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Pre-show testing found the game working, but the picture focus drifted after an hour. Repeated focus adjustments corrected it but it failed to stay sharp, needing backwards & forwards tweak every hour. The game was side lined for the 2024 show having run out of time to investigate.
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The monitor chassis was removed and focus circuit tested on the bench with a meter. All the neck PCB focus chain fixed resistors measured OK but the focus pot itself was open on all pins. I'd never seen any of this type of focus pot for sale, a "Ruwido 50M Lin 15 074 00 3", however it could be opened & cleaned. A fiberglass pen was used to clean the oxidized track. The open connections between the corroded pin rivets and ceramic track were sanded to metal and solder bridged.
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Whilst the pot was out of circuit, I took the chance to measure it. The component was divided into three track segments since the track appeared to be one continuous run of resistive material. The "output" segment was 24.71MOhm, center pot track 19.46MOhm and "input" segment 12.75MOhm with a measured total "input" to "output" 56.76MOhm.
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Whilst working on this chassis I noticed that IC2 (TDA2593) was in an old white socket. I'd previously discovered that if IC2 or its socket fails it can activate the drive transistor to DC and burn out transformer TH2. Whilst replacing socket IC2 I also noticed that pins 1 and 2 were shorted by a small solder bridge, apparently having no effect, that was also fixed.
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Retest of the chassis back in the cabinet still found the focus drifting despite the repairs to the open focus pot.
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Suspecting a CRT issue, the BK 467 rejuvenator measured the CRT as having good gun drive and life. The specific focus test also didn't show any problems, and I now suspected that the HV bleeder was bad.
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The original bleeder had no part code markings, and the manual didn't list a usuable part code. I'd bought a selection of four potentially matching HR bleeders to test out. HR 1687 appeared to be a good physical match and was chosen to try first.
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The bleeder had two easily accessible solder connections, one for focus and one for sheath braid ground, that allowed the bleeder to be replaced without removing the chassis. On power on there were no sparks and the picture display looked OK. An adjustment of the focus pot yielded a very sharp picture, better than it had ever been before, that was stable over many hours.