*** M E M O R U M ***

OK, so I don't know what's causing the issue then.

Anyway, 99% there. Once the new 74LS273 appears on Tuesday hopefully all the keyboard will work. I'm not sure about this one as it came off the machine I fixed a couple of years ago and seemed iffy, so I swapped the keyboards over. Anyway, we'll see.

So, I removed the original chip, fitted a socket and modified a 4164... IMG_1456.jpg Success! However, it's not reliable. The machine starts up with either a blank screen or corrupted graphics on screen a lot of the time. It's probably due to the 4164 being 150ns RAM and the ITT stuff being a heck o...

Dave wrote: ↑11 May 2019 21:41 Oh, you mean just like this then ?

http://www.primrosebank.net/computers/m ... mavram.htm

Oh! Yes

Hmmm... I've just seen a neat hack to use 4164 chips in place of 4116 chips...

http://atariage.com/forums/topic/257923 ... liability/

Basically cut off pin 1, bend pin 8 over the top of the chip and solder it to pin 9.

As the system gets warm some of the memory bits come back to life. Using a freeze spray on 4G returns it to the worse state, which suggests that it's this chip which is knackered.

With the VRAM fault it almost looks like it's in Italian.

Getting somewhere at last!!!! Part of the PAL board problem was due to the wire on the back of the video BNC connector coming off and I didn't notice. Doh! So now I have at least some video output. :-) Secondly, after swapping the LS273s over, with the PAL and diag ROM installed I get... IMG_1453.jp...

I think my next step this weekend, pending the arrival of replacement LS273 chips is to swap 3A and 5A and put the run with the PAL and see if that fixes the ROM select line issue. I don't need the keyboard at the moment so it's no a major problem if it's not working. As for the PAL board, if needs ...

It definitely looks like it's a problem with 5A as the input to the flip-flop is going up and down nicely. With the PAL in circuit the line is floating, which is probably why I was getting intermittent ROM selects randomly. The GAL pulls the line high. If I bridge pins 8 and 9 on 5A I get ROM select...

If Pin 13 is constantly high, the ROMs can never be enabled. It’s controlled by the Page Port (0) (The pin name means ROM enabled when low, CPM [RAM mode] when high) Ah. OK, so maybe it's the 74LS273 then. Only of the very few chips I've (a) not replaced and, (b) not got in stock. Time for an order...