I've been a lurker for a long time, but I've got an Apple IIe board here that made me decide it was time to create an account and ask for the help of the experts.Before getting to the current issue I'm facing, the short story of this board:What I have here is an 820-0064-B that came from with a lot of various other discarded computer motherboards from an ham radio fair, and that was probably 10 years ago.It sat forgotten in my storage room until I had to move a few months ago, when I "rediscovered" it in a box, wrapped in an antistatic bag.
It originally came as board only, pretty clean, no scratches but obviously tampered with:
- Video, CB and EF roms were replaced with EPROMS by previous owner
- IOU pin 21 was broken and it was "repaired" by sticking a short piece of rigid wire into the socket to make contact with the stump from the chip
Things I did before firing up the motherboard for the first time:
- unsocketed the IOU, soldered a thin piece of the leg of a resistor to act as a pin
- Removed the original and damaged IOU socket and replaced it with a turned pin socket. Lucky the original sockets are very easy to desolder
- dumped all the roms and checked the MD5 of their dumps. Video matched 341-0161-A, EF matched 342-0134-A, CD matched 342-0135-B, character matched 341-0132-B (this was still original)
- Checked the HAL chip. I have a custom tool that can analyze registered PALs, the equations I got out of it are the same ones you can find on applelogic, so I considered it ok.
So, the motherboard is of an unenhanced IIe.
You can see the board under test in attached a2e_board_under_test.jpg
Time to fire it up. Aaaand... garbage on screen. See a2e_screen_garbage.jpg . Removing the EF ROM made very little difference, so to me it looked like the CPU wasn't executing much.Lucky me, I had a few 6502s around. Replaced it with another and finally got a looping self diagnostic (I was testing with the keyboard disconnected, and according to https://support.apple.com/kb/TA38047 it's normal to get a loop).
Fine, I needed I keyboard to go on with the testing, and maybe a speaker. I have the carcass of another IIe, so I pulled it out and connected both the speaker and the keyboard.Got the "]" prompt, and when running the diagnostic I now got a "KERNEL OK" message, still I noticed something weird: no beep from the speaker nor CR2 led lighting up.
While I was at it, I decided to see what would happen by converting the machine to an enhanced variant:
- Installed a 65C02
- Burned and installed a 342-0303-A for EF
- Burned and installed a 342-0304-A for CD
- Left the char 341-0132-B untouched, will get to it in the future
This got me the new prompt (with "Apple //e") and now the diagnostics report "SYSTEM OK", but still no speaker sound nor CR2 lighting up.
Had to give another look at the schematics, and found out that speaker is toggled by pin 8 of the IOU, that then goes into a small driving circuit for the speaker.After I verified that pin 8 was not shorted to GND, I attached my oscilloscope probe to the end of C79 that is connected to pin 8. What I saw is in picture a2e_iou_spkr_noise.jpg To me, it looks like noise, and doesn't change while performing the self-test.
I also attempted to input a program that should generate a continuous note, but did not see changes in the output.
So... Either the IOU has a dead pin or something before it is not allowing it to decode properly. Any idea of what else to check here? Any hints? Things that I could have missed?And if we get to the worst, does anyone here has a spare IOU chip and is willing to sell it? Or knows of a CPLD reimplementation?
Thanks for taking the time to read through this!