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My Gigatron build

Posted: 15 Sep 2020, 20:14
by cde
This build is based on the kit from BudgeTronics with the addition of:
- low-profile DIL turned pin sockets for easier replacement of chips and attachment of logic probes
(I noticed afterwards that two chips are 14-pin and not 16-pin. So I had to cut off the legs of two 16-pin sockets to make it work).
- black 40-pin ZIF socket with small pins (ARIES 40-6554-10)
- rubber pads on the underside to increase height (necessary to plug in Pluggy McPlugFace)

I've also bought a TL866II+, EPROM eraser and a few AT27C1024 for easier reprogramming.

The board worked on the first try after powering up which was quite satifying! :-)

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Re: My Gigatron build

Posted: 15 Sep 2020, 21:07
by bmwtcu
I gotta say, that black PCB looks really sharp!

Re: My Gigatron build

Posted: 15 Sep 2020, 21:18
by walter
The first picture I see of a built Budgetronic Gigatron. Thanks for sharing!

Re: My Gigatron build

Posted: 15 Sep 2020, 21:54
by cde
Thanks Walter! By the way I just tested the Tetronis demo and the music seems really strange, much too high pitched. Then I tried Tetronis in the emulator, and heard the music as it should be. I had a look at the audio DAC with a multimeter and measures 10 KOhm for pins 2 3 4 5 but 5 KOhm for pin 6. Is this to be expected? Please have a look at the following video: https://imgur.com/a/T3ilK2p (note that by default, sound is muted).

Re: My Gigatron build

Posted: 16 Sep 2020, 06:26
by walter
I hope you understand, but I cannot give support on Gigatrons bought elsewhere..

Re: My Gigatron build

Posted: 16 Sep 2020, 13:23
by cde
Right, well anyway Budgetronics messed up and they provided an incorrect part for the DAC. They have provided a 6X-1-103LF, which is a simple resistor network whereas the correct part is the 4606X-R2R-103LF. Interestingly the 4606X-R2R-103LF is pretty rare and out of stock until at least december. Instead I found a 10-pin R2R 10K network on eBay which I believe will work by cutting off the extra 4 pins (7-10).

Re: My Gigatron build

Posted: 16 Sep 2020, 17:24
by cde
As I suspected it was indeed the DAC at fault. I desoldered the invalid part and replaced it with a resistor ladder I made based on the schematics and now the Tetronis music sounds as it should.

For reference, music with invalid DAC part: https://imgur.com/a/T3ilK2p
Gigatron with proper resistor-ladder DAC: https://imgur.com/a/YXwUHhN
(note that imgur mutes videos by default)

Here's a closeup of the ladder:

ladder.png
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Re: My Gigatron build

Posted: 17 Sep 2020, 10:44
by bmwtcu
cde wrote:
16 Sep 2020, 13:23
... I found a 10-pin R2R 10K network on eBay which I believe will work by cutting off the extra 4 pins (7-10).
I suspect that won't work since the last 2 pins are wired differently from the rest of the SIP. I also built one a while back out of discrete SMT resistors because I couldn't think of a better solution at that time.

Re: My Gigatron build

Posted: 17 Sep 2020, 17:23
by cde
Thanks, you're probably right. Anyway I have contacted Budgetronics and they have found a temporary solution for other Gigatron purchasers: they will provide a ladder based on discrete resistors (in a more compact fashion than mine), until the needed component gets back in stock.

Re: My Gigatron build

Posted: 22 Sep 2020, 18:38
by cde
bmwtcu wrote:
17 Sep 2020, 10:44
cde wrote:
16 Sep 2020, 13:23
... I found a 10-pin R2R 10K network on eBay which I believe will work by cutting off the extra 4 pins (7-10).
I suspect that won't work since the last 2 pins are wired differently from the rest of the SIP. I also built one a while back out of discrete SMT resistors because I couldn't think of a better solution at that time.
I've just tested the modified 4310R-R2R-103LF with last 4 legs cut. The last resistor is indeed a bit off (60 Ohm instead of 50), but it nonetheless sounds fine to me: https://imgur.com/a/fEG4gnS

In the video above you can also see the ladder kindly sent by Budgetronics who I thank for their quick response (it is the ladder with yellow plastic and the first pin marked by a black dot).