Altair-what do I do first

From: Bob Shannon <bshannon_at_tiac.net>
Date: Fri Sep 27 09:57:01 2002

This thread is covering a lot of interesting ground, but most of it does
not apply to an Altair at all.

The Altair uses a linear power supply of course, and there are
well-known issues with old filter caps causing
problems.

But realistially, how often do these old caps really fail? Lets limit
this to the main filter caps in an Altair or
Imsai class S-100 box. Should these caps really be 'reformed' be
default when a machine is returned to service?

Probably not.

The first thing that should be done to such a power supply is simply to
test the caps. There is a good probability that they are perfectly
servicable as they are. Testing such caps can be as easy as charging
them from a battery and watching the voltage decay through the bleeder
resistors with a DVM. If the caps are shot, they will fail such a test.

If the caps pass this sort of test, there is little justification for
removing parts and risking dammage. A few resistance checks is all
thats needed before you can feel quite safe before you flip the power
switch for the first time.

So what would I do with an Altair for the first time, the exact same
thing I did when I got my Imsai 8080. I spent about 10 minutes with a
DMV, and I used a small bench supply to check the caps. Checked a few
resistance readings, then fired it up. It ran, and still runs
perfectly, without any re-forming of caps or major disassembly.

And no one mentioned one of the most common Altair failures, the
original rectifier diodes had a too-low PIV rating and often fail when
the machine is powered up at the wrong point in the AC power cycle.
 Imsai's also have this issue to a lesser degree, and its a good idea to
upgrade these diodes with something a little stouter than stock.

Check the diodes!
Received on Fri Sep 27 2002 - 09:57:01 BST