TRS-80 Model I

From: Charles E. Fox <foxvideo_at_wincom.net>
Date: Mon Nov 24 05:14:15 1997

At 05:13 PM 11/23/97 +0000, you wrote:
>
>
>On Sun, 23 Nov 1997, Charles E. Fox wrote:

        Thanks, Tony, I will give it a try.

                                        Charlie
>
>>
>> On a TRS-80 Model I, no expansion unit, I am getting the left half of the
>> screen filled with graphics and the right half with scrambled text. Does
>> anyone have a suggestion of where I should start looking for the solution
>> to this problem?
>
>Well, it's been 5 years since I read the Model 1 technical reference
>manual, but I can still remember some bits of it (I hope)..
>
>Start by opening the case, laying the 2 boards out component-side up, and
>connecting power and video. If you want step-by-step instructions on how
>to do that, please ask. Turn on the machine. Is it fixed? If so, suspect a
>solder-ball short (these were quite common one some batches, I believe) or
>a problem in the keyboard cable, which carries the Z80 bus. In fact,
>checking that keyboard cable for continuity (with the machine off, of
>course) wouldn't be a bad idea.
>
>OK, still not fixed? Check the PSU. The 0V line is the -ve side of the
>largest capacitor on the board, and all the supply rails go to the DRAM
>chips (-5V on pin 1, +12V on pin 8, +5v on pin 9).
>
>Now switch off and pull the shunt block (it looks like an IC, but has
>metal shorting bars on it only). That disables all the DRAMs. Turn on
>again. On a Level1 machine (and I believe a level 2 machine, but my manual
>isn't that clear) you'll get a 32*16 display of colons. If that occurs,
>then you've probably got RAM or RAM addressing problems.
>
>Switch off again, and pull the ROMs (or on a level 2 machine, pull the 24
>pin ribbon cable from the ROM socket). Turn on again. The Z80 data bus is
>pulled high, so the machine executes continual RST38 instructions and
>fills all of the memory with 39 00 (the return address, of course). The
>display will fill with alternate '_at_' and '9' symbols, in 64*16 mode. If
>that works, then you have ROM trouble, I think.
>
>If you still don't get the right display, reseat the Z80, and then use a
>scope/logic probe/LogicDart to find out (a) what the Z80 is doing, (b)
>what the buses are doing and (c) what the video controller is doing.
>
>
>>
>> Thanks
>> Charlie Fox
>>
>>
>
>-tony
>
>
>
Received on Mon Nov 24 1997 - 05:14:15 GMT