Predicta Holiday - First power up attempt - Page 6 - Videokarma.org TV - Video

madlabs · #76 09-19-2016, 10:06 AM

Good idea, I will. Should have when I was putting it together but I was too anxious to see if it worked. And some RTV on top sounds about right. Off to work for my 48 hour shift so no work on the set for a couple of days.

Many questions about improving the picture.

It takes at least 5-10 minutes before the picture stabilizes. The horizontal hold is fussy and the picture shifts around. Once warm the horiz. is fine and the shifts are much less but still there.

Even when warm there are faint diagonal lines that are mostly apparent when there is nothing on the screen.

The bottom of the screen is not filled. I'm sure that is just adjusting the picture correctly.

The left side is a little compressed ad the whole screen seems a little blurry.

And early ideas on those issues? I'll get a test pattern downloaded and a screen shot taken of that later this week. I was also planning on subbing tubes one by one and seeing what I get there.

Progress! Talk about a roller coaster ride yesterday. The thrill of victory and the agony of defeat!

Jonathan

Electronic M · #77 09-19-2016, 11:06 AM

Those diagonal lines are called Retrace Lines. They can be caused by two things 1.) driving the electron gun to the circuitry's limit (which is usually done with a weak CRT to make it look better), or 2.) blanking. Blanking is a circuit that feeds pulses from the vertical to the video amp/output or one of the elements of the electron gun...To prevent the gun from writing in the time it takes after the end of the last field for the beam to return to the start point...when the beam does write it creates retrace lines.

Re bottom of screen if you did a full recap, but did not touch the vert size/height and vert linearity controls then try those first...You want a test pattern with evenly spaced horizontal bars. Adjust the set with that pattern so that the space between bars on the sets is even and fills the screen...You may also want/need to adjust the centering rings while doing this.

As for the side being compressed look for horizontal size/lin controls and see if that helps...If minor it could be natural for this set. Some non-linearitys if minor can or should be swept (pun coincidental) under the rug....If the set has a lot of overscan, and the picture is not centered such that more of the bad side is shown changing the centering to move the non-linearity off the screen may be worth doing.

madlabs · #78 09-22-2016, 11:57 AM

Back home from work. I should be able to get some time in on the set today.

Tom, when you say driving the electron gun to its limits, do you mean having the brightness cranked or upping the CRT filament voltage? Right now I'm running the brightness at 3/4 and the CRT at normal voltage. I'll start looking into the blanking circuit, any troubleshooting tips on that?

Today I will let the set get properly warmed up and try to adjust the picture with the controls to see how good I can get it. Then I want to start digging into why there is no audio. On non hot chassis set I would feed an audio signal into the input of the circuit. With the set on an isolation transformer and the sig. gen. NOT on the iso transformer, can I do it that way?

I have been running the CRT filament from a separate power supply, a 12VAC transformer/rectifier/LM317 setup. This has been handy, it helped wake up the CRT by running the filament at 3V for a few minutes. I'd like to milk what I can out of this CRT and being able to nudge the filament voltage should help. And when I do install my replacement tube I'll be able to easily up the voltage to 6.3V without disturbing the series filament current. I was wondering if I could tap into the series filament string at some point and pick off 6-12VAC so I could ditch the transformer?

Thanks for all the help folks!

Jonathan

bandersen · #79 09-22-2016, 12:08 PM

The blanking circuit picks a signal off the vertical output, runs it through the K1 couplate applies it to the CRT grid. So it could be the couplate is bad or the CRT is so far gone, the blanking signal isn't effective.

Yes, you hook up the sig gen that way. Only the TV get's plugged into the isolation transformer. Nothing else!

Wire the new CRT filament exactly as the old one was. You don't need to change anything. The resistance in the tube filaments vary with the current running through them. Sort of self regulating. Just try it and you'll see what I mean. The new CRT will have about 6.3 VAC across it without any changes needed.

Electronic M · #80 09-22-2016, 03:41 PM

The limit varies from chassis design to chassis design. Some sets it is over %50 brightness and or contrast, on others it is outside control range. ...On sets where it is not in control range normally, if it dips into range it means the CRT is weak....I've yet to operate a predicta of that era so I can't give you a concrete target.

madlabs · #81 09-29-2016, 01:35 PM

Well, last week I suddenly stopped getting video. I suspected that it might be the RF modulator I was using so I went and got a small test set to try it on and wound up with four of them. And while the test TV's all had their own issues I could see the same video issue and no sound from all of them, so the modulator is not working correctly. So this morning I finally bought a Blonder Tongue AM60-550A. So when that arrives I can get back to work on the Predicta.

Jonathan

madlabs · #82 10-02-2016, 08:29 AM

So, it turns out I didn't entirely lose video as I thought. Maybe a loose connection or some sort of operator error. I got an Eico 633 CRT tester/rejuvenator yesterday and hooked it up to my CRT. Some of the leakage and continuity tests were on the edge and the quality test was well into the red and the life test caused a ~50% drop. So, I did the first boost procedure and retested and whaddya know, now on the high end of green and the life test drops it less than 5%! Wowsers. And no, I don't expect it to last forever.

Turned the set on and the pic was indeed brighter! However, everything was color inverted, black was white and white was black. I'm guessing that is my dying modulator, which is indeed having issues. Are there any other causes of color inversion?

Another interesting thing: After rejuvenation, the picture now fills the screen. Any ideas why that would be?

Thanks!

Jonathan

Electronic M · #83 10-02-2016, 11:17 AM

There are several things that can cause inverted luminance....Setting contrast and or AGC too high is one of them. Try minimizing contrast is that still does not fix it note the AGC setting and then try backing it down till the video un-inverts...If you loose signal before then, it indicates there are other problems and in that case it may be worth wile to return AGC to it's previous setting. Personally I would avoid monkeying with it until you have a decent modulator set up.

madlabs · #84 10-02-2016, 01:17 PM

Agreed. I'm not monkeying with the set until I get the new mod and then try it on one of my tester TV's first. So instead I'm playing with an Eico 147A signal tracer I picked up yesterday. Got it working, so if my audio issue continues after I get the new mod (which I doubt) I'll have tube test gear to track down the problem! I'm using a Heathkit audio generator IG-18 to drive the tracer with. Fun stuff.

madlabs · #85 10-07-2016, 09:00 AM

OK, I think it's time for a recap on this thread.

I have replaced all the resistors and paper caps on the main PCB. The couplates were replaced with ones made by Bob. Under the chassis I replaced all the paper caps and most of the resistors, I left a few that tested good. K7 is original. The IF and tuner sections have not been touched. My 21EAP4 was/is weak, zapped it with the rejuvenator and it tests well into the green now and passes the life test easily.

Got the new modulator. I'm glad I had the test TV to get it set up correctly with, it took a few minutes to get the sound set right and so on. Hooked it up to the Predicta and I was getting a very unstable picture and no sound still. Since I'm more comfortable with audio, I started trouble shooting that and soon discovered that the 12CA5 looked ugly, I'm surprised I hadn't noticed that already. Checked voltages and the 140V supply was way low. Replaced tube and voltage came up but still no sound. Injecting an audio signal into the 12CA5 did produce a tone. Started working my way back and found a broken wire on the volume pot. Fixed that and still no sound. Disappointed, I was fiddling around with the fine tuning and what do you know, I got sound! The picture is a little unstable, but when it is locked in it is pretty good.

Remaining issues:

I have to tune for not quite the best picture to get sound. Best picture = no sound. The set tunes with the tuning control very close to one end. I see there is an adjustment pot next to the DIP switches on the modulator. Maybe tweak that to get the channel tuning centered? I found a manual for the modulator when at work and can't seem to find it now. I am feeding the signal to the set with coax and a balun, not transmitting and antenna.

When the picture has a lot of white, the sound gets way more buzz.

Horizontal control is unstable. It will lock for a while and then roll, sometimes re-locking, sometimes not. When rolling I get diagonal re-trace lines, sometimes across the entire screen, sometimes just in sections. It's easier to lock the picture when I have it tuned for best picture but then I get no sound.

12CA5 looks like it runs hot to me, it's lit up like a Christmas tree. And that is the socket that had to be replaced and the components around it look like they have been baked.

So many thanks for all of the help! Rio and I watched the set for about 3 hours last night and while the rolling was annoying it was still so much fun to watch. And when the picture is locked in it is more than acceptable. Once I get the dang thing to lock it needs some linearity adjustment, things are a little tall and skinny but not too bad. Really great to see this project start to come together!

Jonathan

Electronic M · #86 10-07-2016, 09:41 AM

On the modulator, sound buzz on white is a symptom of video over-modulation. Feed the modulator a mostly white buzzy image then turn the video level control down till the buzz disappears or is minimized while maintaining decent video.

By the 12CA5 'lit up like a Christmas tree' is it orange glow from the heater perhaps being ran hard, is it blue/purple glow from the tube being gassy, or the plate glowing red/orange? The first may be a problem but is not as bad as the latter 2. If gassy get another tube. If red-plating you have a bias issue that should be corrected immediately....If you run a red-plating tube for more than a minute you may permanently damage/ruin the tube. Deal with this first if it is the latter 2 issues.

Lastly you probably have minor audio alignment drift. Get a set of plastic alignment tools set the fine tune somewhere between best picture and best sound (maintain some sound to know if the adjuster is improving it's level or reducing it), and adjust the sound IF for best sound, walk the fine tuning towards best picture and repeat. Note all adjuster settings and how much you change them so you can return them to original if no change occurs or the problem gets worse. Do NOT touch the video IF adjusters (those require lots of calibrated equipment, skill, and practice to adjust correctly).

You may want to look at voltages in the synch separator and horizontal AFC/osc sections too...Can you flop the horizontal out of synch on either end of the sweet spot? If not (assuming the hold control is a pot) you may want to center the pot and adjust the osc transformer to synch with the pot centered, then re-check.

bandersen · #87 10-07-2016, 01:54 PM

The modulator adjust is likely for the modulation or output level, not the carrier frequency. There is a slug in the tuner for each channel that can be reached from the front of the tuner. Set the fine tuning in the center of it's rotation and tweak that stations oscillator trimmer. Adjust for best picture then go through the audio alignment procedure outline in the service info. It's all done by ear and pretty simple to do.

madlabs · #88 10-10-2016, 11:06 AM

Hi All,

Well, not much time to work on the set this week. I did get the audio tuned in better, but it is still weak. A9 makes a little difference and A10 does nothing. A11-12 is blocked by something inside the can, I'll have to look into that.

Fiddling with the tuner adjustment to center the fine tuning knob does very little. As do the vertical and horizontal linearity adjustments. A test pattern looks really bad and adjustments seem to make very little difference. On the good side, putting the HV cage back on and swapping the sync seperator tube seems to improve the roll and retrace lines a little.

Off for my 48 hour shift, should have a little time to play with the set this week.

JOnathan

bandersen · #89 10-10-2016, 12:16 PM

The tuner adjustment should have a profound effect as it changes the local oscillator frequency. There are twelve of them - make sure you're adjusting the correct one for the selected channel. Vertical linearity control should also have a large effect. There is no horizontal linearity control or do you mean the "T" bar magnet inside the CRT housing ?

madlabs · #90 10-17-2016, 11:40 AM

Hi All,

Whew! Busy fricken' week here! Not much time in on the Predicta this week but I do have 40 gallons of apple cider fermenting!

The thing blocking the A11-12 audio adjustments turned out to be part of the slug is broken off. Crap. What are the chances of a repair? Or should I just start looking for a replacement? I'm still having trouble getting good sound with no 60 Hz hum. The buzz control doesn't affect the hum at all. It is driving me crazy and is the biggest thing preventing watching the set being fun. Maybe C34? Or some other ceramic cap?

On the upper 1/4 to 1/2 of the screen is (sometimes) a whiter area. It moves with contrast/brightness changes in whatever you are watching. If the scene is darker it disappears.

The vertical linearity control does nothing. Neither do the vert centering or horiz centering up on the CRT.

And a bonus non-electronic question: Ordered the Novus polish. Do y'all hand polish or use a small buffing wheel? I may have to break out some sandpaper for some scratches.

Thanks and sorry for it being so long since I updated the thread.

Jonathan

EDIT: Forgot to add that cleaning the tube sockets and pins in the tuner seems to have resolved the horizontal hold issue, it is locked in quite well now. Is it likely cleaning the sockets was the fix?