[Coco] Broken CC3

[Bob Devries]

I should tell you the story of a similar experience I had back in the 
day....

Someone brought a coco3 and disk drive to me (remember, I'm an Aussie), 
complaining that the disk drive always returned I/O errors. Since I was the 
local Coco hardware guru, I agreed to check it out. They brought the 
computer and the disk drive to me.

I set it up at my house, and lo and behold, everything worked fine. I showed 
the unit working to them, and they agreed that maybe it was just a glitch, 
so they took it home. Next day, they called me to say that the setup still 
didn't work at their house. I agreed to visit them to check out the 
situation. The computer just would not read disks, and DSKINI failed at the 
verification phase.

I took the computer to my workplace where I had a more extensive electronics 
workshop, and set it up. Sure enough, it didn't work there either. So I set 
to work connecting an oscilloscope to the disk drive to check some logic 
levels. Now here's the interesting part:

As soon as I connected the ground clip of the scope probe to the zero volts 
of the drive, it worked! Go figure.

I went through the disk drive electronics and compared it to the schematic, 
and found that there was a connection missing between the chassis ground and 
the zero volts of the PSU of the drive. I installed the required link, and 
the unit worked flawlessly after that, even at the owner's home.

So why did the system work at my place but not at the owners house and not 
at my workshop? Simply this: both the owner and my workshop used a 
television set as monitor, but I was using a CM-8 monitor. Neither TV set 
was grounded through the mains, but the (aussie) CM-8 was. My assumption is 
that the lack of a ground connection to the computer, and the missing ground 
link in the PSU of the drive, somehow made a ground hum loop, which 
interfered with the working of the drive.

In Australia, our mains powered equipment normally has a ground pin 
connected, unless that equipment is double-insulated. The disk drive power 
supply is not double insulated, and has a 3-pin power plug, as does the CM-8 
monitor. Most TV sets are double-insulated, however.

Why was the link missing? I heard much later that Tandy Australia imported 
and re-powered a quantity of US built disk drives (FD-500) and sold them 
here. Obviously, someone omitted the link in the conversion to aussie power.

----- Original Message ----- 
From: "George Ramsower" <georgeramsower at gmail.com>
To: <coco at maltedmedia.com>
Sent: Tuesday, September 16, 2008 8:28 AM
Subject: [Coco] Broken CC3


> 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
>
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