IBM 1620 Project Journal

HW Notes




The work of restoring the physical 1620 hardware began on January 9th. In general, we will be having work days every other Saturday. See the Calendar for specific dates.

The team has been generating some reference material which document various aspects of Computer History Museum's IBM 1620. Available are:


Below is the current list of things to do at the next and subsequent work days. Many of these tasks can be done separately so we won't be tripping over each other.

Pre-Power Up Tasks

  1. Visually inspect the machine for broken wires, loose connections, etc. Correct any problems found.

    Several quick checks have been done over several portions of the machine, but one complete and careful check should be made.

  2. Re-jumper the thermal line to reflect missing peripherals.

    The correct jumpers have been determined and verified, they just need to be moved. In the process it was found that all of the main core memory thermal switches are bad.

  3. One by one, disconnect and "bench test" the power supplies. Correct any problems found.

    We have a clearly defined procedure for thoroughly testing the power supplies. The -36v supply, the +12v supply, the +30v supply and one of the two -12v supplies are fully tested and are ok. The other -12v supply had a problem which was traced to be two bad transistors. After replacing the bad power transistor (with a vintage 108 that Tim donated) and swapping regulator cards, the supply seems to be ok. The regulator card will be repaired (with a Tim supplied 026 transistor) and then the power supply will be throughly tested. Preliminary testing of the unregulated +48v supply found it producing ~65vdc - more work is needed on it. Some testing of the other unregulated supplies (24v, 3v margin) also needs to be done.

  4. Trace out the disk interface and connectors. The goal is to determine if we think that the machine has an integrated disk interface.

    This task is not needed for bringup and is very low priority since we don't have a master 1311 disk drive at this time.

  5. Make a replacement for the missing A2 cover.

    This task is almost completed.

  6. Purchase and install a replacement typewriter timing V-belt.

    A replacement V-belt has been ordered.

  7. Clean, lubricate and adjust the console typewriter.

    I plan to ask a professional office machine repair person to help with this.

  8. Install console typewriter.

    The loaner typewriter has arrived in good condition and with all cables & connectors. It just needs to be plugged in.

  9. Install replacement air filters when they arrive.

    No word yet on the air filters.

  10. Identify and rejumper the addressing wrap-around.

    Tim has explored this and it looks like a lot of work to rejumper the machine for 20,000 digit operation. Since we may be restoring the 1623 Extended Memory Unit, it may not be worth making this change.

  11. Reverse engineer new SMS card types.

    As a result of the survey of SMS card in each of the logic gates, 10 previously unknown card types were found - ARW, AYX, DEQ, DES, DEW, DFB, DFC, DFE, DFG and DFP. Their function needs to be determined.

  12. Repair the thermal switches and fan in the core memory.

    While debugging the thermal chain, we discovered that all three thermal switches and the fan in the core memory are bad. These need to be fixed in order to power up the machine.


Power Up Tasks

  1. Disconnect power from the logic gates.

  2. Apply power and debug the power up sequencing.

    Tim has studied the entire power sequencing logic and has created a detailed "bring up" plan.

  3. Verify that correct voltages are present on the logic gates.

  4. Reconnect power to the logic gates.

  5. Power up the machine.


Miscellaneous Tasks

  1. Make replacements for the two missing power supply cabinet covers.

  2. Repair the wooden case supporting the spare console panel.

    This is not needed for the main restoration work but to safely preserve one of the 1620 artifacts.


Here's a list of the tasks already completed.

Remember to document everything that is done to the machine.