My Gigatron build

[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.

[jbailey]

Congratulation... very nice case you built too.

[giobbi]

did you find what was wrong with it?

[james7780]

Congratulation... very nice case you built too.

Thanks - the case was a lot of work, but came out well.

[james7780]

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.

[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.

[james7780]

My Gigatron is now 100% complete, here are some more pictures:

IMG_20210914_173254.jpg (532.8 KiB) Viewed 18034 times

IMG_20210914_173338.jpg (582.12 KiB) Viewed 18034 times

The top of the case is a sliding acrylic panel:

IMG_20210914_173438.jpg (461.8 KiB) Viewed 18034 times

DIY ladder DAC:

IMG_20210914_173529.jpg (722.54 KiB) Viewed 18034 times

[lb3361]

"DIY ladder DAC"