Zenith 25" System 3 console - vertical problems

[Adam]

It has the 9-181 main board, model SB5557GC, from 1985. I found this at a thrift store, and I was just so surprised to see a real CRT television at a thrift store that I brought it home.

But it has a lot of issues...

1. The picture was dark, too much color, tint way off, ... At first I thought the controls were bad or dirty because they only worked intermittently or not at all, but that wasn't it, the controls themselves were fine. I finally just re-soldered every joint in the board they are mounted to and cut off the clips on the wires that hook to that board and soldered them all directly on there - and that seems to have fixed it.

2. retrace lines and vertical foldover

First, I noticed those two 150k resistors looked extra-crispy (circled in the first pic - the video output schematic), I replaced them and the filter cap in that 217V source (which is not shown on that schematic) - but this didn't make much difference.

Secondly, I've just been replacing electrolytics in the vertical output circuit (look in the second pic, I circled the ones I replaced) The one I circled in green was the only one that made a difference - got rid of all the retrace lines and most of the foldover.

Now, I can adjust those two controls (with the blue arrows pointing to them in the schematic - one is vert height, the other is just labeled 95V adj) so I have either: not enough vert size, the picture folding over just a little at the bottom, or not enough horizontal width - I can't adjust it in such a way that I have neither of those problems. (The horizontal width control is maxed out) And there is no way to adjust it to get the vertical linearity quite right. In the image of the tv working, it's adjusted so there is not enough vert size, and if you look closely you can see that the v. linearity is off in such a way that the picture is scrunched together vertically at the bottom.

And if I adjust it so there is not enough horizontal width you can see that the picture is sort of wavy at the edges - kind of in the way you get with bad filter caps in the power supply, but it appears to be 'being wavy' too fast to be from 60Hz.

I was almost ready to part this out, but my son said he wanted it for his room, so I'm trying to figure it out.

[zeno]

First be sure its a "plain" 9-181 & has no dash ##.

The big main cap is common fail. Usually has puss coming out.

Dont remember any vert problems. So double check values & be sure
you got all the 'lytics. Also check the Vert B+ for bad 'lytic etc.

Control PCB is double sided & the griplets go cold, common.

The 200V filter cap a good call. At this point I would change all
on all brands. Some of the symptoms are quite subtle & can fool you.

73 Zeno
LFOD !

[liammc00]

I would do the steps that zeno said. As zeno has helped me out with good advice in the past.
Also hope you get the set working again.

[Adam]

I'm looking at sams #2472-2, which I just got by cross referencing the model number. According to a sticker inside the cabinet, it's a 9-181-10, the schematic says it's for a 9-181-11. The only difference I've noticed so far is one capacitor C2148 is labeled on the schematic as 680mfd, but the one in there was 0.68 mfd. A 680 wouldn't even fit in that spot, so I figured it was a mistake and replaced it with another 0.68.

I replaced the big main cap with two 330mfd caps, it did fix the wavy picture.

I've replaced all the electrolytics that have anything to do with vertical output, and am still left with one weird problem:

Looking at the picture with the color test pattern everything looks fine, but then look at the picture with the snow and some of the vertical trouble returns. It does the same with an all black screen.

[old_tv_nut]

Does this chassis have a customer Vertical Hold control, or is it totally automatic?
To me, that screen with no signal kind of looks like the vertical running slower than normal while looking for sync. Can't say if it's anything to worry about.

[Adam]

There is no vertical hold control, its all automatic.

I found the problem, remember I said in the last post it looked like there was a .68mfd capacitor where the schematic said there should be a 680. There's a picture where it divides the main board into a grid and then tells you which square each component is in. It said the 680 was in sections F-9 (where the 0.68 is), but it's really in section F-19. I replaced the real 680 and that fixed it.

[zeno]

Well done !

As for the 9-181 look for a little paper tag on the PCB. Dont remember the difference but plain 181's had plug in IC's & R-G-B outputs. A real PIA.
Anyhows the jug looks near mint, nice catch !


Zeno

[old_tv_nut]

Originally Posted by Adam

There is no vertical hold control, its all automatic.

I found the problem, remember I said in the last post it looked like there was a .68mfd capacitor where the schematic said there should be a 680. There's a picture where it divides the main board into a grid and then tells you which square each component is in. It said the 680 was in sections F-9 (where the 0.68 is), but it's really in section F-19. I replaced the real 680 and that fixed it.

Great! A tidbit about the Zenith sync IC: it worked by looking for the whole vertical interval pulse pattern, and was so noiseproof it could sync on a signal so weak that you couldn't recognize the picture in the snow. The first models, though, expected only FCC standard sync and had trouble with video games until a redesign was done.