[Coco] UK PAL CoCo1 Repair

[Paul Shoemaker]

Have you checked the power supply?  In my (limited) experience troubleshooting PCs, dodgy power supplies can often make it appear like other components are the problem (sneaky).
According to the Color Computer Technical Reference manual:
A "dead computer (garbage on the display or no display) indicates a major malfunction of the machine.  In this case, the first thing to check is the power supply voltage levels.  These may be checked at the following points.
+5 volts - test Point 12-5 volts - Test Point 11+12 volts - Test Point 9-12 volts - Test Point 10Ground - Test Point 3
Or if you are having problems finding these test points, check the voltage son one of the RAM chips (U20 - U27).
Pin 1 = -5 volts (minus 5)Pin 8 = +12 voltsPin 9 = +5 volts



      From: R Jones <rjones at biketech.org>
 To: coco at maltedmedia.com 
 Sent: Wednesday, September 20, 2017 9:25 AM
 Subject: [Coco] UK PAL CoCo1 Repair
   
Hi,


I'm trying to fix a fault on my Color Computer 1 (ECB), I broke this 
over 30 years ago while playing with a diy cartridge slot board. I put 
it to one side and never got round to fixing it. It now powers up and 
show random characters on screen, but no prompt. I'm just trying to work 
out the power line requirements for the major chips. Are schematics 
available for this version of the board?

I have now socketed all the ICs on the board. As I have a Dragon 32, 
which uses mostly the same chips, I have swapped out most of the ICs in 
to the Dragon and seem to work fine, such as the cpu, 74ls783, 6821, 
6822, 6847, 74ls138, 74ls273, 74ls244. I also have tried 2 other sets of 
working ram chips which do not seem to make any difference. I put the 
meter on most of the resistors and they check fine, I have also taken 
most of the capacitors out of circuit and they check fine on my basic 
tester.


This is the board, a 26-3003: http://www.biketech.org/coco_board2.jpg


I have checked the power and ground to all ICs and they are fine with 
the exception of this chip which I cannot find a datasheet for 
http://www.biketech.org/unknown_ic.jpg I see that Dave Jones of EEVBlog 
recently did a teardown of a CoCo1 with what looks to be an identical 
board but this chip has a different number on his.


I have a scope and checked for the signals suggested in the SAMS 
diagnostic pdf and they are present.


There doesn't appear to be any damaged or broken traces. I have tried 
the basic rom in the Dragon and it power up with the usual Tandy Basic 
text and appears to work fine.


Any advice on where to check next would be appreciated.


Thanks.



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