I recently posted this on another Gorf thread so I just pasted it here since it applies.
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It could be a flaky pot on the neckboard, specifically one of the two associated with blue. The black level pot for blue might have failed.
I once had a Gorf monitor that had no red. The monitor worked okay in another machine. On one of the Gorf boards, the final drive transistor (Q7) that controlled red was shorted. After replacing it, the red would not shut off completely so it generated a faint red raster covering the entire screen. In this case, the pull-up resistor (R68, 100R) attached to the same transistor had burned open, likely due to the shorted transistor pulling current through it 100% of the time. Replacing both the transistor and the resistor finally solved it.
I mention this just in case the same sort of pull-up resistor in your machine is going bad and causing the continuous blue raster.
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Here is an excerpt from my repair log...
• Red missing entirely. Replaced R68 (100 ohm) resistor on game logic board. It was burned open.
• Red is now full on all the time. Traced R-Y signal from R68 to Q7 (now stuck full on) and then back to CPU board to Q7 (Motorola H69 [PNP]). Its case is broken. If it doesn't turn on, red will be full-on all the time.
• Replaced Motorola MPSH69 (PNP RF transistor TO-92) with MPSA06. Result was bad. Red now works but transistor is too slow:
current-gain bandwidth products:
MPSH69: 2000 MHz
MPSA06: 100 MHz
Video is badly smeared on "astro battles" blue screen. Tek scope showed B-Y signal as very square shaped pulses. The R-Y signal was very rounded, obviously the transistor is too slow.
Fairchild 2N5771 not fast enough, only 700 MHz.
Fairchild MPSH81 not fast enough, only 600 MHz.
• Ordered MPSH81 from Newark just to try it anyway.
• Installed new MPSH81 transistor. Had to bend leads to swap B & E (part is BEC, board is EBC).
• New transistor works fine. Red is fixed. No smearing is visible. I would still like to replace it with MPSH69 to because it has the proper pin order.
• Found a source for MPSH69 at:
http://www.goldmine-elec-products.com/prodinfo.asp?number=G43211
• Installed MPSH69, works fine.
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The main board set produces two video signals, a B-Y and a R-Y. The little video converter board takes these two signals and outputs the familiar R,G,B. Since my red color was the problem, I probed the R-Y signal first and it was not active so I traced it back into the main boardset. It your case, try scoping the B-Y signal on the converter board at pin 7 of the 12-pin header. If it is active, then the amplifier chip on the converter board may be the problem. If it is not active, the problem is in the board set. Since your problem is blue, it'll be the B-Y signal. That leads all the way back to the "CPU" board transistor Q6 which is driven by chip U15. From there, B-Y goes to the "game" board and through transistor Q8 with associated resistors R70 (2.2k pulls up base to +12V) and R71 (100R pulls up collector to +12V) and R69 (330R pulls emitter down to -5V). From here, B-Y goes to the little converter board. That covers it. I hope this helps you solve your problem.
Bill B.