Monday, July 31, 2017
Monday, November 7, 2016
- Pin 11 of the socket must be connected high (or to pin 12, or to a soft switch to enable video scanning on row E of RAM).
- Pin 11 of IC F2 must be disconnected and that line (not the IC pin) must be connected to ground instead (See my hand-drawn schematic).
It seems to have worked! The computer shows the APPLE ][ greeting just as expected. I need to connect a keyboard and a disk controller card to test it all out and verify that all 48K is being accessed.
So, I now have an Apple II with 48K of 4164 DRAM in just one bank of RAM. Now for 64K. Or 128K...
|2 banks of RAM installed for future 128K modifications|
Tuesday, November 1, 2016
I ran jumpers wires on the back of the Apple II+ logic board to route the various signals I needed (PHI_0, AX, A14&A15) to the J1 socket area. Fortunately since only half of J1 is used by the Apple II+, I can use the unused pins of that socket to route my signals through to the new circuit I am adding.
Warning - I don't know if this works yet.
After I ran the wires, I started trying to solder a tiny adapter board to translate the 74LS257 signal locations to the new 74LS153 IC. That was taking too much time for something that might not work at all, so I put the circuit on a bread board. Here it is:
Unfortunately, this did not work. I suspect that the dip jumper cable may be too long and adding some capacitance or delay. Or, the RAM timing may have been thrown off by the internal logic of the new IC. My Apple II+ is not booting and is in a similar state to when the RAM was in the wrong row. I'm sure I would have figured this out, but Halloween took priority.
Better news is that my Apple IIb is in a pretty final state. I think I will start buying supplies to actually build it.
Until next time, RetroChallenge!
Saturday, October 29, 2016
Friday, October 28, 2016
I spent a number of hours last night and tonight trying to figure out why my modified Apple II+ was not beeping, and instead showing a screen full of question marks (or sometimes a white screen) on start-up, even after I added a row of 4264 DRAM. I swapped all of the ICs back and forth with my functioning Apple II+ and still had the same problem. At that point, I realized that I should have done some basic testing before I modified the logic board. I removed my modifications, and still had the same problem.
I found out that I could get very similar symptoms on my working Apple II+ if I removed the F2 74LS139 decoder, which selects which RAM bank to use. So I started checking continuity on my non-working board. I discovered that here wasn't continuity between the low bank selection line and the row that I had my RAM in. I thought I had found a major fault in the board. Then it hit me. The apple II+ rows are labeled A through K, and start with A on the bottom ROW. I had put my RAM in row E (the high address bank), instead of row C (the low address bank). I moved the RAM chips and after swapping out a bad chip, BEEP, and APPLE ][ at the top of the screen.
I felt so dumb. Hours wasted. So now I'm re-doing my modifications. Still time for some progress tonight. At least it's working. That logic board has't been fully functional in probably 30 years.
Tuesday, October 25, 2016
Sunday, October 23, 2016
Here are the two Apple II+ logic boards that I have for repair. They were mostly stripped of integrated circuit chips, and even have some ports and connectors that were de-soldered to repair other boards (back in the 1980's).
I chose to repair the board on the left due to the missing D6 RAM socket on the other board and a possible damaged trace under the F8 ROM.
I swapped the power connector and added a missing audio jack. These are still available on eBay as "panel mount mono jack":
A word of caution. I have found that the solder on these old boards has the potential to make you feel sick and hung-over. Drink some milk before you solder and work in a very well-ventilated area.
I was able to replace most of the missing IC on the board from the box of spares I had, and from the other board. I have no idea if they work, so that will probably require a lot of troubleshooting.
The only IC I could not replace from old pulls was the one labeled "9334" in the J14 position. Fortunately, this IC can be replaced by a 74LS259, which I had on hand from my early experiments with my SPI based Disk II emulator.
Finally, I did not want to have to troubleshoot with old, possibly bad RAM, so I have some 64K 4264 DRAM chips on hand to see if I can replace the original 16K 4116 chips. This will require some modifications to the logic board (due to the different power requirements and slightly different pinouts), which I hope I can complete before the end of RetroChallenge.
Sunday, October 16, 2016
Saturday, October 15, 2016
|The parts donor Apple II+ I am trying to repair is really a mess. Even the rubber feet were harvested for another computer. Fortunately, I found some decent rubber feet on eBay. I bought two packs of 12 20mmx20mmx8mm and it is a great replacement part for Disk II and Apple II feet.|
Regarding my other project, I was pretty scared to connect the repaired IIGS Upgrade board to a power supply, half expecting everything to start smoking. It boots! First time in about 15 years:
|Composite video works||and RGB works!|