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TTL Parts


Site Administrator
Staff member
Apr 27, 2003
San Jose, CA
I have several machines in my collection that have been repaired by simply replacing TTL components on various boards.

One Altair 8800 was fixed when a friend helped me by tracing through the TTL on the front panel to isolate and replace a bad component or two.

Another Altair 8800 that I recently recieved had bad TTL on both the processor board and the front panel board. Fortunately both of those were socketed so I was able to isolate the bad parts fairly quickly.

The same was true of an Altos in my collection. One bad TTL part kept the machine from booting.

With that said, I have two questions:

1. What causes these parts to go bad? On the second Altair and on the Altos there was no reason for the parts to fail other then pure age. No spikes, static or other abuse could/should have caused it since there was none. The other Altair may have been poked the wrong way at some point.

2. Having replaced these busted components with newer ones, where can one find older date-coded parts without tearing up other old machines? I'd like to keep my collections as close to pristine as possible which, to me, means keeping the date codes for the parts within reason.

Any ideas or comments?



Jun 28, 2003
Aberdeen, Scotland
re: TTL Parts


Older TTL (before mid-1980s) was *extremely* voltage sensitive. While you say there were no "spikes", a warm PSU that allowed the 5V line to drift over about 5.25v could easily have killed them.

If you really want to stick with original parts, you're going to have to do the same thing NASA do when they want ceramic-packaged 286 and 386 chips for the shuttle program - go on eBay and buy old equipment you can strip them out of! :)

Luckily, as TTL was both flaky and expensive in those days, pretty much all of it was socketed, making for easy stripping down of old boards. For bulk TTL, I'd suggest keeping an eye out for old telephone exchange boards circa 1979-1982.

If you're willing to compromise with newer devices, most machines will accept LS series chips (Low-power Schottky(sp?)) which have a greater voltage latitude, but being based on CMOS technology need to be handled with anti-static precautions. Also, they're no good for circuits where a single output line drives three or more inputs; in that case you need the original part.


New Member
Jul 17, 2003
"LS series chips (Low-power Schottky(sp?)) which have a greater voltage latitude, but being based on CMOS technology ........"

Sorry Charlie, LS TTL chips are not CMOS, nor based on CMOS technology.

CMOS = Complimentary Metal Oxide Semiconductor technology.

LS series is still BTL technology.