HP9836C colour alignment (grey scale tracking)

[...] I forgeot to expand that footnote, so here it is : [1] The PCB has overlapping 16 and 18 pin IC positiones, for either 2112 (256*4) or 2114 (1K*4) SRAMs. In the latter case 2 of the address lines are grounded, so only 256 locations are used. Presumably HP fitted whichever chips where cheaper/more avaialble at the time (I've seen boards with 2112s, I've seen them with 2114s. I've never seen one with one RAM of each type, but there's no logical reason why this won't work). Incidentally, this PCB is unusual (for HP) in that the silkscreen has, next to each IC positionm the last 4 digits of the HP part number _and_ the standard number (like 'LS164'). [...] I also didn't mention that I had problems with the brightness control. It's quite simple, just a 500 Ohm (IIRC) pot with a thumbwheel glued on the spindle, which can be turned through a slot in the bottom of the case. The pot is mounted on a little metal bracket screwed to the metalwork around the vidro PCBs. The 3 connections of the pot are wired to a socket which fits onto a header plug on the graphics PCB -- why it connects there is , I assume, to make it easy to get to the plug, since all the grpahics PCB does with those 3 signals is route them to pins on a connector ot the text PCB under it, where they go into the DAC circuitry. Anyway, when I got the machine, turning that thumbwheel did nothing. It turned out the potentiomenter's spindle was very stiff to turn, and the glue holding the wheel in place had failed. So turning the wheel simply caused it to slip on the spindle. There was also minor corrosion damage to casing screws in that area. So I pulled the wheel off and remove the pot from its bracket. It's a physically small thing, a cylinder about 1/2" in diameter and 1/2" long, with tags on the face opposite to the spindle. The 'top' -- the face where the spindle came out, was held down by bent metal tabs, covered with some kinder of selant. Most of that was scraped off, the remainder removed by heating it with a soldering iron, and the tabs bent up. After removing the top, the 'wiper' was seen to consist of a carbon bock backed with a spring wire fitted into a plastic moulding on the end of the spinde. These parts were removed and the spindle slid out from the top. And yes, there was minor surace corrosion here. Cleaned up all the parts and put it back together. The pot tested correctly electrically, and turned a lot more freely. I was not happy about glue the thunbwheel back on, though. I don't trust glue. The thumbwheel consists of a disk with a tube moulded onto it, the potentiometer spindle going into this tube. The outside of the tube was slightly tapepred, so the first thing I did was machine it straight. I then made a brass collar to fit over this part of the wheel, with a radial hole tapped for a grub screw. A clearance hole for said screw was drilled i nthe tube and finally a flat milled on the spindle. That's not going to slip!. IMHO that's how it should have been made in the first place. WHy HP cut a little corner here in otherwise well-designed and well-made machine I will never know. -tony