QX10 graphics board

OK, here's the explanation (and why I think it was a man-made fault). I bought a pair of QX10s at a radio rally, one working, the other for spares. Of course I had to find out what was wrong with the second one... The problem, as I mentioned was that the display was corrupted after a screen scroll.. And the fault was clearly on the graphics board. So I looked at that. It contains a 7220 graphics processor, 16 DRAMs, a bit of TTL, and a gate array (early versions had more TTL + charcter generator EPROM in place of this, alas mine are the later version) Now the DRAMs can eitehr be 4116s (giving 16K*16 video RAM) or 4164s (giving 64K*16). There are some links on the board that you set depending on which RAMs are installed, 3 of them select the signals on the 3 pins that are different between 4116s and 4164s : Pin 1 is either -5V or not connected; Pin 8 is either +12V or +5V, and pin 9 is either +5V or an address line. They were all set correctly. And the board basically worked. The display at boot-up was sane. Suspecting a RAM problem, I put in a complete new set of 4164s -- no change. And I tried the other socketed parts -- the 7220 and the gate array, in the working machine -- they were fine too. And all the TTL checked out. Oh yes, the power lines were fine too. After a lot of thinking, I spotted it. Pin 9 on the DRAMs was a power line on 4116s, and had decoupling capacitors on it. 8 (or maybe 16) 0.1uF disk ceramics to ground. Needless to say, when 4164s were fitted, and it bacame an address line, the poor little TTL chip that was driving it hadn't a hope of switching it fast enough with that sort of capacitive load. At boot up, this extra address line was always low (I think, anyway, it didn't have to switch), so everything was fine, after scrolling, it had to switch during the memory cycle, couldn't manage it, and thus the memory was not addressed correctly After removing the 'extra' capacitors, it worked fine. Be warned that these were not shown in the technical manual I have (which is why it took me so long to realise what was going on). My guess is that originally the board had been fitted with 4116s. Either one had failed, or the owner had wanted more video memory, and had done the conversion to 4164s, but hadn't removed the caps. The board behaved oddly, the machine was offloaded to me at the rally... -tony