[Coco] Broken CC3

[Robert Gault]

George Ramsower wrote:
> Well, I'm stumped.
> 
> I found the power supply was clean after all, I think.
> 
> The computer seems to work fine as long as I don't access the disk drive.
> 
> To bring you up to date I've changed:
> 
> The CPU, Gime, RAM Board, disk controller and disk drive and C14. That's 
> the one just after the .1 ohm resistor from the voltage regulator 
> transistor. The .1 ohm resistor is not in the circuit as, I'm using an 
> external power supply.
> 
> AC noise on the 5V line is < 20mv. However, I'm seeing that noise on the 
> ground also. So I suppose this is just noise on the power line to the 
> scope.
> 
> Right now, from power up, when I DIR drive 0, it steps over to( I think) 
> the directory track and then crashes. After a crash, I reset twice 
> because the first reset doen't reset to RSBASIC. The crash gets better 
> and more colorful. Upon the second reset, I get the initial startup 
> screen. This time, when I do a dir, the disk doesn't find the track, 
> tries again and then crashes when it( I think) gets to the dir track.
> My 3.5 drives, upon power up, moves the head to track 0(I think). So 
> after the crash, the drive isn't where the coco is expecting the head to 
> be. Therefore, it goes to the wrong track, doesn't find the directory 
> and tries again after moving it back to zero, tries again and crashes. 
> I'm used to hearing this and I'm pretty sure it is stepping the correct 
> number of tracks before it crashes.
> 
> I hope my info is not to vague or stupid sounding.
> 
> Suggestions are welcome!
> 
> Is it possible the ROM in the coco could have failed? Has anyone ever 
> had a ROM failure in a CC3?
> 
> George
> 

George,
You need to take a step back, calm down, and approach this problem with 
some logic.

1) Remove the disk controller from the Coco and try all the Basic 
commands. Try the RESET button. If everything works normally, then the 
problem is not in the Coco but in the disk controller.
2) With the disk controller installed, don't issue any disk commands but 
just test the RESET button. If it works correctly, the problem is not 
the DOS ROM but probably the Western Digital chip of the actual drive.
3) What happens with the disk controller installed but the drive off 
when you issue drive commands? How does the RESET button work in this case?

In short, you need to isolate the problem to as narrow a range of 
hardware or software as possible. Then you will know what needs to be 
tested or replaced. Heck, the problem could even be oxidized contact 
strips on the controller or CART socket.
Much of this can be done without test equipment. However, for best 
results at least some test equipment will be needed.