July 6, 2014

Reconstructing an AT&T 7300

Several kind folks on comp.sys.3b1 and the MARCH Yahoo Group have suggested that, in order for us to gain a better understanding of the workings of the rare and elusive MightyFrame, that we should get familiar with the AT&T 7300 / 3b1.  

Many say that this machine, manufactured by Convergent Technologies for AT&T, is "functionally equivalent to the MiniFrame".  Even though the MiniFrame and the MightyFrame have significant differences, this certainly seems like a great next step for us.

So, thus begins our journey....

So far, we have been able to acquire the bulk of a complete 7300 system, in pieces.  Just this weekend, we were able to piece together most of these pieces, and see what it does.

Here's the list of pieces that we are working with.  "Yes" means was part of the 7300.  "No" means I've had to find the part somewhere else.  I try to detail out the part and its source:

Yes - Motherboard

Yes - Upper system Tray

No  - Hard Drive: We've purchased 2 MiniScribe drives on eBay and await delivery.

                             A 20Mb Model 3425  and a  32Mb Model 3438

Yes - Monitor

Yes - Full Plastic Enclosures

No  - Floppy Drive: Using a Teac FD-55GFR (More modern than the Teac model FD-55BV-06-U...will it still work?)

No  - Power Supply:  Made a custom connector to a modern power supply

No  - internal peripheral connector cables:  Constructed our own using pieces of other cables

No  - Mouse:  doing without until we find one

Yes - Keyboard

No  - Keyboard cord:  Made our own...did we do it right?

No  - OS & Software install floppies: Have yet to hunt down sources.

Many thinks to eBay seller refurbparts3009 for working with us on these parts.

Power Supply Reconstruction:
Special thanks to DoN. Nichols and Dave McGuire for giving us the pin voltages.

Keyboard Cord Reconstruction:

This diagram shows how we wired the connectors to each other.  We must have it backwards or otherwise wrong, because when the keyboard is plugged in, the ends get warm, nearly hot (particularly on the motherboard side), as if there is some voltage short.  Here, we chose a "mirror image" connection.

If anyone has access to one of these cords, it would be great to know how to properly wire these connector ends.  

Internal peripheral connector cable reconstruction:

As suspected, we have not yet achieved success on our first try...but this is a long and adventerous journey for us.  We are not easily discouraged.

Monitor "Snow", then calms to blank.  No characters or cursor ever appear on the screen:

Motherboard LED indicator Lights:

On power up, I get different results when trying multiple times.  I have not yet determined if there is any pattern.  So far, it seems random.

Sometimes no lights light on power up.  (no subsequent clicking after power on)

Sometimes the green light lights up.  There is a click, as if a relay is engaging somewhere, 1 second or less after power up, at which time this green light turns on sold, and stays on.

Sometimes the (dimmer) red light on the right lights up.  (again no subsequent clicking after power on)

So far, if there is any "pattern", it goes:
Power up attempt 1 (cold) - no lights, no clicks.
Power up attempt 2 (still cool) - green light after click.
Power up attempt 3 (warm) - dim right red light, no click

Not sure if this really is temperature related, but it seems like it might be.

Can anyone direct us to a document or manual that defines what these lights are indicators of?

Or maybe share your insights?

Motherboard picture details:

For reference, here are hi-res shots of all of the motherboard chips.
(Soon, I hope to have a graphic of the full motherboard, where one clicks on each section to see a hi-res image of just that section, and then further, link each chip it's datasheet.  In the meantime, please bear with my numbered links to these pictures)


While powered on and waiting for nothing to happen, it seems that these IC chips get the warmest:  

AM2149-35DC 8517HPP

Is this normal?

Wanted:  Keyboard Cord & Mouse:

As mentioned above, there is an issue with how I wired this keyboard cord/cable.  More on that soon.

Thanks, all, for your input!


PS.  Thanks to DoN. Nichols for directing me to the AT&T UNIX PC Technical Reference Manual.  This should prove very helpful in answering some of these many questions.

Requests made at:
The MARCH Yahoo Group
Vintage Computer Forums (vintage-computer.com) Unix/Xenix/Linux
ComputerForums.org (banned for spam?)
VintageComputer.net (Bill Degnan's Personal Blog)
Reddit Retro Battle Stations  http://redd.it/2a8554

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