The 520ST One Meg Upgrade


To do the upgrade, you will need: 16 256K RAM chips, 150 ns access time, e.g. NEC 41256C-15 or TMM4256-15 which is what I used. You can order them from MICROPROCESSORS UNLIMITED, INC.,24000 South Peoria Avenue, Beggs, Oklahoma 74421 (918) 267-4961, or if you know of a place locally.

A good miniature soldering iron, with a small tip. Approximately four foot of wire. If you have it, use wire-wrap wire. But you can use ordinary thin single strand wire. De-soldering equipment. E.g. a sucker. Normal tools, such as a screwdriver, tweezers, pliers etc.

How it's done:

  1. Open up the 520ST case; pull off the keyboard connector; if there is shielding round the circuit board, take that off, and remove the board.

  2. Identify all the RAM chips


Desolder and remove all the capacitors adjacent to them. This is to give you more room to do the soldering of the chip pins. Using a solder sucker, open up the holes that used to be occupied by the capacitors. Do this step carefully to avoid damaging your ST board.

  1. Take each of the new ram chips in turn. Bend pins 4 and 15 so that they are horizontal to the package, and cut off the tips of pins 4 and 15 so that they are about half length. Place the RAM chip on top of the old one (same way around!)with all the pins except, 4 and 15, touching the chip underneath. Carefully solder each pin, except 4 and 15. The best way to do this is to put the tip of the iron on both pins, heat them both for a second(no more) and add a little solder, which should flow around both legs. After doing each chip, check all pins with a magnifying glass.

  2. Replace all the desoldered capacitors and solder them back exactly as they were. Please note that your should not attempt to power up your ST until you have completely finished step 6.

  3. Now for the wires. The first one connects the RAS on pin 4 of all the new chips, to pin 18 of U15. If you are using wire-wrap wire, then strip off five inches of insulation, Solder the end to pin 4 of the new U45. Measure the distance to the next pin, and shift over that much insulation before soldering. Continue until all the ships are done. Route the wire through the hole in the PCB next to U15 and connect to pin 18 of U15.

  4. The second wire connects all the pin 15's of the new U45-U32 to U15 pin 22. The third connects from pin 15 of the new U30-U16 to U15 pin 21.

  5. Now sit back and think. Read through these instructions again. Have you done everything? look at the board. Are all the joints good? If you have a meter, check for continuity on all the wires. Applying power with errors might destroy a lot of chips. If you are confident, then put the board back in the case (your workbench is probably covered with bits of wire and solder), connect it to the monitor, disk and power supply, and switch it on.

  6. If it boots, then you are probably there. If you have SID (supplied to the developers only) or Kissed, then look at the phys-top variable($42E) which should read $100000. If it is only $80000 then you have only 512K. If you don't have SID, load basic and type in this program.

                     10 def seg=0
                     20 loc#=1070
                     30 print peek(loc#)

This should print a result of 1048576. If you get half this (524288) then you have only 512K.

DISCLAIMER: The author of this doc file or ST Report are not responsible for your workmanship, success or failure on this upgrade project.

Russell Vail
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