap. Good enough to see if I can get this set working. Then we'll work out a better quality system.

Thanks!

Jonathan

Electronic M · #48 09-08-2016, 04:47 PM

I'd keep only the set on the isolation transformer. The set should have isolation in it's internal antenna transformers, but some don't, and others the isolation sometimes fails.

The modern gear don't have hot chassis issues so it does not need an isolation transformer. The reason you isolate ONLY the TV's power input is so that it will play nice with the 'chassis ground' of ANY piece of gear you connect to it hot or cold. Also if you have recent test equipment like an o'scope NEVER plug BOTH the set and the scope into the SAME isolation transformer....If the scope is grounding to neutral and the TV chassis is grounding to hot, damage will occur the instant you ground the scope to the TV chassis to try to measure the TV with the scope.

Any working mod is good enough for testing in my book.

For example if you had 2 hot chassis sets one with +70volts chassis with respect to earth, and another with -98volts with respect to earth and you wanted to feed them with a cold chassis (0V with respect to earth) signal source. the signal source would get plugged into the wall, +70 would be pluged into isolation transformer #1 and -98 would be plugged into a second, different isolation transformer (#2). If you plug devices with different chassis voltages into the same outlet/source you CAN NOT directly connect them together without causing damage....If you did connect them it would be roughly equivalent of taking a household plug jumpering one prong to the other and then plugging it in!...(granted there are parts in the sets that tend to burnout before the breaker trips).

When in doubt measure voltages (AC and DC) between grounds and hot leads in signal wires you wish to connect (toggle power states on any hot chassis gear while measuring). If you see more than 5V think twice or consult someone.

Bill R · #49 09-08-2016, 06:11 PM

You could just connect an antenna to the modulator and a antenna to the set. Put the modulator antenna near the tv antenna and tune the tv to the modulator channel. Not strong enough to transmit any distance, but I have used this setup to test sets before.

madlabs · #50 09-09-2016, 09:33 AM

And it is... a drum roll please....

THE THRILL OF VICTORY!!

Well holy cow. The set actually works! The picture is dim and there is no sound, except hum. That may well be that the cheap rat shack modulator or HDMI to composite converter isn't working. I'll have to dig into that.

However, the pictures is dim. Part of it that the inside of the cover is filthy. Even so, it should be brighter. I have read about cooking the filament a little to help bring the tube back to life, any recommended voltages and duration? I am planning on removing the shroud today and cleaning the screen and at that point I should know for sure which tube it is, although it must be one of the 2.35V tubes. And I do have a 21FAP4 NIB but I'd like to see what can be done with the original tube first.

Overall, what a wild success! The controls seem to work ok, horizontal hold locks in. Picture is poorly focused and needs some shaping. But I did watch an episode of Peter Gunn on it! Although with the sound coming out of the computer.

Thanks SO MUCH folks! Never, ever could have gotten it to here without the help I have kindly received on this forum.

Jonathan

Electronic M · #51 09-09-2016, 10:06 AM

Great! I'd check the voltages at the CRT socket against what is called for in the schematic, measure the HV, and test the CRT before trying to cook the CRT. Dimness can be gun bias circuit problems, low HV, or a weak electron-gun. Knowing which will give you a path to the most effective solution.

IIRC my B&K 466 has in it's instruction manual a 'cooking proceedure' that states to multiply the designed heater voltage by 1.4 and let it sit that way for 5-25 min (I've gone longer on hard luck cases)....That works best in test mode with it reading emission (at least with my rig).

I'd limit possible causes to a weak gun before cooking it, and I'd verify tube number and designed heater voltage before trying to test or cook it.

madlabs · #52 09-09-2016, 10:33 AM

Tom, I don't have a CRT tester as yet. How can I test the HV? I do have a HV probe for a voltmeter but haven't used it in a long time and IIRC it would load a flyback down too heavily.

I'm going to get the shroud off and find out which tube we are dealing with and take what voltage measurements I can.

Jonathan

Electronic M · #53 09-09-2016, 04:41 PM

You need a HV probe or HV meter. You want a HV probe good to at least 20KV (30-40KV is better especially if you plan to do color work).
I'm not sure where you came up with the notion that your probe will load it too heavily....If it is designed for 15-20KV duty it should be fine....Most HV meters TV techs used were essentially an HV probe with a volt meter tacked on... Look at the HV spec on the schematic and select a probe that can do about 5KV or more above that number.

You should be able to find a half decent CRT tester for $10-$50...I got my trusty B&K 466 for $25 working.

Compare neck voltages with schematic bearing in mind that some vary with control settings, and those settings are often noted on the schematic.

madlabs · #54 09-09-2016, 06:47 PM

OK, we are dealing with a 21EAP4. Looking at voltages:

B+ = 273, should be 290
Pins 2&6 of CRT, 405 should be 420
pin 7 of CRT, 27 should be 35
Filament = 2.28VAC
HV = 14.4kV, should be between 14 and 15.5kV
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I do hear some crackling/popping down in the well of the HV rectifier. Should get some dielectric grease in there. Not sure what my HV probe is good for voltage-wise but its a foot long and has creepage disks near the handle so it is good enough for this.

Looking at the voltages, they all seem close enough. One thing is that the line going to the CRT plate (pin 7) has an RF choke and it appears to be inline under some plastic tubing. It looks like it may have gotten hot. And the contact was corroded, did what I could for it.

Another thing, I had to substitute 3*1200pF caps in parallel for a 3900pF for C42. The idiot that does the ordering around here (me) screwed up and ordered 0.039 instead. Don't think that would make much of a difference though.

So, I am leaning towards a weak picture tube. I am going to have to buy a tester, maybe I'll go post a want in the classifieds.

Here is a shot of Peter Gunn.

Jonathan

Electronic M · #55 09-09-2016, 11:08 PM

Probably is a weak CRT.

Good luck.

old_coot88 · #56 09-10-2016, 12:26 AM

It's got the classic 'silvery' look of a very weak jug.

madlabs · #57 09-10-2016, 10:33 AM

OK, in the absence of a CRT tester/rejuvenater, I am looking at options for trying to wake this tube up.

1) In circuit, reduce the ballast resistor value, either by replacing one of the sections with a lower value or putting a variable power resistor in parallel. Check the voltage delivered with the CRT filament out of circuit and a 3.9 ohm resistor subbed in. Target voltage 2.34 volts * 1.4 = 3.22 volts. 5-25 minutes. Questions: Video signal or no video signal during rejuve? After rejuve, increase nominal filament voltage by about 15% (2.8V) as with a tube brightener?

2) Make a rejuvenator like one of these two, one is a link, the other a PDF attached. While similar, both have some differences. I have the stuff for either. Seems like it is a more of a real rejuvenation as opposed to just upping the filament voltage.

http://www.thevalvepage.com/teletech...ej/crt_rej.htm

I'm thinking the answer likely is to try #1 before moving on to #2. It would be real nice if I could get some life out of this tube.

Thanks for the help!

JOnathan

bandersen · #58 09-10-2016, 11:42 AM

Keep in mind that all the filaments are in series. Lowering the ballast will increase the current through all the tubes and the voltage will be split between them in the same ratio as before. There will be very little increase in the CRT voltage.

Video signal makes no difference. I've found that if a CRT is going to wake up, it will do so even at it's normal operating voltage, it just takes a while longer than using elevated voltages.

Rejuvination works by blasting a layer off the cathode. There's not much material there to begin with in these short necked CRTs and it's liable to make things worse.

I hate to say it, but I've encountered about a dozen of these 21EAP4 CRTs and all were deep into the bad. No waking up or rejuvinating helped

madlabs · #59 09-10-2016, 12:01 PM

Duh on the series resistor. Then why did Philco say to bypass part of the ballast resistor for the 6.3V tubes?

OK. So I might as well try an external rejuv circuit, what have I got to lose? Opinions on which circuit or techniques to use in a last ditch effort?

On the next expedient. I do have a 21FAP4 NIB. In other threads the thought was that this tube should work but nobody had yet, at least as far as I could see. Looking at the data sheets, it seems the main difference is that the 21EAP4 has an extra grid tied to the anode. And that G1 and G2 pins are swapped and that G3 on the 21FAP4 is the same as G4 on the 21EAP4. So does all of that boil down to swapping G2 & G3 on the CRT socket?

Thanks!

Jonathan

After thought: What about putting a 3.9 ohm resistor in place of the CRT filament and then making a separate power supply for the CRT filament?

bandersen · #60 09-10-2016, 12:12 PM

The 6.3 volt CRT has a higher filament resistance. If you don't bypass part of the ballast, the current through all the tubes will be slightly reduced. So slightly I doubt any difference would be noticed. In fact it will prolong the life of the tubes to run them a little on the lean side.

Although I haven't tried a 21FAP4, I picked up to of them with the intent of using them in Predictas. I don't see any reason why they wouldn't work very well. I'm, not sure about the socket rewiring.

Sure, you could power the CRT filament separately if you want.