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MITS 8800b Turnkey Restoration (#3)


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  MITS 8800b Turnkey Restoration (#3) by Bill Degnan - 04/25/2015 16:09
PCC MITS Altair 8800b turnkey front
Started Pertec / MITS Altair 8800b Turnkey system restore project. After doing an initial power test I disassembled the unit for cleaning. Click image for larger view.

The system itself works pretty well, with 48K RAM. The only functional issue remaining is the front panel. I cannot reset nor start the system using the switches. I removed the front panel card by desoldering the switch posts connected to the rear of the card from the front of the chassis so I could slide the card out. I also desoldered the two connection wires and made notes. The alternative way to remove the front panel is to remove the switch nuts and keylock mechanism. Click image for larger view.

All of the LEDs measured resistances of greater than 15 ohms. My test unit had resistances closer to 1.5 ohms. But that does not mean much, I have been told "..their resistance is not linear with voltage the way a normal resistor is..The easiest way to test an LED is to use a 220 ohm resistor to hook it up to a 5V supply..."

This helped

More photos

So, I ran the LED test and they were all ok. The diode on the board is a diode is 1N914, I think I have one of these and if so I will replace assuming process of elimination dictates it must be the diode that is at fault. The poles are cruddy, may have shorted out caused by the exposure of storage.

  Turnkey Board Repairs by Bill Degnan - 04/28/2015 09:47
I put a diode on a protoboard and two wires. With this I touch pins 99 and 100 to ground them and reset the system. The LEDs on the front panel light up when I do. Now at least I can reset the system, a requirement for loading BASIC after running the bootstrap.

BASIC requires 9600. I repaired the previous owner's soldering damage the best I could to set the baud rate to 9600 from 300. The original metal connection for the jumper pin was worn away for pin S0 (top pin). I put in a copper wire to connect the jumper to the next point in the circuit. That worked, now I have 9600 board. Click image to view larger image.

With these two hacks I can boot BASIC on this computer.

Next: Turns out the diode in the front panel was OK after all. My guess is that I have to locate bad solder joints or repair the switch, or clean it. I lost the diode (fling! - oops!)...waiting on a replacement in the mail. After I finish the front panel repairs I will clean everything nice including CLR-ing the turnkey or maybe repaint it silver.

  Restoration Complete by Bill Degnan - 04/30/2015 11:39
I ended up using a different front panel instead. The switch was bad on the unit I was trying to fix.

Unit is now fully functional and cleaned up.

Here is a video



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