G08 repair issue

komodo

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Donor 2019, 2021
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Working on a G08 for a friend and I'm having an issue with the image. The monitor was brought to me with a horizontal collapse. The monitor had been working when purchased at an auction site but upon delivery, the monitor displayed a vertical line. The owner had tried changing the 2n6259 transistors with no luck. He brought it to me for further analysis. I pulled the chassis and began to go over it and found one set of the 10 watt 1.5 ohm ceramic resistors had broken free from the chassis. I dug thru my stash and had exactly 2 of the necessary ceramic resistors. I replaced the broken ones and went over the rest of the chassis. All transistors checked out fine. Paddle boards test fine. These paddle boards have large ceramic resistors attached to them with mods to a pair of transistors on the deflection board which I had not seen before. The deflection board is a revision 2. All header pins have been reflowed. Replaced the smaller caps as they were original and had relatively high ESR. Nothing else appeared to be out of place. Upon power up, I had an image but not the one I suspected. I am using the Vector-labs vector test rig and had tried to get a cross hatch pattern. Instead I am getting a jumbled pattern of what appear to be random lines. I pulled the chassis and rechecked my work but I am not seeing anything out of place. I rechecked my cap work and found no solder blobs or solder bridges or torn traces. I was very careful on my work as the FAQ had been very clear of the delicate traces.

As I don't work on many of these monitors, I am wondering if the image I am seeing is a result of a bad IC600 or something else. I did not power up the monitor before rebuilding it as I had a description of the issue from the client. I would appreciate any assistance from the G08 monitor gurus. I am enclosing a photo of what I am seeing on the monitor. Please note the 4 white dots on the upper left corner of the monitor are a reflection of the ceiling fan lights in my garage. I have not tried it with a working board set yet as that would require me to drag the monitor to my game room and hook it up to my Eliminator cocktail which might prove to be a bit difficult. It is currently pouring rain here and don't want to risk slipping and falling and destroying a hard to find monitor.


IMG_2148.JPG
 
I don't use the Vector Labs test pattern generator so I'm unfamiliar with its options and settings. Is this supposed to draw a white cross hatch or an individual color like red or green? The white dot in the center along with all the white retrace lines that should be blanked leads me to believe there is something wrong with the biasing voltage going to the neck board transistors. Even if all the solder joints have been reflowed I've seen several neck boards with broken foil traces undetectable to the naked eye. I would point-to-point check everything on the neck board, including the wires that go between it and the main deflection board that carry supply voltages.

While I never rule out IC600 in any problem, I've seen really only two major failure modes. The first is where IC600 has saturated X and Y outputs and will drive a boatload of current through the rest of the deflection board even with no input video signal present. This will cause the main fuses to blow after a few seconds. The second is where the monitor will produce an image but it will be excessively bowed or slanted along the perimeter. Since IC600 does pincushion correction and there are no external adjustment for the geometry, the only solution is to replace it.

Examine the neck board closely and see if you uncover a circuit issue. I don't think you are too far off. And for the record, revision 2 is a very early issue. The hacks and bodges could have been legitimate field service kits installed 40 years ago. This is one of the major barriers to troubleshooting the G08 because you can't tell what is real and what might be from someone blindly grasping for straws out of desperation in a prior repair.
 
Thanks. The vector labs kit is supposed to display a cross hatch in this mode. Any other mode displays something similar to the image shown. This is my first time using this kit with this monitor

The weather has moved on so tomorrow I'll take the monitor inside and try a known good signal before I chase a red herring. The information you've provided is very helpful. I'll report back on what I find
 
OK, an update on the monitor. IC600 appears to be fine. I pulled the chassis again and de-soldered the wire between the deflection board and HV unit to allow me better access to the boards. Went over all of my work and rechecked all of the transistors on the deflection board. The neck board was fine. There were no pulled traces anywhere. I cleaned up some pre-historic flux on the header pins for the paddle boards, replaced the wire from the deflection board to the HV unit with something a little bit longer and reinstalled the chassis. It now works properly. I honestly don't think that anything I did "fixed" the issue, but apparently something did fix it. I have since pulled the chassis 2 more times to do further work on it and it is still working fine.

My biggest problem now is the image appears to be too large on screen. Adjusting the knobs on the deflection board helps but the image is still too large. I've hooked the monitor up to my Eliminator set, but the boards are not set properly as there are jagged vectors etc. I don't have any other working g80 boardsets right now to use as a reference and the vector test rigs I have I've been through all of the documentation and nowhere can I find the value to set the HV at. 19.5 KV is the standard for Atari monitors. The schematics show 22.5kv to the tube. Typically, a higher KV will make the image shrink. Right now, the lowest I can get the tube to is 20 KV. I don't particularly wish to glow at night so if someone can show me where the correct value is listed I would appreciate it.
I found the G08-105 manual which references 21KV so I will try that.

The last time I worked on these monitors was at least 10 years ago and I don't recall having this much of an issue with them. Again, any nudge in the right direction would be appreciated. When I get this monitor finished, I will have to pull my Eliminator monitor and go through it.
 
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