| Posted By
Muzer
on 2021-01-27
17:26:07
|  Plus/4 black screen diagnosis (not CPU/TED/PLA)
Picked up my first Plus/4 recently, and unfortunately it's not working, with the dreaded black screen (it IS outputting a composite signal as my TV picks up the PAL signal, but there is nothing but blackness displayed). HOWEVER, there is a silver lining - I have a Commodore 16 and I've tried swapping out all the socketed chips that are common to both (CPU, TED, the ROMs that are common to both, and the PLA). All these chips work fine in the C16. So I've managed to get a Plus/4 with a working CPU and TED, which I'm reasonably pleased about! I've also tried just plain removing the extra ROM (the one with the Plus/4 applications on it), again to no avail.
Where should I go from here? My next obvious guess would be the RAM chips, though I suppose it could be the 8551. At this point is it worth buying one of those RAM testing ROM chips that I've seen? I've also seen someone with 8551 problems - for testing purposes, if I were to remove this chip entirely (obviously requires desoldering) would the Plus/4 boot without it? Any other suggestions?
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Posted By
Retroshire
on 2021-01-28
06:28:46
| Re: Plus/4 black screen diagnosis (not CPU/TED/PLA)
Last time I repaired a C16 with a black screen, the problem was the Kernal rom. Easy to try though.
Otherwise you can also check the socket of the CPU. Once (other than the above example) I kept swepping CPU's with no results. When putting a CPU replacement CPU (FPGA) suddenly it worked (longer en thicker legs). Though this regards a C16, they sometimes have the inferior sockettype. In a Plus/4 a have not noticed them (yet).
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Posted By
Muzer
on 2021-01-28
07:38:47
| Re: Plus/4 black screen diagnosis (not CPU/TED/PLA)
Thanks! I've already tried swapping out the Kernal with the C16 though and that works fine. Good idea on the socket, perhaps there could be a cracked solder joint or similar? I haven't yet taken a look on the bottom of the board. I've ordered a Diag264 chip off eBay (don't have an EPROM programmer currently so I thought this would be quicker) so hopefully that will be useful.
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Posted By
Muzer
on 2021-02-03
15:15:05
| Re: Plus/4 black screen diagnosis (not CPU/TED/PLA)
Ran the test today, seems like one of the bits is buggered; screen was alternating between the checkerboard and "D" rather than checkerboard and "@". However leaving it running for a while partway through some seemingly-random areas of the screen changed to displaying "?" and the horizontal line graphics character (not entirely sure which one). Ordered a complete set of new RAM chips in case there's more than one having problems, in that case.
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