Tuesday, July 31, 2018

Plus 4 Adventures - Part 2

If you read the Plus4 Facebook group you'll know that I had all but given up on my Plus4. It displayed garbage on the screen at startup. Pressing reset occasionally got back to the boot screen but then garbage would return almost instantly. After a few minutes things would get progressively worse until, after turning it off then back on, only a black screen was displayed. The list of things I did is below for posterity. It was heading for the bin...

Then, in stepped Ian from Mutant Caterpillar...

Ian (not to scale)

Long did I wander, searching for the mystical stranger, known only as 'Ian'. Through valleys and forests, across mountains, down the A44 (turn right at Rhayader), until the distant horizon stretched out before me into the deep, foreboding ocean. Wind whipped around me as my hands clasped the precious package, strange smells emanated from open doors, mixing and polluting the air (the local KFC - well worth a visit). I knew I must continue - after I'd finished my boneless bucket - for the end of my journey offered hope of redemption and a new life. Finally, I came to his door and threw myself on his mercy.


And, Lo! he did take my Plus4 board and it was re-made in the forges of Aberystwyth! And strong was the tape loading, and long was the soak test! And through it all it became known as 'working again'!

My board with 8551 removed
(Pic from Ian on FB)

Loading from tape!
(Pic from Ian on FB)

Yes, Ian at Mutant Caterpillar fixed my Plus4 in less than a day. He traced the fault to the 8551 I/O chip spewing random crap on to the data bus. There was also a dodgy RAM chip (not a surprise). I had thought about that particular chip - the 8551 - at some point early on in the process of all this but I'd not done anything as it didn't really connect to anything that I was going to use..

So, what have I learned in all this?

1) If you have a computer that needs repair you seriously can't do better than Mutant Caterpillar
2) No, really, re-read point 1.
3) Plus4's are delicate beasts but, ultimately, worth the effort as they're getting rare (in a working state)
4) Don't discount anything unless you've tested it!
5) Did I mention how great Mutant Caterpillar are?


My escapades retained for info below:

My Plus4 is faulty and I'm having a hard time finding the fault. Note that it runs a 6510 rather than an 8501. The 8501 was identified as dead early on. I also have a replacement kernal which includes modifications to accept the 6510 (in case you were wondering).

Here's a video of the fault in action:

Although I said it dies, it actually lives... A while after this video was shot I tried turning it on again and it worked for about 5 seconds before I got a repeat performance.

So, things I have done, not necessarily in chronological order:

1) Checked the power supply - there is a steady 5.23v coming from the supply which reads as 5.13v at each of the chips. There is just over 10vac on the other part of the supply which shows up OK on the board. Power supply seems OK.

2) Just in case I ran it with just a straight 5v 3A supply. Same symptom as in the video.

3) Replaced TED socket. The original was a single wipe and I couldn't see the condition of the contacts so I just replaced it with dual wipe socket. No change. Same symptoms as above.

4) Replaced TED. At least, a known more or less working TED. The 'new' one I have does not like joysticks but otherwise operates as expected in a working machine. No change. Same symptoms.

5) Replace CPU socket. Although the 6510 adaptor board has nice turned pins, the original socket was another single wipe affair. I replaced it with a shiny dual wipe socket. No change.

6) Replaced TED socket. Same again really. The original is a single wipe and I replaced it with a dual wipe. No change.

7) Removed 3+1 ROMs. These prompt a white screen when run anyway so I just removed them (but keeping them safe!). No change.

8) Replaced PLA socket. Now out of sockets. No change.

9) Obtained a Diag264 cartridge. This seems to start but then stops after a few seconds presumably because of the fault.

10) The Diag264 cartridge managed to run almost a complete set of tests before the fault appeared. Here's the screenshot:

11) Re-capped it. More or less. This is controversial but at least if with re-capping it then I will not be wondering if an out of spec cap is causing all my trouble.

12) Start looking at the RAM (more unobtanium). The strong suggestion is a RAM fault, so I may do this first. Strangely, the MT4264 chips in my Plus4 are actually 4164 1*64 DRAM which I think I have few of somewhere... I didn't but I managed to get some from Retroleum.co.uk.

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