This problem, called the blooming effect, is when the monitor screen keeps growing after power up. It fills the screen making it seem like you have the monitor set to magnify. After complete warm up you will only be able to see the center of the image in very large size. The adjusting pots will not help in this case. It is most likely caused by a bad high voltage (HV) diode. On the Electrohome G05-802 or Wells Gardner 19V2000 monitor the problem lies with the HV diode (D904). This diode is located in the HV cage. It is in-line with the HV transformer T900 and the suction cup attached to the monitor. Make sure to install the new diodes cathode side (usually marked with a line) towards the suction cup. This diode cannot be checked with a normal diode checker.
In the Electrohome G05-801 monitor there is no D904 diode in-line with the suction cup. The 801 uses a part called VD900,. This is a voltage doubler. This part is no longer available; but thanks to some creative thinking, it has been duplicated in the form of 2 HV diodes and HV cap. See diagram below for simple set up and parts list.
Parts and repairs are available here.
Here is the rear of an Electrohome G05-801 monitor. It is commonly found in games from Atari like Lunar Lander, early Asteroids and some full size upright BZ games.
Step 1 - Remove the HV cage from the monitor.
Here is a picture of the HV cage from a G05-801 monitor with the shield removed:
Step 2 - Remove VD900
Remove the VD900 part. Make sure to cut the cable going to the VD900 as close as possible to the VD900 leaving as much cable as possible on the HV transformer and the cable going to the suction cup. Notice I removed the board from the metal surround making it easier to work with. Make sure you mark the 2 transistors located on the side of the metal case so you know which order they go back (Q903=TIP31B and Q902=MJE51T):
Step 3 - Install the 2 HV diodes and 10pf (102) 10K capacitor. The HV diode leads are very fragile and brake off easily, use caution when assembling. Here we have the 2 HV diodes and 10K cap attached to the cage board. (Addition: To avoid arching from the underside of the board to the chassis, first build a base on the board with silicone Goop before assembling the parts. Make sure to cover up 2 of the 3 small holes on the board with Goop.)
Step 4 - Seal the parts with silicone
In this step get some silicone sealing called "Goop" for automotive application (available in most hardware stores) or something similar, here is their link: "Goop". Apply about 4 layers to cover the parts you just installed completely plus the base you build earlier in step #3. You need to wait about a day between each layer to allow for the silicone to harden completely. If you don't allow enough time for drying between layers you won't get a good seal from the HV that will pass through these parts and you may have problem with arcing after power up. If you do get some arching turn off power, wait longer for your silicone to dry and/or add more silicone. You may also want to check your 90V by hooking up your positive lead of your volt meter to the D903 cathode (white line) and your black lead from your meter to ground (the metal case works fine). Adjust R912 to 90VDC:
Here is a diagram of the parts added to replace the VD900:
Warning - The above procedure involves high voltage and should be approached with caution.