Dead B+128

[tom_seddon]

(A dear diary kind of thing. Any suggestions welcome, but even if nobody has any ideas I'll be poking at this as I get the chance, and updating the thread with any additional info.)

I've got a B+128 that suddenly died. It was working fine, and I was testing some ROMs in sideways RAM that I was planning on writing to EPROMs. All looked good, so I programmed the ROMs, and plugged them in, and powered it on, and: brrrrrrrrrrrr Unfortunately removing the ROMs didn't help...

With the standard MOS ROM, it never boots up properly, but sometimes the display will get initialised to something vaguely sensible-looking, in that there's a flashing cursor and it looks like it might be one of the standard modes (but exact mode is not consistent).

After poking about with some test code in an EPROM in the MOS socket, it looks as if the system is actually basically working, but it seems that bit 1 of RAM is just noise. I got a bit lost trying to get any further than that with my logic analyser though; the exact timing for valid outputs on pin 14 of the DRAM ICs are a bit fearsome, and trying to connect the clips in that region is a huge pain due to the B+128 riser board. At least it isn't bit 7 I suppose.

For now, I'm not going to overthink this. I'll order some replacement 64 Kbit 120 ns DRAMs and some sockets, then replace the offending item, and see what happens then.

--Tom

[eelco108]

That sounds familiar (with random startup problems, or no startup) - I had the same problem with one of the RAM bits (not bit 1), also just noise. The desoldering was the most annoying part, but it is certainly fixable.

[tom_seddon]

I wasn't sure that random noise was a usual failure mode for DRAMs, but if you've seen it too, then that's promising! Thanks for the note.

I've got a bunch of bits on order now, so I'll get desoldering when they arrive. I'm not looking forward to working around that stupid riser board - but at least it's all standard through hole stuff. Just a question of being careful, I hope.

--Tom

[eelco108]

I'm not sure it's a 'usual' failure mode, but you're certainly not the first ...

Best to socket the new DRAM chip, although the next failure will surely be one of the other ones

As for desoldering - I just cut all the legs of the faulty DRAM chip (as close to the body as possible) and then desolder all the leg remnants one by one. Much easier - just heat and pull out with a (small) plyer.