I am putting together my 3rd Apple one.
Last time I built one, I used MK4096 DRAMs which were affordable.
I noticed their price has gotten ridiculous, so I am considering MK4027N-1 or MK4027N-2 parts.
I have two questions:
1) What is the difference between the two, I am guessing access time ? Which is a better choice for the Apple 1.
2) Who is a good source for them ? I see they are going for about $2.25 to $2.50 each on eBay.
I have used both Motorola MCM4027-3 and various Mostek MK4027, and some Intel DRAMs in my own Apple-1 builds, and as long as you follow my advice about reliability mods posted on Applefritter, all of them should work, provided that the CAS timing oneshot is set right. The 480ns timing seen in the original Apple-1 manual is (mathematically) correct for a 1 MHz 6502.
But the window to hit is only 70 ns wide, which is questionable with the tolerances seen in the resistors and the 74123 itself.
I have seen the 480 ns needing resistors between 20KOhm and 24 KOhm. Never had a case where 27KOhm (as specified in the schematic) was correct to get 480ns.
If you use 6502A or 6502B, the timing window you need to hit widens considerably. But don't use CMOS versions.
The actual DRAM speed grade IMHO is irrelevant. At the slow speed the Apple-1 works, all of them can do it.
The fundamental difference between the MK4096 and MK4027 is that the MK4027 is the second generation part, which introduced "page mode". That neither helps nor hurts the Apple 1. The timing parameters for the speed grades are very similar. Pretty much any 4K DRAMs from any vendor should work. Faster speed grades will give more timing margin, but shouldn't be required.