Sunday. Too much time on my hands. Impatience. Project. Completed. Ramifications? (no pun intended)
SUCESS. So my deductions were correct. On any 512K board, by simply adding a jumper at W1, the 512K circuitry is bypassed and the Mac only recognizes 128K.
Pumped from the success of that experiment, I went to an original 128K board which had been upgraded to 512K, and using the same rational, placed a solder bridge across pin 1 & 2 at E3, which trace is otherwise required to be severed completely when upgrading to 512K with a mini-mux board (seen in the image above). This has the same result as installing the W1 jumper on the 512K board, and the Mac only recognizes 128K RAM, without removing any of the other modifications.
That's right, despite the presence of 256 chips and multiplexing circuitry to decode 512K RAM, with the jumpers in place, the Mac ignores all of that and functions only as a 128K.
Clearly, for those of us who want to be able to switch between 128K and 512K without maintaining two machines, or swapping logic boards, this makes a simple switch an effective solution. Open the circuit, 512K. Close the circuit, 128K. The switch could be easily run through a vent slot on the right side of the Mac.
BUT DOES THIS HARM THE MAC? Well, that's now the question. Obviously it works, and everything functions normally with the jumpers in place, but is this healthy for the Mac? Is the additional circuitry truly being bypassed from the electrical path, or is it still affecting voltages across the system? I didn't know what I was doing to begin with, and I have no idea what is actually happening now. Another question, is it authentic? In other words, will 256 chips without the additional decoding circuitry behave exactly like the 64 chips?
Nevertheless armed with too much confidence, I'm gonna move on to working on the 64K/128K ROM switch concept. As others have confirmed, it should be as simple as a single switch enabling one set of ROMs or the other. In theory, these mods should allow a 128K through 512K Plus (with SCSI board), to operate uniquely within the same box, including the fabled 128Ke. I'd like to find some killy clips for testing, to install the 128K right on top of the 64K ROMs. This allows the stacked ROMs to be used with a SCSI board. Unfortunately it might be too thick to fit in an unmodified chassis.
SUCESS. So my deductions were correct. On any 512K board, by simply adding a jumper at W1, the 512K circuitry is bypassed and the Mac only recognizes 128K.
Pumped from the success of that experiment, I went to an original 128K board which had been upgraded to 512K, and using the same rational, placed a solder bridge across pin 1 & 2 at E3, which trace is otherwise required to be severed completely when upgrading to 512K with a mini-mux board (seen in the image above). This has the same result as installing the W1 jumper on the 512K board, and the Mac only recognizes 128K RAM, without removing any of the other modifications.
That's right, despite the presence of 256 chips and multiplexing circuitry to decode 512K RAM, with the jumpers in place, the Mac ignores all of that and functions only as a 128K.
Clearly, for those of us who want to be able to switch between 128K and 512K without maintaining two machines, or swapping logic boards, this makes a simple switch an effective solution. Open the circuit, 512K. Close the circuit, 128K. The switch could be easily run through a vent slot on the right side of the Mac.
BUT DOES THIS HARM THE MAC? Well, that's now the question. Obviously it works, and everything functions normally with the jumpers in place, but is this healthy for the Mac? Is the additional circuitry truly being bypassed from the electrical path, or is it still affecting voltages across the system? I didn't know what I was doing to begin with, and I have no idea what is actually happening now. Another question, is it authentic? In other words, will 256 chips without the additional decoding circuitry behave exactly like the 64 chips?
Nevertheless armed with too much confidence, I'm gonna move on to working on the 64K/128K ROM switch concept. As others have confirmed, it should be as simple as a single switch enabling one set of ROMs or the other. In theory, these mods should allow a 128K through 512K Plus (with SCSI board), to operate uniquely within the same box, including the fabled 128Ke. I'd like to find some killy clips for testing, to install the 128K right on top of the 64K ROMs. This allows the stacked ROMs to be used with a SCSI board. Unfortunately it might be too thick to fit in an unmodified chassis.