MITS ALTAIR #1 Restoration


The front panel is misbehaving, but we can get the processor to run and blink the lights on the front panel. Pressing EXAMINE does not load the switches into the address, and increments the address. Pressing RESET sets the address to 0000. Pressing EXAMINE NEXT increments the address and displays some data

The D3 LED is always on
We checked the SN74lS04 that drives the LED, and we see a 30uS pulse when RESET is pressed, so that means the LED driver chip is OK. we pulled boards from the system hoping that we would find one that was hanging the D3 bus. With all of the boards out we individually grounded DI0 through DI7 pins and each LED went out. After fiddling with the boards the D3 LED decided to behave normally


We removed all of the boards to take pictures of both sides and document the jumper settings. After reassembly the front panel looks like it is working OK now.


We picked up the ASR-33 Teletype that was used with the Altair. The printing unit was not mounted to the base and the H-bar that connects the keyboard to the printing unit is missing. It looks like this Teletype has both current-loop and RS-232 interfaces. It is going to take some work to get this Teletype working again


We analyzed (fiddled with) the ASR-33 Teletype today. If you manually hold the part in the printer assembly that connects to the H-bar, clutch disengages correctly. Pressing a key on the keyboard and manually allowing the clutch to engage prints a character. The same is true for the paper tape reader. Next week we will visit the warehouse and get the missing parts, and hopefully get this working correctly. We will also swap the connector on the serial cable and the Teletype to match what we use on the DEC computers .


We raided the warehouse for the missing ASR-33 Teletype parts. After some cleaning and reassembly it seems to be working nicely.

We even connected it to a Windows laptop through the RS-232 interface and printed a test page. We also found a spare Cromemco 64k memory board that we can use in the Altair if the MITS 4k memory boards misbehave.


Alex cleaned the top cover of the Teletype, and it looks very nice. We repaired a crack in the cover with epoxy, looks better

We punched a paper tape of Altair BASIC V3.2 on the Teletype. Time to see how the remote reader control works.


It looks like the additional reader control circuit is only controlled by the 20 mA current loop interface, and not the RS-232 interface. There is a small switch added to the reader control circuit to enable the reader when the RS-232 interface is used. The distributor clutch is constantly being tripped when there is paper tape in the reader. The magnet is the reader is not being activated, so the paper tape is not being read or advanced. There is a switch on the distributor that opens when the shaft is released to advance the paper tape and engage the reading pins. We need to check the whole circuit from the power supply in the base, through the reader control board, and the distributor switch to insure that everything is working.


Back to working on the Teletype. Too many visitors today to get much accomplished


This shows the paper tape Reader Control board that allows an external computer to control the reader with a 20mA control signal. Our DEC computers and the Altair 8800 have the 20mA control signal for the paper tape reader. Since this Teletype has an RS-232 adapter, and the RS-232 interface doesn't have the control signal for the pa