Another Project? - Page 4 - Videokarma.org TV - Video

leadlike · #46 03-31-2010, 08:49 PM

I found an adjustment for the horizontal on the schematic (L21) and began twisting away. It functions almost as a course horiz. frequency adjustment, so the picture quickly pulled into sync a faint image of my B&K test pattern. Hooking up a dvd player revealed a scrambled mess of a picture. the difference in sources was enough that I had to readjust everything again. Now I have an image to play with!

GEDC0169.jpg

Vertical lock is pretty well nonexistent, but I am more concerned about the faintness of the image. For this tv, since all the contrast control does is ramp up the plate voltages on the IF, I will need to find another way to tighten up this image. retrace lines are everywhere, too, but the lack of vertical hold is probably contributing to that.

bandersen · #47 03-31-2010, 09:02 PM

Awesome! I had to make the same 'coarse' horizontal adjustment on my last set too.
Are you feeding the signal into the video amp tube or the antenna terminals ? If it's the video amp, perhaps the level is too low ? That could account for the lack of vertical lock too.

leadlike · #48 03-31-2010, 09:12 PM

It's being fed right into the video amp's grid. I would suspect that the piddling signal from the dvd player just isn't enough to drive a decent picture to the single video amp tube to the crt.

OTOH, the IF strip could be swamping the dvd signal. Should I pull the final video IF tube to drop this potential interference? Also, does the dual diode do anything for a signal added to the video output grid as I am doing, ie do I get dc resto?

bandersen · #49 03-31-2010, 09:34 PM

Here's a link to a partial schematic for the 802. The DC restorer comes after the video amp so it should be OK. Have you tried doing this using the B&K ?

Set the RF selector to 'IF'
Set the IF frequency to somewhere around 22 MHz
Inject the video signal at the grid of the 3rd video IF (pin 4)

Do you get a picture now ? (you may need to fiddle with the IF frequency). Is it any better ?
Likewise you can work you way back through the 2nd and 1st IF amps.

leadlike · #50 04-01-2010, 08:58 PM

I replaced some more caps and resistors, focusing on the vertical section this time. Nothing too terribly out of line, except for a mica cap (c136 from Banderson's schematic) was open. I've found a number of open micas in this set, something that I rarely find in other tvs.

I believe that the tuner has at least one open one in its signal path, hence that is why I have no fm or television reception. The service manual describes the tuner as "easily removed from the chassis for servicing" well, it took about two hours to remove the tuner, mark the dozen or so leads that needed to be pulled, and even then it appears to be physically impossible to get to those mica caps.

Anyway, back to inputing composite video to the video ouput grid. Here are some results:

GEDC0170.jpg

GEDC0171.jpg

The distortion in the first picture comes when I install the dual diode (6h6 on the schematic, but mine is a 6al5). Lots of nastiness sweeping from the left to the right.

When the 6al5 is pulled, you have the better looking result in the second picture. Vertical ALMOST wants to lock. I have a pretty good contrast balance, considering that now twisting the contrast knob just throws off the horizontal sync. I do get a bit of distortion-a pretty variable black shadowiness with jagged edges (not unlike a lesser version of what was happening with the dual diode installed) that changes size with the material on screen. I believe this may still be an issue with the horizontal lock, as the horizontal blanking bars have also been creeping in unless the horizontal is dead on.

bandersen · #51 04-01-2010, 10:00 PM

Weird that the 6AL5 would cause problems. Just curious - what are you testing the mica caps with ? I have a couple vintage capacitor testers that use a bridge and eye tube, but I don't really trust them.

leadlike · #52 04-01-2010, 10:11 PM

I have an old Heathkit cap tester. There was an online tutorial on how to fix this particular model, and I've never had any issues with it since it was recapped/repaired. Typically, when I test a cap, I test the potential new one going in, and then test the old one going out. In the case of the 240pf cap that was dead in the vertical section, it didn't register as anything on the magic eye tube.

jeyurkon · #53 04-01-2010, 11:03 PM

Does your set call for a 6AL5? I mean is it on your schematic or the tube chart? The 6AL5 and 6H6 have different pin-outs.

John

bandersen · #54 04-01-2010, 11:14 PM

Quote:

Originally Posted by leadlike

I have an old Heathkit cap tester. There was an online tutorial on how to fix this particular model, and I've never had any issues with it since it was recapped/repaired. Typically, when I test a cap, I test the potential new one going in, and then test the old one going out. In the case of the 240pf cap that was dead in the vertical section, it didn't register as anything on the magic eye tube.

Nice - I should restore mine.

Quote:

Originally Posted by jeyurkon

Does your set call for a 6AL5? I mean is it on your schematic or the tube chart? The 6AL5 and 6H6 have different pin-outs.

John

I was wondering about that too. A 6AL5 is a 7-pin miniature and a 6H6 is octal

leadlike · #55 04-02-2010, 07:42 AM

Indeed those dual diodes do have quite a different pinout. The socket install looks factory, but I'll check the pinouts to make sure the 6al5 is hooked up properly. GE made a fair amount of changes to these sets during their production run, and I could see this being one of them. However, it is on none of the documentation I have been able to find for this set.

leadlike · #56 04-02-2010, 09:57 PM

Not too much time to work on this tonight, but I did discover that the plate voltage is pretty much missing from the clipper half of the 6sn7-sitting at about .3vDC when it should be at 11vDC. The voltage is supplied by a 330k resistor, the supply voltage is 215vDC on one side of the resistor and .3 on the other. I've tested this resistor a couple of times, and it always tests at about 340k- a little high, but no reason to shut out voltage like it does. But having a freaky clipper certainly would explain the sloppy sync I am getting....

In other news, I emailed the screenshots that I have been getting of the set to the original owner, who only grew up with it in his house and never saw it running in the fifty years he has had it before giving it away. He was pretty pleased to see it working at all.

jeyurkon · #57 04-02-2010, 11:23 PM

I'd be intersted in seeing the sync clipper portion of the schematic if someone could post it.

The grid voltage should probably be about -0.5V. It's usually derived by grid leak from the sync pulse. If the sync pulse isn't getting to it the grid will be too positive and that would give you a low plate voltage.

Or you could have a problem with the 6SN7. It could have a leaky grid. But, in this sense it emits electrons, rather than collecting them from the cathode, so it becomes too positive. You need a tube tester that can check for grid leaks or a special tester like the Sencore LC-2 leak tester.

Not having seen the circuit, I might have said something stupid.

John

bandersen · #58 04-02-2010, 11:57 PM

My schematic comes from a Wallace Telaide with their ledger size pages. Does this scan cover the areas of interest ?

jeyurkon · #59 04-03-2010, 12:39 AM

Yeah, it does, thanks. It looks like C40B could also cause the problem he's seeing if it's too leaky or shorted.

John

jr_tech · #60 04-03-2010, 12:49 AM

How about C40B... perhaps leaky/shorted/installed backwards ??
Also R-28... perhaps it is way too low ?

jr

oops! it looks like John beat me to it