MITS ALTAIR 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 paper tape reader, a bypass switch was added
Wayne from the Greenkeys group recommended that we check the fuse in the reader power supply

Click on the image for a larger view

Wayne from Greenkeys suggested we check the fuse in the paper tape reader power supply
It was blown, so we replaced it
The reader still didn't work
We disconnected the wire harness from the reader power supply
The reader feed contact and the reader feed magnet tested OK
Of course, after we reconnected the wire harness the paper tape reader works!

Next up, get the 20mA interface working
The RS-232 adapter is wired to plug #2 which is in parallel with the barrier strip
Just unplugging the RS-232 adapter will let us use the 20mA interface
We compared the the Molex connector wiring to the Teletype that came with the PDP-12
The transmit and receive wires were swapped, so we fixed that
We tried the Teletype as the console on the PDP-9, and it didn't work
More debugging to do

To do:
  • Document the jumper settings on all boards (took pictures)
  • Determine how the serial port is configured (110 baud, 20 mA Current-Loop)
  • Implement the second serial port (need to buy a UART)
  • Disassemble the software in the EPROMS in the Cromemco board
  • Repair the front panel (done)
  • Get the ASR33 Teletype working and connected (picked up 12/31/19, everything OK but the paper tape reader)
  • Punch Micro-Soft BASIC on paper tape (done)
  • Try loading Micro-Soft BASIC from paper tape on the Teletype (This will take several minutes)
  • Program Micro-Soft BASIC and a loader into the EPROMs on the Cromemco board