Back in the days of the Apple II, you could apparently run off to Akihabara and get the same computer with possibly better components for less money. Or just the parts you needed.
I had a chance to look at such an Apple II Plus clone that wouldn’t boot into BASIC. It beeped and displayed “Apple ][” at the top, but didn’t go anywhere from there. Pressing Ctrl+Reset made it produce another beep, but nothing happened on the screen. Sometimes it beeped twice. Once in a blue moon it would enter the monitor. And as I found out through trial and error, it would also enter the monitor when I pressed Ctrl while turning on the power switch on the PSU.
Before I did that though, I checked the voltages, and they were pretty low, near 4.6V, even with all expansion cards removed! (I also checked for hot spots to make sure there were no almost-shorts.) Fortunately the PSU supplied in this computer has a potentiometer that allows users to adjust the voltage without even opening the computer!
When you’re in the monitor you can check the memory, ROM, and do what you want! And since this is a popular system, you’re just one Google search away from getting some really cool incantations that you can paste^Wpeck in there.
For example, I found something that’ll perform a memory test! This test passed with flying monochrome colors.
If the RAM is good, the ROM could be bad, or there might be a problem with a buffer chip, etc. So how do you test the ROM? Well, you go to https://www.scullinsteel.com/apple2/ on a computer (or phone), select the appropriate machine (][+ in this case) and enter the monitor by typing “CALL -151” (or “CALL-151”?). Then you type, e.g., “D000[return][return]”, and you’ll get the contents of the ROM at D000-D007. I had a look at the first 8 bytes of D000, D800, E000, E800, F000, F800. And guess what? E000-E007 were different!
E000 has 4C in the emulator, 48 on the real machine. E003 is the same. E004: 3C/38.
0x4C is 0b1001100.
0x48 is 0b1001000.
0x3C is 0b0111100.
0x38 is 0b0111000.
As we can see, it looks like one bit is refusing to come up.
I took out the ROM for E000-E7FF (which is conveniently labeled E0 on the PCB), cleaned the pins and the socket, and put it back in. And that’s all that was wrong with it! (I’ll replace at least one cap in the power supply at a later date. Unfortunately it’s one of those huge high-voltage high-micro-Farad ones that aren’t available in my parts cabinet.)
