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Thread: Sol-20 S-100 bus dilemma.

  1. #1

    Default Sol-20 S-100 bus dilemma.

    I have an apparently normal working Sol-20, with no cards plugged into the S-100 bus.

    I note, that just at the moment of power up, for less than about 50 to 100mSec, a brief faint image appears on the CRT, of what appears to be initial video ram data, before it vanishes and the computer starts normally.

    I have three 16k processor technology (PT) ram cards, each checks out normally on a memory test protocol, or with entering and dumping data. With the three cards plugged in (each correctly set to cover the 48k address range) just once in about 20 computer power up's the initial video ram data appears and stays on the screen, the computer then being locked and immune to pressing the upper case and repeat keys (that sends a hard reset to the 8080 cpu). I have also checked the voltage regulators on these boards and added extra filtering on them with no effects. Also the power supply is normal to testing with no RFI or any measurable issues.

    If another (4th) S-100 card is added, while the three PT 16k memory cards are present, the problem is worse, in that then most of the time it boots up in a locked up state (picture attached) . If I change the video ram IC's it comes up with a different pattern of symbols. Oddly if I change the CPU IC (with an IC still known to work) it comes up with the 9090 repeating pattern seen when the Solos personality module is not plugged in.

    I also observed that if the S-100 boards are powered first and I power the main board second, there is no problem. Also if I de-power the S-100 bus (by disconnecting its power connector) a similar fault occurs in that it locks up on power up, presumably because the S-100 board is loading the bus in its un-powered state.

    It appears as though a total of 3 or more of the PT 16k boards affects the bus in some way in that if the loading on the bus is enough, just at the moment of power up, this lock up problem occurs. One thing I cannot explain yet is why, in this state, the CPU does not appear to respond to a hard rest on its reset pin (the CPU's voltages and clock pulses seem ok) and why in this state the video ram does not clear.

    In addition, I have another generic 16k memory card it does not seem to affect the bus as much as any of the other 3 PT 16k cards. Also I observed that, say running the three 16k PT cards, with the Northstar controller card, unplugging any one of the three PT cards reduces this initial turn on problem. In other words it does not appear to be due to one of the PT 16K ram cards in particular.

    Has anyone seen this effect before with three PT 16k cards plugged onto the bus ?
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    Last edited by Hugo Holden; October 26th, 2018 at 03:12 AM.

  2. #2


    I found the problem after burning the midnight oil and for a while I thought it would never be fixed. It wasn't an easy one. It turned out to be the cumulative loading effects of the three 16KRA ram cards on pin 99 of the bus (the /POC signal) used by PT, and not used by Seattle Computer on their 16k cards. The loading upsetting a 8T97 buffer IC U77 that is also used extensively in the clock & timing circuits. In case anyone else strikes this issue, I just wrote it up. Not all SOL-20 computers might display this fault as it is borderline and depends on a particular IC specimen and would mostly require three 16KRA cards plus one other card loading the /POC line present on the bus.

  3. #3


    Great inspection, research, and repair! That can be a very rewarding dopamine hit.

    In regard to 8T97ís: I canít tell you how many times a degraded 8T97 has been the root cause of an intermittent or strange problem Iíve had to hunt down. The 8T97ís have a penchant for ďfailingĒ without totally failing. I go straight to a 74LS367 replacement and donít bother with another 8T97.


  4. #4



    That is very interesting about the 8T97 IC's being degraded but still working. My tests demonstrated it produced the malfunction with a load capacitance a little lower than it was characterized for on its data sheet, which suggests that this is in fact a degraded IC at least to some extent. And the 74LS367 was much better as a buffer in the U77 position.

    For the most part working with TTL IC's I'm not very used to the notion of relative failure, as they tend mostly (at least for typical 74 series ones) to work or be faulty. Relative failure would show up though in cases of higher loading on the IC's output. Also, in this IC, buffers are linked together to some extent by the internal tristate controls (even though the inputs to those are not used). I tried loading the buffer that the offending one was associated with (pin 13 & 14) and that also produced a similar effect, so maybe there is some cross coupling via this circuitry inside the IC.

    What you have said agrees with the observations & test information. It is upsetting too, as I have always been very fond of vintage Signetics TTL logic IC's. I wonder what has happened to them at the transistor junction level, most likely something to degrade the transistor's hfe. It means though, that a SOL-20 restoration should probably include replacing ALL the 8T97's with 74LS367's.

    Are there any other TTL logic IC types where you have observed this relative degradation ?



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