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My Gigatron build

Posted: 11 Jul 2021, 19:05
by james7780
Here are some pictures of my Gigatron TTL build I just finished this weekend.

When the Gigatron first came out, I found it vaguely interesting, watched a few youtube videos, but never intended to build one.

But then 2 years ago I ended up with a BasicEngine kit, built that, and then this year I also built a clone Jupiter ACE (Cees Meijer's version), and then recently a friend gifted/traded me a Gigatron PCB, and it's bad form to waste an empty PCB :)

All the components were sourced locally (Cape Town). Here are the specs of my board (mostly based on which components I had already):

Clock 6.5 MHz (extra from the Jupiter ACE build).
Most IC's are 74LS. The clock "conditioning" IC (U1) is 74HCT04 and also using 74HCT377s.
Power consumption: 430mA ?

Surprisingly it worked first time when I first powered it up! However after a few minutes it locked up and now only runs for a few seconds before crashing.

Re: My Gigatron build

Posted: 11 Jul 2021, 21:24
by jbailey
Congratulation... very nice case you built too.

Re: My Gigatron build

Posted: 16 Jul 2021, 23:19
by giobbi
did you find what was wrong with it?

Re: My Gigatron build

Posted: 01 Aug 2021, 13:32
by james7780
jbailey wrote: 11 Jul 2021, 21:24 Congratulation... very nice case you built too.
Thanks - the case was a lot of work, but came out well.

Re: My Gigatron build

Posted: 01 Aug 2021, 14:36
by james7780
giobbi wrote: 16 Jul 2021, 23:19 did you find what was wrong with it?
I actually only came right with it today, after spending weeks trying to diagnose what was wrong. I even borrowed a real Gigatron from a friend so that I could compare signals/voltages/etc.

Basically I removed all the chips, washed the board thoroughly in warm soapy water, then rinsed the board, shook the water off, forgot to rinse with IPA, and dried it with a hairdryer. Then the next day I very carefully re-inserted all the chips, making 100% sure they were properly seated in the cheap sockets.

Powered it on and it now seems to run reliably :) (I hope it stays like that).

My McPluggyFace did not work with a keyboard, so I dumped the code/data/fuse from my friend's ATTINY85 and wrote it to my ATTINY85, now it works fine :)

The only thing left to fix is the DAC (for some reason I just assumed it was a simple resistor array), cannot find that part locally.

Re: My Gigatron build

Posted: 01 Aug 2021, 17:31
by bmwtcu
The audio DAC is an R2R resistor ladder and isn't a simple resistor array, but you can literally copy the symbol on the Gigatron schematic and build one from 10k/20k resistors. In my experience, unactivated flux is conductive, so perhaps that was the original problem you had.

Re: My Gigatron build

Posted: 14 Sep 2021, 15:49
by james7780
My Gigatron is now 100% complete, here are some more pictures:
IMG_20210914_173254.jpg
IMG_20210914_173254.jpg (532.8 KiB) Viewed 41 times
IMG_20210914_173338.jpg
IMG_20210914_173338.jpg (582.12 KiB) Viewed 41 times
The top of the case is a sliding acrylic panel:
IMG_20210914_173438.jpg
IMG_20210914_173438.jpg (461.8 KiB) Viewed 41 times
DIY ladder DAC:
IMG_20210914_173529.jpg
IMG_20210914_173529.jpg (722.54 KiB) Viewed 41 times

Re: My Gigatron build

Posted: 14 Sep 2021, 18:16
by lb3361
"DIY ladder DAC" :-)