Funky 540 Video board on 4P

[nama]

I am unfortunately having an issue with my 4P. It was working fine about a month ago, but today when I went to power it up to test some disks the screen was quite funky. It's like a single line of random characters (although it seems the random characters are the same on each power up) with the screen scrolling super fast vertically, therefore it seems like the screen is full of the same line of characters repeating. Thats the best way I can describe it.

Doing a reset produces a blank screen. Through swapping boards around with my 2P I have narrowed the cause down to the 540 video board.

I now have my two machines, the 2P and the 4P, setup identical with no other cards installed except the 540 video cards, so I can cross check between the two. The 4P video card of course has the colour circuitry installed which makes it a little different from the 540 in the 2P.

So far I have checked the main clock and it seems to be pulsing normally. The bad 540 board produces the same symptoms when installed in both the 2P and 4P, so I think this can eliminate any power supply issues.

I have a multimeter, logic probe and an O-scope to test with. Hoping that someone could point me to the right area of the board for testing.

Cheers
Philip

[dave]

Hi Phil,

From the picture, you have the same line over and over again. However, the column counters seem to be fine, as do the row counters as far as the 8 rows of each character. That should prompt a look at the row counter chain farther down the line; perhaps U5H at the end of the the row divider chain, or maybe even (but less likely) a bad pin 11 or pin 15 (carry out) of U5F (although good row signals are coming from that chip.). Since the vertical sync will depend on the lowest frequency row counter bits, that could also explain the apparent vertical instability. I'm betting the RAM chips are good, and probably the multiplexers as well. You know the oscillator is good, and so is the chargen and video output circuit.

Let us know what you find.

Dave

[nama]

Hi Dave,
Thanks for the insight.
I checked U5H and U5F. Using a logic probe U5F seems to be functioning correctly and I cross checked it with the working board, and it seems consistent.

However, as you predicted, U5H seems a bit strange. Basically all the output lines (pins 11, 12, 13, and 14) seem to be stuck high. I checked this with the working board and they are certainly supposed to be pulsing.

U5H from working board
(pin 1 top left. 5=5v. G=GND. HL= high low pulse. L=low pulse)

5 ____ 5
HL ___ L
G ____ HL
G ____ HL
G ____ HL
G ____ HL
HL ___ L
G ____ 5

U5H from broken board:
(pin 1 top left. 5=5v. G=GND. HL= high low pulse. L=low pulse)

5 ____ 5
HL ___ L
G ____ 5
G ____ 5
G ____ 5
G ____ 5
HL ___ L
G ____ 5

Dave, did you find the culprit IC on your first try?

Phil

[nama]

Dave...you are totally rock and roll!
I decided the evidence was fairly well weighted towards the U5H being dead, so I went ahead and changed out the 74LS163 as I had some spare in my parts bin. Dropped in a nice machined socket first of course. Now the machine seems to be working 100%.

Interestingly enough, when I first fixed this machine about a year ago there was another 74LS163 I had to replace on the video board to get it working. I guess these 74LS163's are prone to dying.

Thank you very much for your time. Now I can get back into figuring out my floppy drive issue.

Cheers

Phil