HP9836C colour alignment (grey scale tracking)

Oh yes. With explanations of what's going on too ! Oh don't get me started on that. If we go back to the machine that started this thread, the HP9836C. One of the faults in mine was that it reported 'Floppy Disk Missing' in the power-on test. In other words it hadn't found the floppy controller card. Now, the normal way that add-ons are detected in these machines is by seeing if said device assets DTACK/ or whether there's a bus timout, but the DTACK/ for the floppy controller is producted on one of the other boards (basically all intenral I/O gets the DTACK/ produced there) So it wasn't that. In fact it didn[t take me long to rrealise that it detected a floppy controller iff it could do a read/write to the first byte (I think) of the 256 byte sector buffer on the card. As soon as I spotted this was implemented with 2114s [1]s, I knew what to do. I remplaced them, and put the card back in, not conencted to any drives, and tired again. A great improvement. It then reported 'Disk drive not responding' or something like that. Which made sense as there were no drives. So I cabled up drive 0 [2] and tried agqin. I then got a 'Floppy Drive Failed' error.. After much testing, putting the drive on an exerciser, looking at signals round the FDC chip, etc, I realised that being a 9836 (and not a 9826), it was expecting 2 drives. If you gave it 2 drives, it correctly reported '2 Floppy Drives'. If yoy gave it 1, you didn't get '1 Floppy Drive' you got 'Floppy Drive Failed'. And this isn't in the boardswapper guide either! [2] In this machine, the drives are jumpered to be permanently enabled, and have separate cables back to the controller board [3]. This means the controller board can continually monitor the Wprot/ line to detect when disks are changed. [3] The first version of the controller, which I've never seen 'in the flesh' had one 34 pin cable soldered to it, and could only be used with one drive. Thus this controller is used in the 9826 only. The second version has a 34 pin cable soldred to it for drive 0 (in 9826s and 9836s) and a 34 pin header plug, into which a second cable is pluged for drive 1 in the 9836. You would ahve liked the way I had the machine set up for this. I wanted to be able to work on the PCBs easily, which essentially meant not having any case parts in the way. So what I had was : The monitor on the bench, with the computer's motherboard on top of it. Monitor cable plugged into the back of the motherboard. Video boards (and brightness control cabled up in the normal way. CPU board in its normal slot. Floppy controller in its normal slot. 1M memory board in the slot for the expansion backplane (I didn't need any more slots, the pinputs are similar enough -- jsut 2 signals, not used on memory boards different -- for this to work Keyboard connected by normal ribbon cable and placed between the edge of the monitor and the top of a bench PSU (see later). PSU PCB in the normal motherboard slot, but not connected to the HP transoformer/rectifier bits. Instead I wired a spare edge connector to take power from that bench PSU to the regulator board input. [...] Actually not. It was just that the picture got ever brighter and more out of focus as the monitor warmed up. Checking the CRT pin voltages revealed the obvious (A1 going up, and I think focus coming down, but I wouldn't bet on that).Further tests showed the problems was alsmost certainly the flyback and not leakage in the CRT, replacing the flyback proved it. -tony