Help with getting an original Altai running: 8080A clock circuitry and MITS88 2-SIO board

drlegendre . drlegendre at
Sun Feb 7 19:51:57 CST 2016


Assuming the initial visual inspection goes well, the PSU is +always+ the
first thing to check.. and that goes for everything from digital computers
to vacuum-tube radios.

Do you have a general background in electrical / electronic
troubleshooting? Do you have a Variac on-hand, or at least a
current-limited AC supply - such as the old light bulb socket in-series, as
was popular in the days before Variacs became affordable to the common man?

On Sun, Feb 7, 2016 at 6:15 PM, Robo58 <robo58 at> wrote:

> Hi Brent, Drlegendre and Eric
> Brent:  I took your advice and did a little program to output "A"'s and
> monitoring Transmitter empty before looping again.  That works fine and it
> will run for hours.
> Drlegendre:  Thanks for the excellent document link.  My buddy has the MITS
> 1K static ram and the MITS 4K dynamic ram cards.  I've been doing my
> testing
> with the 1K static ram to limit potential problems.
> Eric:  This Altair has the smaller PS with the smaller Electrolytic caps.
> They were all replaced and the PS brought up without cards with a Variac.
> My buddy bought Kemet Capacitors on the basis of very good ripple current
> rating and low ESR.
> Thanks Ron
> -----Original Message-----
> From: cctalk [mailto:cctalk-bounces at] On Behalf Of Brent
> Hilpert
> Sent: Sunday, February 7, 2016 4:14 PM
> To: General Discussion: On-Topic and Off-Topic Posts <
> cctalk at>
> Subject: Re: Help with getting an original Altai running: 8080A clock
> circuitry and MITS88 2-SIO board
> On 2016-Feb-07, at 6:17 AM, Robo58 wrote:
> > c). I hand assembled some code to exercise the 6850.  I know it has a
> > software reset and then you set attributes.  I tried my buddies echo
> > program.  It looks for a received character and then echo's it.  I'm
> > using a laptop and PuTTy along with an RS-232 breakout box.  I can see
> > characters going in but nothing coming out.  The 6850 appears to drive
> > the bus for a finite amount of time and then turn off its drivers.  I
> > don't have any experience with the part and I don't know if that's the
> > way it's supposed to work or that the Altair has a bus timing issue.
> How about doing a program that simply repeatedly outputs a character first,
> getting that going before trying to deal with input - cuts the problem
> space
> in half.
> The repeat loop should give you some consistent bus activity and observable
> waveforms on a scope if you have one, rather than trying to catch events
> initiated from external input activity.

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