-*- text -*- This is a list of possible replacement parts for OSI board repairs. History: 11/18/2011: updated part numbers on molex connectors, clarified some notes on buffers. 2/17/2012: updated to reflect WDC 40-pin DIP parts. 5/27/2015: added information on keyboard replacement switches. Finding parts: If you have a part number for a currently made part, you can search for distributor inventory on: http://findchips.com http://octopart.com If the part is no longer made, I suggest: Unicorn Electronics, http://unicornelectronics.com - excellent service, and have an amazing number of vintage computer parts American Micro Semi, http://americanmicrosemi.com - larger operation, all kinds of obsolete semiconductor parts, excellent quality and service. Jameco, http://jameco.com - A perennial standby for classic computer parts. Reliable, good service. Replacement for keyboard switches (542/542B/542C keyboard, 543 keypad, 600/600D superboard): This replacement switch was discovered by Bill Dromgoole and by Grant (from the forum, creator of the 600/610 replica). Forum link: http://osiweb.org/osiforum/viewtopic.php?f=3&t=204 Switch: Futaba MD-4PCS This can be purchased from Electronic Surplus: http://www.electronicsurplus.com/futaba-md-4pcs-switch-p-b-no-keyboard-package-of-20 The switches were also sold by George Risk Industries with the part number KBM-01-01. The data sheet is here: http://osiweb.org/datasheets/GRI-KBM-01-01.pdf Replacements for OSI Molex PCB connectors: 12-pin right-angle (horizontal) edge connector (sockets): Molex 09-52-3121 12-pin vertical molex connector (sockets): Molex 09-52-3123 12-pin header (pins): Molex 26-60-2120 6502 CPU The Western Design Center currently makes new DIP CMOS versions of the 6502 and 6521, available from major distributors. The part number is W65C02S . For vintage chips, there are a large number of online vendors, including Jameco and Unicorn. Notes on PIA: 6820 PIA can be hard to find. The 6821 and 6521 are electrically improved direct replacements. New old-stock parts can be ordered from Unicorn Electronics (http://unicornelectronics.com) The Western Design Center makes a DIP CMOS versions of the 6521, available from major distributors. The 40-pin electrically and mechanically compatible version is the W65C21N. You can find distributors for this chip by searching on http://findchips.com or http://octopart.com Potential replacements for 8T26/8T28: 1) note that the 8T28 and 8T26 are basically interchangeable in OSI computers, as long as a machine has uniformly one or the other. For RAM boards, it doesn't matter which you use: you can use either chip, or you can use an 8T26 in one socket and an 8T28 in the other. 3) 74LS240 and 74LS241 (and 74HCT240/1) are perhaps the closest replacements for the 8T26 and 8T28, respectively. These are 20-pin devices instead of 16-pin devices. There are two "halves", with opposing physical orientation, and complementary enable lines. Unlike the 8T26/8T28, which which tie the driver output and receiver input to a common "bus" pin, these chips have an input and output pin for each direction. This is why these chips have 4 more pins than the 8T2x chips.Tying together neigboring input and output lines on the "left" side of the chip (2+3, 4+5, 6+7, 8+9) results in a logically identical replacement for the 8T26/8. However, the chips are not electrically identical. In particular, the 8T26/8 have a much higher drive current on the BUS lines. For most OSI designs, this should not be of consequence, but care should be taken with respect to the fannout. For a direct replacement, an adapter would be required. 2) 74LS242/3 (or 74HCT24/23), 14-pin quad bidirectional inverting ('242) and non-inverting ('243) transcievers. These can be used in many OSI boards, since on many boards, the input and output buffers on the 8T28 are tied together. Like 8T28, these have complementary read/write enables. Differs from 8T28 in that there are no separate inputs/outputs; just A1-4 and B1-4. The i/o pins are tied together on most OSI boards, except the 542 keyboard. Where it can be used, an adapter would have to be made. 3) Nat. Semi DS8641 (formerly Dallas Semi. part). Similar to 8T26, except pinout is different. Open collector outputs require resistor termination. Also apparently obsolete, so not a great choice.