Subject: TRS80 mod 4 ver1 video/memory problems

From: Jim Davis <[email-masked]>
Date: Sat May 31 18:21:16 2003

Hi,
I recently picked up a model 4 ver 1 with a problem. The system originally
worked and booted, after moving the system it started to display random 
characters
on power up and failed to boot.. I removed socketed  ICs and  cleaned 
the connectors.
The system now displays a blank screen on power up ( although the drives 
spin, and
it could be booting ). If  I disconnect the FD controller from the main 
board, the
system jumps into basic ( with  video ) and seems to operate. Print 
fre(0) returns
somthing over 48K with the fd disconnected. Any hints?
Jim Davis.

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From: [email-masked] (Tony Duell)
Date: Sat May 31 18:34:23 2003

> somthing over 48K with the fd disconnected. Any hints?

It sounds like the CPU board is basically (!) OK -- the ROM BASIC only 
uses 48K of the RAM (no matter how much you actually have fitted).

When you reseated socketed chips, did you also do the ones on the FDC 
board? Have you checked -- carefully -- the ribbon cable between the CPU 
and FDC boards? I've had a lot of problems with those cables in M3s and M4s.

-tony

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From: Jim Davis <[email-masked]>
Date: Sat May 31 18:36:22 2003

>and FDC boards? I've had a lot of problems with those cables in M3s and M4s.
The floppy cable is indeed fragile, Having been forced to use the same type
on a project 20 some years ago. When I had it disassembled, I pulled the fdc
controller, burnished the leads, cleaned the socket and the fdc I/F 
connector
on the cpu board with 90% isoprop. I'll check cable continuity and fdc lead
to controller trace.
Thanks,
Jim.