Posted By
PaulK
on 2018-08-13
09:41:57
|  Plus 4
Long time since I last posted, think it was over 9 years ago. Anyway received a Plus 4 today which was supposed to be fully working but as what normally happens it displayed a black screen. On opening it up I noticed the U19, 23 & 25 rom chips were missing. So before I try a different Ted etc would these 3 chips missing give the black screen fault
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Posted By
gerliczer
on 2018-08-13
10:22:50
| Re: Plus 4
U19 is the PLA. It is responsible for genearting chip select signals for peripherals like ACIA, Parallel port 6529B, keyborad latches, and ROM select logic. U23 is BASIC ROM and U25 is FUNCTION LOW ROM. There's no chance for your machine to start, as far as I can understand.
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Posted By
PaulK
on 2018-08-13
11:23:28
| Re: Plus 4
Thank you
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Posted By
MMS
on 2018-08-13
14:48:14
 | Re: Plus 4
Maybe worth to buy a REALLY defective Plus/4 or even C16 really cheap.
Usually the TED or the CPU gives up, sometimes PSU or RAM.
I think PLA is the Top3 or Top4, but usually ROMs are OK.
So, I think there is a high chance(>70-80%), that from one or two really defective C16s/+4 you can repair this one's PLA + ROM
(and there would be a low chance to have finally a spare TED or a spare CPU).
I think BASIC ROM is equivalent in C16, PLA is the same, the U25 I am not sure. (I am not on my normal PC to doublecheck document)
BTW there is a retrofit (more reliable) PLA available, but absolutely not cheap.
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