Mark 8 microcomputer reconstruction

I am in the process of getting all the pieces of my Mark 8 microcomputer working again.  Several people have asked about what it looks like, what hardware is included etc.  I will be, on occasion, posting progress reports here as things get restored.

The Mark 8 was written up in Radio TV Electronics Magazine in 1973 as a home construction project.  It was based on the first 8-bit micro, the Intel 8008. (not 8080, 8088, 8086.....)  I built mine in 1975 /76 because at the time an 8080 chip was about $150.00 and I built the whole Mark 8 for about that (at least the basic section.)  When I finished playing with it, it had 15K RAM, 1K EPROM (2708), a TV character generator (8x32 from the Digital Group), a cassette interface (also from the digital group) Scelbal basic and other miscellaneous software and hardware.  It has an 8008-1 and at one time, I had it running at 800 KHz vs the standard 500Khz.  It is currently at 500KHz and that's one of the things I'll work on later.  It is up and running with the ROM monitor I wrote and I am currently working on getting the tape interface working.  I am also having a problem with the old parallel keyboard in that one bit seems to be not working.  I will need to find a new chip or parallel encoded keyboard to complete the restore but that is a later worry.

A picture of the computer " on the bench"  shows the power supplies, front panel and most of the boards installed.  The back two boards are the TV character generator board and the cassette interface board.  A second picture shows the front a little better. A  video  showing the machine running the memory test gives an idea of the speed (or lack thereof) and a second  video shows the front panel lights as the same test is run. The videos are currently AVIs since that's what my Fugi camera gives me. I'll try to get the converted to mpegs soon.

I'll add more as I get a chance but this gives an idea of what is going on.

The cassette interface is now working. I was able to read old tapes of SCELBAL basic and run print statements to the screen. See video1 and video2 for some pictures of this. The screen is really not quite as slow as it seems since I have it switched to output to a 110 baud ASR-33 in addition to the screen. Not really there yet because the BASIC requires a little over 8 K of memory and I have 8 Kin blocks one and two plus 3K and ROM in the forth block. When I plug in the 4K board for the 3rd block to make it 15K RAM and 1K ROM, the +5 volts drops to about 3.8 volts. I know I ran it with that configuration before and the system did work. May be leakage in PS capacitors. I may just have to get an old PC PS to get enough +5V. Still trying to find the keyboard chip. (AY-5-2376) I did the search as suggested but every where I looked, it seems that they want all kinds of info before I can get a price. Any suggestions on power supply tests or keyboard chip happily accepted.

Update 6/3/05 Someone located a keyboard chip and is sending it to me
I'm copying many of my audio tapes (Digital Group tape interface) onto my PC to put on a CD.
In looking at the specs for the 2102, it looks like each 4K board takes about 1 amp of +5V. This may mean I need to put a bigger PS in the computer or replace the 2102 with low power versions. Not sure which will be "truer" to the original.

Update 6/13/05   Keyboard chip is in and working. Now working on getting more 5V so I can get the 4th memory board and getting an RS232 interface to the 110 baud ASR-33 output I used for printing. I'll just add a line driver for it and capture the stuff on a PC terminal program.
Found my old 2708 EPROM programmer that I designed to interface with the Mark-8 and will try to get it running also. It's really just a wirewrap board with connectors to interface to the Mark-8 I/O ports and uses 3 9V batteries for the 27V but it worked back then to get my EPROM monitor working. Even found the software on tape to let me use it. Just have to get some old flat cables replaced and I can try some of the new 2708s I just bought.

Update 4/11/21   Now retired and hoping to get the system checked out and running. Stay tuned.