[Coco] [Color Computer] Video garbage on CoCo

[Robert Gault]

Either a hardware or software failure but probably a "minor" one. The 
Coco is still functional but incoherent. This is a typical result of the 
CPU executing random code.

Open the Coco and look for anything abnormal: signs of overheating, 
poorly socketed chips, exploded capacitors, etc. If you have a service 
manual, there are troubleshooting procedures that can be used. Here is 
an example or two from the Coco3 manual.

4) Random Character/Clear Screen/No Sign-on
Check RAS* (TP4), CAS (TP5)
Check pin 1 and pin 2 of IC15
Check pin 20 of IC2
Check IC16-19 (Dynamic RAM)
Check the items in 6.4 Processing Problem

6.4 Processing Problem
Check OSC Circuit (IC6)
Check TP2 and TP3 (ECLK and QCLK) (Waveform 7)
Check TP4 and TP5 (RAS* and CAS*)(Waveforms 8 & 9) pins 10 and 11
of IC6 (WE0*, WE1*)
Check Address decode circuit (IC9)
Check S0 - S2
Check pin 40 of IC5 (HSYNC*), pin 18 of IC5 (VSYNC*), and pins 37, 38
of IC5 (IRQ*)
Check pin 40 of IC1 (HALT*), pin 2 of IC1 (NMI*), and pin 4 of IC1
(FIRQ*) - these pins are normally high.

Testing is typically done with a multimeter or an oscilloscope that can 
handle at least about 20MHz input. A service manual will have waveform 
examples.

If the above is causing your eyes to glaze over :) , then toss the Coco 
or find a friend that tinkers with electronics.


brunkfordbraun wrote:
> My CoCo is outputting random video junk, does not respond to
> keypresses, and does not go to the command prompt. There are many
> different things the video has done, but here are two samples:
> 
> http://img56.imageshack.us/img56/4391/junk17wt.jpg
> http://img56.imageshack.us/img56/3977/junk29os.jpg
> 
> Is this a common problem? Is this something I could fix with my own
> soldering skills, is there someone out there who still fixes these
> things, or am I just better off tossing it? 
> 
> 
> 
> 
> 
> 
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