Scratch Built Maximite


After two weeks on holidays in Tasmania, I was itching to get back to the soldering iron. Here’s my latest Maximite, built on a PCB made from the original Gerber PCB files (thanks Trippyben!) with a 32MX795 chip on board and various support parts. If you build this way, you will need the PicKit3 to initially program the PIC. The Altronics and Dontronics kits are pre-programmed.

Notes on my construction…

Soldering the PIC. If you’re accustomed to soldering it is quite easy, believe it or not. Just allign the PIC with Pin 1 to Pin 1 on the mask, tack a few corners and solder away. Firstly i ran some liquid flux down the row of pins I was a bout to solder, then I ran the iron gently down each line of pins, using a reasonable amount of solder. This produced a few bridges, especially at the end of the row, which i cleaned up with some thin solder wick. Overall, I was happy with the result. Silicon Chip Magazine in March this year gave some instruction on soldering this type of chip.

Programming – be aware that the PicKit3 will not fit over the ICSP pins once the keyboard socket is mounted on the PCB (see photos above). I remedied this with a short jerry-rigged extension lead, which now resides with the PicKit for future use. See the pictures above.

Initially the PIC programmed all right but would not start up. I immediately though my soldering wasn’t up to scratch but it appears that there is a problem with the Core voltage filtering. My build, like a few others, would not kick over with the Vcap 10uF capacitor, C5 – even though it’s measured ESR was at around 0.3 ohm. This has a work around with a 22uF Electrolytic capacitor between the positive lead and the 3.3V supply, this sort of kick starts the Maximite on power up. I will be replacing the 10uF/22uF combination with a ceramic type mounted right next to the chip when I get time and hopefully this will be a better fix.
As far as the ability to program but not to run… this is not unexpected as the clocking regime is different internally for programming as it is for running.

Once it was running, I upgraded from 2.1 to 2.4 using the USB upgrade method.

I used an LM3940-3.3 in place of the LM1117T-3.3 regulator specified, this works but you need to contort the legs into the different holes. The LM1117 goes (L-R) Gnd, Out, In whereas the 3940 is the standard In, Gnd, Out. My regulator looks like it’s part way through a game of Twister. I also screwed a bit of aluminium to the 5V regulator to help it with the thermal load when the supply voltage is 12V.

The SD card socket has to be the original specified (from Altronics) so it fits the footprint on the board. I quite like it with its spring loaded eject. The case is also the original specified from Altronics.

The VGA connector sticks out a fair way from the back, which means that if you want to cut out the rear panel to accommodate it, there is very little plastic left. My case has the rear panel permanently missing!

Oh, and that grey dongle looking thing sitting on the VGA connector is a DB9-15 plug to RCA socket for getting composite video out to my monitor.

Any questions? email me.

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