Here is a really strange one about a problem with the tube itself!
Focus starts out sharp but slowly gets fuzzy and then suddenly snaps back into clear focus (repeat every 10 seconds; eventually will stay out of focus all the time).
I didn't see the problem until I extended the wiring harness out of the back of the machine so the monitor was sitting on a stand, because there WAS arcing occurring, but it was inside the picture tube socket. There's a white, cubic lump attached to the end of the neck (where the neck board attaches) where all the pins are and I could see a faint glow out of it when the monitor freaked. The focus line from the HV power supply connects to the pin nearest the cube. Inside of it, a sort of spark gap is implemented by running the focus wire under and in contact with a small metal plate (the wire eventually connects to the socket pin). A second formed wire is suspended above the metal plate with the point of a V bend in the wire forming a spark gap with the metal plate; this formed wire is connected to chassis ground. Its purpose is to form a spark gap with certain pins in the socket; the intent is to prevent damage to the gun of the CRT during power down. In my case, the focus wire had lost good contact with the socket pin, and some internal arcing of some sort appeared to have released the metal plate from where the plastic housing was supposed to retain it, causing the spark gap to become too small. So it appears one problem led to another. The connection to the focus pin of the picture tube socket had become intermittent. Some arcing caused the built-in spark gap to come apart. A similar problem with the same symptoms can develop causing a dead short to ground of the focus electrode. Within the socket, there is a plate in the back of the socket which is grounded. If any moisture or conductive crud gets in there, you can get a low resistance path between the grounded plate and the focus pin. To fix it, you have to desolder the socket and break it open. The socket simply pries apart but it was originally staked, so don't be alarmed if you see small pieces of plastic flying off when you pry the sucker open. I found it went together tightly enough that I wasn't concerned about it falling apart but you could use a touch of glue just to be sure. I soldered the metal piece to the focus wire, placed it back in its retaining channel, cleaned out the gap area and soldered the socket back in place. The part that comes off the bottom of the socket to gain access to the internals touches the PC board when the socket is installed, and is retained just fine when the socket is in place without gluing.
All the above is assuming you just plug the game in and it doesn't work. If you happen to be playing the game at the time it fails, you have a little more information to go on. If you hear a loud bang like a firecracker, then check the capacitors on the HV board first because they can be loud when they burst. If you see a little bit of smoke inside the cabinet and smell a hint of "electrical smell", then check the fuses first. If you see a ton of smoke inside the cabinet, then check the large transistors on the chassis first. If you start losing quadrants of your screen intermittently than I would advise replacing the corresponding transistor before it fails because it can take other components (usually fuses) with it when it goes altogether. The same advice goes for the blooming caused by ZD902.
Now some things besides the monitor itself. Tempest is harder on this monitor than the other Atari vector games. The attract mode that displays "TEMPEST" (often burned right into the phosphors) really stresses the monitor. For Tempest, I like to do what Atari did when they offered the Major Havoc conversion. Add a fan to the back door of the game. I try to find a small cooling fan that just moves a small amount of air (not one that howls). You have to cut a hole in the back door and position it so it directs air at the deflection board. I usually connect the power for the fan to the wires that head up to the fluorescent light and put a connector in so that you can still remove the back door (with the fan mounted on it) without it hanging on the wires to the fan.
If you would like to greatly reduce your chances of experiencing a monitor failure in your Tempest machine, you may want to "shrink" the screen in both axes using the X and Y "SIZE" pots on the game board. I also have heard Tempest was bad, because it draws a diagonal retrace line (Z/brightness off) from the bottom left corner to top right corner during the attract mode. There was supposed to be a ROM fix, which took this line out and really helped the transistors but I've never seen it. I could probably buy it, as it was supposed to have been pushed for by a local operator.
All of this has assumed that you had a good logic board in the game and the monitor was receiving the vector info. If the spot killer stays on and the monitor seems OK verify the presence of the X and Y signals by measuring between ground and pin 7 of the big white connector for the X signal and pin 8 for the Y signal. This is an AC signal and if either is missing the spot killer circuit is just doing it's job and saving the tube's phosphors.
You might also want to check that you don't have static analog data (voltage) saturating your transistors. To check this, set your multi-meter on the DC voltage scale and measure the voltage on the lead of R600 and R700 which are connected to the yellow and orange wires, respectively. The correct voltage should average out to about 0 volts DC but will fluctuate (both positive and negative) because of the changing deflection signal. If the measured voltage is constant (between 5 and 15 volts positive or negative DC) then check the X and Y amplifiers on the game board.
Now that you have a working monitor (and game), it is time to fine tune your picture. Before making any adjustments, turn your game on and let it warm up for at least 10 minutes. R918 is the HV adjust and my advice is that unless you have a HV probe, don't mess with it. If you have a probe, set the HV for 19.5 kV with zero beam current (i.e. with the game boards disconnected from the machine so that no input signals are being supplied).
Some HV supplies (P329) have a circuit called the HV over-voltage protection circuit. It monitors the voltage of the focus assembly in the secondary circuit of T900. If the high-voltage at the anode of the picture tube increases beyond the threshold set by H.V. TRIP adjustment R930, this circuit shuts off the timer of IC901 and LED D903 turns on to indicate the over-voltage condition. Presumably, this was added to help discourage Tempest from eating HV transformers as it is prone to do since they are by far the most expensive component in the game. If you know your HV is good, adjust R930 so that it is just beyond the point where LED D903 lights (so that the LED is off).
NOTE: The following adjustments are a 2 person job (unless you are very handy with mirrors) because it is nearly impossible to view the screen and turn the pots at the same time. When adjusting your monitor, all adjustments should be made in the order as presented below as many of the adjustments have effects on other qualities and will cause you to have to go back and redo them if you change the order.
On the outside of the HV assembly are focus and brightness adjustments. Adjust the focus until the picture is sharp then adjust the brightness just under the point where the dot in the center starts to show or just under the point where you can see the connecting lines between objects. Be careful to not get carried away with the brightness as you can do permanent damage to the phosphors.
The manuals don't talk much about adjusting the positioning and size of your screen but that is easy to do. There are small potentiometers on the game board that are clearly labeled which control X and Y centering as well as X and Y size.
The following adjustments refer to the 8 tiny blue and/or black potentiometers found on the large game board towards the back on a Tempest machine. If your lines are not meeting at the "joints" properly or your text and numbers aren't lined up properly on the statistics screen then you need to make these adjustments. This information is directly from the Tempest schematic diagram supplements (sheet 2, side B, 3rd printing) so locations/numbering/labeling of the pots may be different for other vector games but the instructions should be relatively portable...
[NOTE: Atari did copyright these documents and they are clearly marked with a copyright symbol.]
[NOTE: All of the following controls exist in the Vector Generator section of the game PCB; THEY ARE NOT IN THE MONITOR!]
+------+ +------+ +------+ |/\/\/\| | | | | | | |\/\/\/| | | | | | | |/\/\/\| | | | | | | |\/\/\/| |--- | |--+---| |/\/\/\| | -----| | | | |01..YZ| | | | | | | |\/\/\/| | | | | | | +------+ +------+ +------+ screen I screen J screen K
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