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Model 4 just began displaying a screen full of 09090909

Dokken

Experienced Member
Joined
Feb 27, 2018
Messages
127
Location
TN
I've been working on a model 4 trying to get the floppy drives working. the computer has worked fine, booted fine until this afternoon. I turned it on and the screen was full of 09090909.

Any thoughts on where to look?
 

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TXNathan

Experienced Member
Joined
Mar 15, 2018
Messages
84
Location
DFW
My model 3 did a very similar thing and it was a bad ram chip. I replaced the set with some from Mav but I tested after each and ultimately found that it was one
 

ldkraemer

Veteran Member
Joined
Mar 14, 2013
Messages
2,096
Location
Chaffee, MO
Here is where I'd start looking. On Startup the SPACE is written to the Display to CLEAR it.

SPACE = 0x20 = 0010 0000

09 Displayed and repeated, which is 0x30 0x39

Code:
0x20    0010 0000
0x30    0011 0000
           ^
    Which is Bit 4 or Ram IC 04 (Starting from Right to left RAMIC{0..7})

 
0x20    0010 0000
0x39    0011 1001
           ^ ^  ^
    Which is Bit 0, Bit 3, and Bit 4 or Ram IC's 0,3 & 4 (Starting from Right to left RAMIC{0..7})

You might try carefully removing those RAM IC's and Re-Inserting them in the IC Sockets.
If you get the same results, try replacing them, or maybe swapping Ram IC 4 with RAM IC 1,
in which case you will likely get 0x2B = +

+++++++++++++++++++++++
+++++++++++++++++++++++


Do you have Private Messages turned OFF? I can't PM you???
Larry
 
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Dokken

Experienced Member
Joined
Feb 27, 2018
Messages
127
Location
TN
thank you both!! I read last night on Ira's site that garbage on the screen could be RAM or the Z80. I have a chip tester, and all the RAM chips passed. I tested the Z80 and it failed. I had a spare, put it in the model 4, and the computer works again!
 

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Robbbert

Experienced Member
Joined
Jan 10, 2019
Messages
318
That was quick. I was dreaming up all kinds of reasons for the failure, but you've fixed it already. Well done!
 

Eudimorphodon

Veteran Member
Joined
May 9, 2011
Messages
4,403
Location
Upper Triassic
Way back in the late 1990's I was in the middle of playing with my Model 4P when it just froze and henceforth was dead. I didn't have much in the way of diagnostic tools at the time but as a shot in the dark I pulled the Z80 out of a Timex Sinclair 1000 that was literally a doorstop and swapped it in, and the 4P lived again. Apparently Z80s just die sometimes.
 

Dokken

Experienced Member
Joined
Feb 27, 2018
Messages
127
Location
TN
Finally, my fully working Model 4 with 2 working disk drives.

model4 1.png

And my spare to continue working on. Something is up with the floppy control. Machine boots fine, but can't get a floppy or gotek to be recognized. Pretty sure it's not the FDC card as I have a spare.

model4 2.png
 
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