HP-85 Display Error on startup

[hogelmann]

Hello,

I recently bought a HP-85 and found the problem, that the display is showing "µ" characters in every second row (located in the middle of the display) on every startup or reset.

Selftest is running without an Error 23 occurrrence.

Programms are running properly but when I override the µ-characters on the display, a "W" (for example) is changed into a "V". Not every character will be changed into another but many.

What could it be?

regards

hogelmann

[Paul Berger (Canada)]

Well the display memory in the 85 is 4 16K dynamic RAMs, so it is nibble wide and each character would 2 nibbles so every two lines on the screen would be 128 nibbles which would mean a different row in the dynamic RAMs so i would suspect that one or more of the DRAMs has a failed row of cells. I would suggest you look at the bit patterns for the characters that get changed and what they get changed to to look for a pattern to determine which DRAM might be suspect. If you wish you may email me through the forum with a list of what you observe and I could help you figure out what may be going wrong.

[hogelmann]

Dear Paul,

thank You for Your prompt answer. I will check the changing characters within the next days and send a report.
I haven´t heard of Halfbytes (nibbles) before. I thought, that the VRam is part of the "normal" Ram. Could You please tell me on wich PCB the VRam ist located?

regards

hogelmann

[Paul Berger (Canada)]

They are located on the the display PCB I looked at my 85A and they are MB8116 chips so are a standard 16K bit DRAM chip. They are in sockets near the bottom just left of the flyback transformer, on the display board on the left side of the CRT. They are U13, U14, U15, and U16 on that board. You could try reseating them but from what you said it sounds like one of the chips has failed. If you turns out you need one of these chips I probably have some that would be suitable that I could try in my 85 before sending them. you could further isolate by swapping around the DRAMs if one is bad and you move it the symptom should change.

Yes the half byte (nibble) wide memory is a bit odd but I would guess it was done since it would fit a standard part and would reduce the chip count. the way the display buffer seems to be arranged the memory is only accessible through the display controller so there would be no convenient way for the self test to find problems with the display memory. You will find the service guide for the 85s on the HP Computer Museum website in Australia along with a lot of other good information.