Posted By azog on December 16, 2012
I would not consider an SX-64 to be exactly “rare”, but it would be more of the “uncommon” type of system. And apparently “desirable”. When listed on eBay, they usually start at $100 or more. I don’t really know how many sell at that price. So when I had opportunity to acquire an SX-6 in “parts condition” for less than the price of a couple of pizzas, it was one of those things I couldn’t pass up. It was in cosmetically nice condition and more importantly, had the keyboard.
Visual inspection revealed no major flaws, so I took the plunge and powered it on. I was expecting the PLA to be blown, at the very least. But to my surprise, it booted right into BASIC. It had some bad memory, reporting 30929 bytes instead of the standard 38911.
So my first goal was to take out the board with the memory and see what the deal is. Unfortunately, unlike the SX-64′s less-portable brother, the SX-64 is not very “user accessible”. Things are jammed at tight as possible, with various daughterboards connecting to one another thru a mess of assorted wiring harnesses.
The board at the far rear is the main CPU board. In front of it, a bit to the left, you can see the expansion cart slot, with a homemade diagnostics ROM. To the left of that is the “expansion” board, which carries the 6526′s, and plugs into the main board via a 40-pin header. Behind THAT, is the 1541 drive control.
When I remove cables, even if their orientation is obvious, I always mark them which you can see on the two white cables at the top of the main board. Then it was a matter of extracting this thing.
I didn’t really get a shot of the whole main board while it was out, but this is where the RAM chips are.
I tried to remove the original RAM in a non-destructive manner, with the idea to test them via the swap-and-replace method on another 64, but that plan didn’t work out. I was more concerned about damaging the PCB, so in the end, I eventually snipped the RAM chips out, which destroyed them. That’s a shame, because some of them were good.
I did find replacement RAM (4164s), and decided to socket them as well. I figure once something needs replacing, there’s a possibility that it may need replacing in the future, and why not? So I picked up a stack of really cheap “machined” sockets.
Well, it goes without saying, but I’ll still say it. When you buy cheap Chinese parts, expect nothing but cheap Chinese parts. It’s hard to tell from the photo, but the pins are flimsy to the point that practically breathing on them will bend them. And the insertion holes are dumb.
Normally I use “real” machined sockets:
They’re usually much more expensive, but for small projects like this, for a do-it-yourself, etc, the cost is so much worth it. I should have just backed out and ordered some good sockets, but I was not in the mood to order $10 worth of sockets with a $10 shipping fee.
Once that was said and done, when I powered it on, I received the expected 38911 bytes free, but the 1541 was not working; the LED and motor stayed on. I did not remember if it was doing this prior to my fiddling with it, as once I saw the memory incorrect, I just jumped in. So I don’t know if I blew the 1541 controller, or if it wasn’t working in the first place.
Ah well, this symptom is apparently one of the most common failures in the 1541, and the first thing you do is replace the 6522s, which fixed the problem.
As far as I can tell, everything else is working, but I’ll have to find some way to put it to more tests. I made a Commodore 64 dead-test diagnostic cart (which you can see above, inserted into the cart slot), but the SX-64 is apparently different enough that it fails quite a lot of tests.
But all in all, I’m quite happy. The keyboard needs a standard cleaning cycle, but that’s nothing to write home (or blog!) about.