[Coco] Resurrecting a dead CoCo 3

[John Eric]

Recently, I acquired a CoCo 3 that had been killed by it's previous owner. An apparent attempt at repairing the CoCo 3 by replacing I.C.'s at random resulted in the destruction of many PCB traces. The actual problem with the CoCo 3 was relatively simple and I'll point that out in a moment, but first, I had to reconstruct the damaged circuit traces.After a few hours with the CoCo 3 Schematic, a notebook and an Ohmeter, I had ascertained all traces that needed to be reconstructed. I began by desoldering every I.C. left on the board that didn't have sockets and replaced them with sockets and plugged those I.C.'s back in (a step that wasn't needed, but I wanted to socket all the chips to make future repairs and experiments simpler.) Next, I took my notes and some kynar wire-wrap wire and repaired all of the damaged traces as noted with the help of the Ohmeter, and I then placed sockets where those I.C.'s went and plugged them back in, except for the CPU
 which I replaced with a 63B09E. Since this board has the 1986 G.I.M.E., I went ahead and replaced the capacitor C64 with a quality WIMA 220pf capacitor, although this probably wasn't necessary. I then plugged the Performance Peripherals 512K Memory Board Back in, hooked everything up and turned it on. It didn't work, of course, because the original problem that caused the previous owner to destroy the board in the first place must still exist. What was that problem? Well, quite simply, the GIME only needed re-seating in it's socket - I took it out, cleaned the pins and socket contacts plugged it back in and voila - working CoCo 3 512k fully socketed chipset :)

I did notice that with the 512K upgrade, the CoCo 3 gets HOT. In an attempt to alleviate this, I replaced all of the logic I.C.'s with 74HC parts, except for the 74LS245 which I replaced with a 74HCT245. I will probably replace the 245 with an F245 later. So now, I have a CMOS CPU, High Speed CMOS logic, so why not replace the PIA's? Well, I replaced the 68B21 with a W65C21 CMOS PIA, but I left the custom keyboard PIA in there, because even though it functions like a 68b21/65c21, it is an Open-Collector Device, with internal pull-ups, for the purpose of making the keyboard more reliable (the membrane in a CoCo 3 keyboard tends to increase in resistance as it is used and this PIA counters that). Eventually I will replace this PIA with a 65c21, some open collector buffers w/pull-up resistors on a small board that plugs into the socket, just as a test. I'm just having fun - the CoCo is my new Hobby - all my friends are playing Guild Wars, I'd rather be
 soldering :)

My next project will be a 512k upgrade card that intercepts the GIME 256 cycle refresh and converts it to a 512 cycle refresh so that I can use the 256Kx4 DRAM chips in my junk box to make a cooler running 512k upgrade. A small CPLD and 4 RAM chips will make up the complete board. So, this brings to mind, a question: Where is a good free webpage that I can set up to bring these projects to you all?

Many Thanks - JEric