[Coco] Coco SDC troubleshooting: no ROM found

[Richard Goedeken]

Hello everyone,

Today I built up two CocoSDCs from kits, and one of them works perfectly but
the other does not.  If anyone could help me get my second unit working I'd
much appreciate it.

The non-working unit doesn't boot up with the SDC-DOS extensions.  It just
boots as Extended Color Basic 2.0 or whatever the default Coco3 basic ROM is.
 The LED will light up and stay lit if there's no SDHC card inserted at power
up.  If there is a card plugged in however, the LED will light for a second or
so after power-on, and then it will turn off.  It flashes if I hit reset.  So
some portions of the system are working.  But for some reason the Coco doesn't
start up the SDC-DOS rom extension, so it is not usable.

I already did quite a bit of troubleshooting.  I swapped all of the chips out
between the malfunctioning unit and a working one.  And after swapping each
chip, the malfunctioning unit still malfunctioned, and the working unit still
worked.  So it's not a problem with the chips.  I examined all of the solder
joints with a 45x loupe and even went through and re-melted all of them and
touched them up, but still no joy.  The DIP switches are all set to off.  I
used a meter to 'ring out' the contacts on the flash ROM socket, and I got
continuity on all of them.  I also tested the pins of the DIP switch, and they
are definitely all not conducting.

So I'm sort of at a loss as to what could be causing this.  A bad socket?  A
defect in the PCB itself; bad via maybe?  Can anyone suggest how to
troubleshoot this further?  Maybe I can do some PEEKs to see if the SDC-DOS
rom image is visible?  What else could cause it not to start up with the ROM
extension?

Thanks,
Richard