Messing with a perfectly good CPU board

Back from vacation! I'll try to get caught up.

In our schematic exercise, we took a good look at how the CPU board routed the data bus using two chips for address decoding. Given this good look at how the hardware is wired together, we should be able to throw a bunch of stuff out and wire in a new controller. Time to dive in and start taking things apart.

First things first. A disclaimer...

Ok, now that we have that behind us....to the machine!

Original Backbox Thumb

The unplugged machine! Unplug it. You read the disclaimer right? high voltage, zap, danger, bad. Once unplugged (you unplugged it right?), remove the backglass, and swing open the upper panel to reveal the CPU and I/O driver boards. We recommend removing the upper panel to provide better access to the machine. All you need to do is disconnect two sets of wires, then lift the panel up, the hinges simply slide apart. Much thanks to whoever thought of that. In fact, we wish we thought of it earlier.

Now you can disconnect all of the wires to the CPU board. Be gentle with it, the board is actually pretty easy to break. Once all of the wires are pulled, loosen the screws holding it in place and lift the board out. Once removed, take it over to your nice cleared off workspace (you did clear off workspace right?) and prepare for pulling off chips.

PulledCPUBoardThumb

There is a bunch of stuff on this board that we simply don't need. Most of the chips are fortunately slotted into chip sockets and can be pulled with a chip puller or very very carefully pried up. If you have any delusions of doing something with the chips we are pulling out, you will want to keep the pins straight. Someone out there may want those Goldeneye ROMs. Oh who am I kidding, I keep every removed chip. There has to be something I can do with it. Lets start by pulling all of these socketed chips

U209 (main CPU), U210 and U212 (ROMS), U4 and U7 (ROMS), U6 (CPU for sound generation), U9 (the BSM sound chip DAC), U21 and U17 (Sound ROMS)

thumb_socketchipspulled.JPG

Rather unfortunately, I also want to take out a few chips that are not socketed. Delsoldering chips is really no fun and I discovered that I'm not particularly good at it. Well, I'm much better at it now than when I started. Be smarter than I am and buy a desoldering bulb. I did my work with only desoldering braid. Also, ventilate! For some reason in the time between projects I seem to forget how nasty soldering is and I fail to run a fan on the first day. The headache is simply not worth it.

thumb_desolder.JPG

The two chips of interest are the two that make up the address decoder. If you don't have an extra 74LS138 in your spare parts kit, you will want to be very careful when pulling the one off the board so you can reuse it for prototyping. Also, try to be careful with the contacts on the board itself. I wasn't and ended up ruining the trace for one of the pins. These things can be worked around, but it is a whole heck of a lot easier if you don't have to.

thumb_removedaddresschips.JPG

thumb_74ls138.JPG

Fantastic! One stripped down and ready to hack CPU board. Next up, adding some connector rails, making a cable or two and preparing our prototyping board.