[Coco] Resurrecting a dead CoCo 3

[Gene Heskett]

On Friday 09 January 2009, John Eric wrote:
>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.

95% of that heat comes from the transformer and the pass transistor, the one 
on the big u-channel heat sink.  So I excised all that stuff and stuck a 4 
pin trailer connector out the back panel and hooked it up to an AT style pc 
psu.  With a 63C09 in the cpu socket, and a disto 2 meg memory kit with cmos 
dimms in it sitting in the memory socket, I can lay a photo thermometer on 
the grill above the memory, throw a furniture blanket over that and walk away 
for a couple of days.  When I do check, the thermometer is up maybe 2 degrees 
F over room.  I doubt my coco is using more than 2 watts now.

>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
>
Depending on your present box, you could do like I have and set up my own web 
server on port 85, blocking vz's port 80 blockage they claim they aren't 
doing.  Sure they don't.  Yup, denial ain't a river in Egypt.  You can see 
what tries to keep this old fart out of the bars at

<http://gene.homelinux.net:85/gene>

My uplink is slow, so it won't load like slashdot, but it works. Probably 
better than me, but I keep plugging along since the alternative is worse. :)

-- 
Cheers, Gene
"There are four boxes to be used in defense of liberty:
 soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author)
"We shall reach greater and greater platitudes of achievement."
-- Richard J. Daley