| Posted By
dmurton
on 2020-01-21
00:44:44
|  Plus 4 acting strangely
Hello from down under.
I'm hoping that I can get some assistance in troubleshooting a stubborn plus 4.
It starts ok, displays the correct amount of ram and will run 16k programs fine. Any programs needing more than 16k, machine crashes.
What I have done to this point is
1 diagnose one of the ram chips is faulty. Replaced chip and socketed. Noticed a broken trace leading to the next chip. Repaired broken trace.
2 removed and tested all other chips, which test fine in another machine and an independent dram tester.
3 replace all electrolytic capacitors
4 removed and tested the 74ls257 chips, tested ok.
5 removed all bar kernel Rom
6 tested with a known good cpu
Using the really handy diag264 cartridge it shows a high ram fault $8000-01 addr.
I'm assuming that this means an address fault at location $8000. However, the ram is good, the 257s are good, my next port of call I'm assuming is the pla or the TED or connections between them.
Any suggestions would be appreciated.
Regards David
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Posted By
crock
on 2020-01-21
02:03:32
 | Re: Plus 4 acting strangely
Hi David, the message you see means that a possible addressing issue is identified on A15. For addressing issues, the $8000 is the address line, rather than a explicit location. If you read how the addressing tests work on the Diag264 web page, it might make more sense.
If you've replaced the 257's, swap them over and see if it changes. Then swap all the DRAM's over. If the message does not change, I would suspect another broken trace or dry joint.
The other possibility is that the signals from the TED which bank out the ROM are not working. From BASIC, try POKEing values in the range above 32768 and see what you PEEK back.
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Posted By
SVS
on 2020-01-21
02:05:42
 | Re: Plus 4 acting strangely
Hi,
you should test the TED chip, that is the manager of the RAM-ROM switching over $8000.
Keep in mind that this chip is responsible of the most HW failures.
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Posted By
crock
on 2020-01-21
07:54:08
| Re: Plus 4 acting strangely
Actually, explicitly testing for the RAM/ROM banking operating correctly would be trivially easy to add to Diag264. Will add it to the ToDo list.
If it does turn out to be your TED, I would buy it off you as I intentionally keep a stock of partially failed chips to test with.
Rob
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