View Full Version : 69 Magnavox can't keep the white point stable


sparks_elec
03-28-2016, 01:27 AM
An example would be if there's green grass in the background it makes everything red and blue tinged. Same for the other colors, almost like something is unstable. Luminance is okay and the picture tube comes up fast from a cold start. Does have poor DC restoration. It also has a bright purple trailing edge (inches long) on anything bright, like any text.

None of this changes with brightness setting. I have the sams and did a grayscale setup, but it didn't effect it. I heard this is a low end set, but for a 25" TV, should it be not much better than a portacolor?

Here's the chorma section

http://i1153.photobucket.com/albums/p517/ygufty/chromacircuit_zps2c529da1.jpg

zeno
03-28-2016, 09:12 AM
Start by turning the color all the way down. Make it as
good a B&W set as you can & watch it for changes.
If it stays good the video is OK. Streaking is usually CRT
but can be in the video. Also if it has a horz blanking tube
try that. It will cause odd problems on old maggies &
is often missed.
Once you have a nice B&W turn up the color & work on
that if needed. Watch the plate resistors on the 6GY6's
they drift.
BTW dont trust a tube checker. Sub tubes with NOS
brand name tubes.

73 Zeno:smoke:

old_tv_nut
03-28-2016, 10:53 PM
The R-Y and B-Y high level demods (6GY6) are DC coupled to the red and blue grids with 470k ohms across the .047 caps. The G-Y amp (6GH8A) has no DC coupling resistor. This would cause the picture background to drift to the opposite of a predominantly green or magenta picture content as you describe. I wonder if this was a deliberate attempt to avoid the long term background drift that is a problem with DC coupled (but not DC restored) demods? You could try putting a 470 K across the .047 that goes to the green grid. Then you will probably have to readjust the tracking, and also may see it change as the set warms up, or at least when you replace tubes.

sparks_elec
03-30-2016, 12:22 AM
I got it a little better, but still has the same issues. I think I know why, just about every resistor is 20 to 30 percent too high. This has to trow off most of the circuits and adjustments. I changed the 6GY6 plate resistors awhile back, they were almost twice the resistance, actually I think one was open. This seems strange as this is a low hour set, it had all the original tubes and the picture tube comes up fast. I tried a resistor across the .047 cap, it throws everything off way too much. Even with a 2M resistor I got 120V green grid, 100V blue, 60V red, so I'll do without that. Normally all the grids are around 60V, the cathodes around 230V, the screens all around 650V.

It has a decent picture, wish I was capable of doing a convergence setup as it's way off on one side. I also noticed the 22 inch crt in the schematic, it seems possible this chassis has a hard time keeping up with a 25" crt. I showed it to a few people who have seen tube TV and they said it has a good picture. I have to remember this was when color was new thing and not perfected. I may be expecting more as I grew up in the late 80's early 90's when they had perfected and refined the circuits.

I do notice it handles older TV shows fine. It has trouble current TV, loaded with flashy graphics and fine text, basically anything that didn't exist when the TV was built. I still find it amazing how they built these things like this, with all the adjustments (65 to 75 total, I counted), and the off tolerance resistors, and tubes, but yet it still produces a decent picture.


I wish I could get my hands on a bit higher end model tube TV, but there are almost none here. Where I live, people consider a vintage TV anything that isn't a flatscreen. The local thrift shop was selling early 2000 CRT TVs for 10 bucks, some digital ready, all working pretty good and it took them a month to move them. Can't sell a working TV for the price of a pizza? I don't get it. Around here, if it's not an LCD nobody wants it.

I'm thinking this TV is working about as it should so I'll leave it at that.

Thanks everyone.

consoleguy67
03-30-2016, 12:26 AM
Please post some pictures when you get a chance.

dieseljeep
03-30-2016, 11:31 AM
I got it a little better, but still has the same issues. I think I know why, just about every resistor is 20 to 30 percent too high. This has to trow off most of the circuits and adjustments. I changed the 6GY6 plate resistors awhile back, they were almost twice the resistance, actually I think one was open. This seems strange as this is a low hour set, it had all the original tubes and the picture tube comes up fast. I tried a resistor across the .047 cap, it throws everything off way too much. Even with a 2M resistor I got 120V green grid, 100V blue, 60V red, so I'll do without that. Normally all the grids are around 60V, the cathodes around 230V, the screens all around 650V.

It has a decent picture, wish I was capable of doing a convergence setup as it's way off on one side. I also noticed the 22 inch crt in the schematic, it seems possible this chassis has a hard time keeping up with a 25" crt. I showed it to a few people who have seen tube TV and they said it has a good picture. I have to remember this was when color was new thing and not perfected. I may be expecting more as I grew up in the late 80's early 90's when they had perfected and refined the circuits.

I do notice it handles older TV shows fine. It has trouble current TV, loaded with flashy graphics and fine text, basically anything that didn't exist when the TV was built. I still find it amazing how they built these things like this, with all the adjustments (65 to 75 total, I counted), and the off tolerance resistors, and tubes, but yet it still produces a decent picture.


I wish I could get my hands on a bit higher end model tube TV, but there are almost none here. Where I live, people consider a vintage TV anything that isn't a flatscreen. The local thrift shop was selling early 2000 CRT TVs for 10 bucks, some digital ready, all working pretty good and it took them a month to move them. Can't sell a working TV for the price of a pizza? I don't get it. Around here, if it's not an LCD nobody wants it.

I'm thinking this TV is working about as it should so I'll leave it at that.

Thanks everyone.
I don't claim to be a big fan of Magnavox and that was one of their poorest excuses. That one and some of their hybrid portables. :thumbsdn: