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IBM 5150 Monochrome Video Card Problem: displays dashes on bottom of the screen

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PCFreek

Experienced Member
Joined
Apr 5, 2013
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322
Location
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I have a monochrome graphics card that is displaying dashed lines on the lower half of the monitor (see photos below). These dashes do not appear when I swap this graphics card with another monochrome card. The computer and monitor are obviously not the problem. I have looked at the board for obvious defects and have re-seated the ROM.

any suggestions?

sorry for the faded pics... I can't take the pic without the flash or it is badly blurred.

P1019544.jpgP1019545.jpg
 
Did you try cleaning the edge connectors on the card ? You can use a clean pencil eraser to clean the contacts
and then wipe with a clean cloth. I've seen some bizarre problems caused by dirty edge connectors.
 
mikey99 said:
Did you try cleaning the edge connectors on the card ? You can use a clean pencil eraser to clean the contacts
and then wipe with a clean cloth. I've seen some bizarre problems caused by dirty edge connectors.
Click to expand...

Thanks... just tried it an it didn't help. good tip though. It shined right up. I have also tried the card in different slots. I have seated and re-seated it several times. I've plugged and unplugged the monitor repeatedly. I've wiggled it and tapped it while it is on. Nothing changes the little dots across the bottom.

One odd observation though... the dots do not appear for the first few seconds after power on. I see the proper flashing cursor in the upper left for a few seconds and it gives me hope... then the dots appear. some of the lines of the dots kind of change brightness/intensity. Tapping and wiggling does not affect the brightness/intensity of the dots.
 
Al Hartman said:
Looks like you might have bad RAM on the card.
Click to expand...

Yes, it could be a heat related issue with a memory chip in the screen buffer. Sometimes you can isolate the bad chip with freeze spray.
You could try to put the card in the fridge for awhile to let it get very cold and then plug/power on quickly to see if it displays properly for
a longer period. Eventually it will heat up and display those bad characters, but that would let you know for sure its heat related.
Then you could continue to find the bad chip with freeze spray.
 
I tried putting it in the freezer and quickly booting the computer with it.... and it seemed to make the dots appear on the screen even faster than before, so I did it again. It definitely makes the error appear faster. The change in brightness/intensity on certain lines corresponds with the floppy drive booting during POST.

I pressed some keys on the keyboard and the characters displayed correctly until they got down to the dotted lines. the dots changed to slashes and as I continued to input keyboard characters, the gibberish filled the screen as it scrolled. see pic below. Then I tried using the down arrow to place the cursor in the gibberish on the screen. I can actually hit backspace in the gibberish area and erase the gibberish. If i go to the bottom of the screen, I can backspace the whole screen clean... odd.

P1019550.jpg
 
UPDATE: I can no longer erase gibberish with the back button. It seemed to only work when the card was cold.
 
Isn't the RAM on the MDA just a single 24-pin SRAM? You could try piggybacking a good on on the one currently installed to see if things change--if so, replace the RAM.
 
modem7 said:
The circuit diagram [here] shows eight 2114 chips, four (U12 to U15) for the character and four (U8 to U11) for the attribute.
Click to expand...

Yes, it has the eight 2114 chips across the bottom of the board. I can get two for $5 on ebay. Which would be indicative of my problem... "character" or "attribute"?
 
Character.

You could check if one of these chips responds to a 'massage'.

Update: It should be U14.

- The second half of character memory is corrupted, that leaves U12 and U14.
- Digits (0x3x) at the bottom of the BASIC screen become '>' (0x3E). So the lower nibble is at fault -> U14.
 
Last edited:
H-A-L-9000 said:
Character.

You could check if one of these chips responds to a 'massage'.

Update: It should be U14.

- The second half of character memory is corrupted, that leaves U12 and U14.
- Digits (0x3x) at the bottom of the BASIC screen become '>' (0x3E). So the lower nibble is at fault -> U14.
Click to expand...

would I be able to piggy back it for diagnostic purposes? I have piggy backed RAM on the 5150 MB with success.
 
If straight piggybacking doesn't work, you can try clipping the OE/ pin on the original to defeat its output drivers and then piggyback the replacement, jumpering its OE/ pin to the board. If it's not the problem, you can solder-bridge the cut you made. No need to unsolder the complete chip for testing.
 
H-A-L-9000 said:
You could check if one of these chips responds to a 'massage'.
Click to expand...

I repeatedly performed a "tap test" on all the RAM chips. The display didn't even flicker a bit. I have ordered a couple of new RAM chips which should be here by early next week.
 
H-A-L-9000 said:
Character.

You could check if one of these chips responds to a 'massage'.

Update: It should be U14.

- The second half of character memory is corrupted, that leaves U12 and U14.
- Digits (0x3x) at the bottom of the BASIC screen become '>' (0x3E). So the lower nibble is at fault -> U14.
Click to expand...

You are a genius!! I piggy backed U14 with a new chip and it corrected the problem. I just finished swapping out the chips and it now works perfectly... thank you so much!!!
 
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