[Coco] An interesting Korean CoCo2 - 26-3134 (no 'A' or 'B')

Brian random.rodder at gmail.com
Thu Nov 15 23:48:36 EST 2012


Greetings CoCoNuts!

I recently (early October still counts for recently, yes?) acquired a 
CoCo2 from evilBay. I just got around to dis-assembly and was met with a 
motherboard I can't identify - it's marked 20261044. I have looked thru 
Sam's Computer Facts for the CoCo2 which covers the 26-3134, but, the 
circuit board layout isn't even close; the diagram has the CPU just to 
the left of the cartridge slot. The CPU on this board is under the 
keyboard, closer to the middle of the board and there are two white 
connectors sticking out and two TMM41464P-15 chips (64k x 4 RAM) in 
between those connectors.

So, not being able to I.D. the board from that publication, I opened up 
Tandy's Little Wonder (thank you Frank!!) and started reading the CoCo2 
section. TLW describes this board pretty close since it does mention the 
two white connectors and a 'RAMSIZE' jumper (J6), which this board has. 
However, there's also jumpers J2 thru J5, apparently used to select 64k 
or 128k? J6 has been 'jumped' and all of J2-5 has been jumped to select 
128k, and there's a wire running from the ECLK pad near the CPU to an 
SN74LS02N which has been piggy-backed on top of IC13 - and since I can't 
find a board layout I have no earthly idea what it is. Additionally, to 
the left of the CPU is J1, another place to choose either 64k or 128k. 
Finally, this machine has been upgrade to ECB - it reports as much at 
boot (along with a bunch of RFI noise...). To the right of the ECB ROM, 
the motherboard is stamped 128k/64k ROM.

Doing a Google search for 26-3134, I came across Techno's site 
(http://users.digitalindigo.net/~techno/coco2.html) which mentions a 
board that sounds similar to mine; however, there's no mention of 128k 
markers on this board. Additionally, he lists the 20261044 as being in 
both a 26-3134A and 26-3127B. He also mentions that this board has a 28 
pin socket for the ROM chip, which was originally shipped with a 24 pin 
ROM containing Color Basic (mine has been upgraded to ECB). Now based on 
only the type of RAM that's installed (dual 64k x 4), I'm pretty sure 
I'm looking at a 64k, ECB upgrade to what was once a 16k CoCo2 with 
Color Basic. Are all of these other jumpers and the wire from ECLK 
necessary for it to use a 128k (27128) EPROM?

The machine does work, and other than this particular motherboard not 
being very well documented, I don't think there's any great mystery - I 
guess it's more of a curiosity thing. If I get the time over the 
holidays, I may do a component layout of the board so at least that exists.


Brian



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