Altair 680, up and running

Bill Sudbrink wh.sudbrink at verizon.net
Fri Jan 22 09:45:18 CST 2010


For anyone interested, after the power supply referb and
chip reinstall the 680 was not fully functional.

The front panel seemed to work properly, but examining
page zero revealed that one of the 2102 RAMs was dead.
This wasn't surprising and a quick replacement had front
panel deposit and retrieval to RAM fully operational.

Examining the contents of the 1702 EPROM with the front
panel showed that the monitor code was in tact, but
nothing was coming out of the serial port when the unit
was reset.  Looking around on page zero with the front
panel after a reset revealed that the monitor code was
running, so probably something was wrong with the serial
port.

I put a scope on the chip select lines of the UART and
could see the monitor code polling for inbound characters.
Was the UART dead?  On a lark I put a probe on the receive
clock pin of the UART.  Nothing.  I put it on the crystal.
It was making a good signal.  Turns out the 4702 bit rate
generator was dead.  It seems to me that that is a very
odd component to fail.

What surprised me even more was that I happened to have a
tube of 4702s in my "random chips" box.  I have no idea
where I got them.  I've never worked with a 4702 until now.
The only small drawback is that the date codes on these
4702s are 1982.  All of the other chips in the 680 have
1975 and 1976 date codes, including the 2102 I replaced.

With the 4702 replaced, the monitor is active on the serial
port.  Now to find some _SMALL_ programs to push to it.

Thanks to all for the power supply help.




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