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TRS-80 Model 1 Repair Help

M

musovern

Member
Joined
Mar 2, 2011
Messages
13
Location
New Zealand
Hi there.

I have brought myself a Model 1 26-1001 looks like its been upgraded to 16k (4116 ic's) and Level II its a 2 ROM set on the pcb board.?
after making a power supply i plugged it in and it started up.
I then had a video sync issue which i found a you tube fix for. thank you.

It now has strange characters on the screen. looks like

!#!#!#!#!#!#!#
!#!#!#!#!#!#!#


full screen ,etc

Has anyone got any ideas what I should check next.?

I don't have any spare RAM to substitute yet. Will look on ebay.

Kind regards
Vernon
 
Hi Ian,
I re checked the wires coming from the ROM board to address lines and did a re-solder of them. Now I have a picture.
I followed the Selection Isolation Flowchat. Figure 16 in the SAMs book. Removed shunt Z71. Now its shows the number 3's over the whole screen. Not sure what that means.?

If i un plug ROM there is no picture. Does this mean i should be looking at CPU type problem.?
Have checked that i have a 1.774Mhz clock and i do.

Any idesa anyone.?

Thank you
 
Last edited:
At this point when I'm repairing these and I have a clock signal going to the CPU, I try a known good set of ram and a known good CPU. If that doesn't help then you need to pull the rom cable and go though the information Larry has pointed out in order to find where to go from there.

Leave the roms cable out until you get the flickering @9@9 screen, there is no need to plug it back in until that point.

Ian.
 
Hi Ian. thank you for your help.
I'll order some RAM IC's 4116. Should I get some 2102A as well.? For video RAM just in case.

I changed the CPU to a known good one. And the screen now shows 9999 across the whole screen. So happy I've made some progress.

Cheers

Vernon
 
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