First post, by Scared8628
I got my hands on a rare XT clone, ARC (American Research Corporation) Proturbo 88 machine.
There is very little information about this machine apart from an incomplete jumper manual (https://theretroweb.com/motherboards/s/americ … bo-88#downloads).
The machine is in a zombie state. Sometimes it may start, make one beep, initialize video, show the its BIOS banner and then hang. Usually it just initializes the keyboard (LEDs blink) and starts making seven short beeps. Sometimes it initializes the keyboar and then makes no sound at all.
This board has its own rare BIOS (ARC BIOS) so I have no idea what this seven beep diagnostic code might mean. POST cards show nothing.
What I did so far:
- Checked all the crystals with oscilloscope, they all definitely do oscillate. The base frequency seems to be 4.8 mHz. The CPU is a Siemens 8088-1-P clone and seems to be capable even of 10 mHz frequency.
- Tried to re-seat all the chips. That changed nothing
- Checked voltages on ISA pins and CPU pins, they seem to be OK.
- Tried to change J12 from "64K" to "8K", that changes startup sound to three short beeps.
- Checked the board and capacitors visually, it seems to be ok
- Checked resistors and capacitors in place. I know that it's not the best idea but at least there are no shorts and no obviously dead ones.
- Checked that the board makes no sounds w/o BIOS chip and w/o CPU.
I'm kinda out of ideas how to progress further without any schematics and manuals. One idea might be to read the ROM chip, disassemble it and try to understand what these seven beeps mean but I have no idea what is this particular chip and how to read it. There is an ARC BIOS dump on this page: http://www.retroarchive.org/maslin/roms/index.html, I'll try to play with it.
My guess that it either can't initialize graphics or memory (or can't initialize graphics because of defective memory, a couple of times I've seen visual artifacts instead of BIOS banner and there were no beeps), but I don't know how to progress from these assumptions.
Could someone give me an advice please?