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28110Re: vintage SRAMs self healing

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  • s100doctor
    Nov 14, 2012
      --- In midatlanticretro@yahoogroups.com, corey986 <no_reply@...> wrote:
      > Tom,
      > I have the same test setup as Mike. I'm not sure it's capacitance causing the "failures", for all we know it's helping revive the chips ;)
      > I have tested known bad chips and had this happen. Some seem to "walk" the memory test failure point from a restart of the test failing at different addresses each time, eventually not failing afterwards and being good. Not all my bad chips but enough to be suspicious. I also know it's not dirty leads on the chips, because first thing I did with the bad memory card after I started having problems was remove the chips and soak them in ISP and brush the leads with a tooth brush. Only fixed a single chip out of a lot of 64 on a card with at least 18 known bad chips...
      > So I pulled out the 2102 ram tester setup and started testing the ram that was in the card individually. That is when I saw this weird effect. I know the ram tester worked, because I tested a bunch of NOS ram before I installed it in another card and then ran a system level RAM test and all was good. The same in system RAM test that failed on the card that caused all my recent testing.
      > I'm really just wondering if no one has seen this before because if they had a bad chip, they throw them out.
      > Cheers,
      > Corey---
      I had not realized, you (both) were using a custom one-chip tester.

      Also - what's "ISP"?

      I'm impressed with the effort you made, to program up a 6522 to test static RAMS. Both an 1101 and now a 2102. Nice go/no-go testing. But what I see in the tester (for those who didn't read the linked page, even I haven't run through the software in detail), is that you have a parallel I/O chip being driven by software, which drives one RAM chip; and you have the chip in test at the other end of several inches of flatcable, and in a protostrip.

      What that tells me - and this is not a complaint, it's a description - is that the RAM chip is being run VERY slowly, well below spec; and there's relatively plenty of capacitance on every pin; and possibly a certain amount of crosstalk between those lines. An actual read/write cycle time could be measured with an oscilloscope; that could also show any crosstalk (incorrect signals or timing). I'd be curious to know the "speed".

      So I'd call it a "go/no-go" tester because it doesn't test for speed, only for function. I"d imagine that any chip which failed that tester, would have to be near dead. But any chip that succeeds, may yet fail if run on a memory board at CPU speed.

      another problem with comparing results under "use", is that some S-100 RAM and CPU cards were not terribly well-designed, or are set up for wait states. So chips that fail ONE board or CPU, may run on another. Even putting a terminator on the S-100 bus can make a difference. I already said Z8's and 8080's run RAM differently.

      Thanks for calling out evidence that it's not "corrosion". If cleaned chips don't run better than "dirty" chips, to a few percent gain, that seems to remove that consideration. There's all kinds of corrosion issues in 1970's technology.

      Again - that's a nice little tester. Go/no-go rapid testing of individual RAM chips is very useful, and can be adapted for different chips as needed. I'd like to point to your tester from my Web pages on repair/restoration.

      Herb Johnson
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