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Thread: Frustrating Problem

  1. #131
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    I have one socket that is a little suspect. I have the front panel board plugged into that one. Yet the contacts all brightened up nicely after cleaning. I used a small piece of cardboard, that I measured about 0.045" thick, to feel the drag on each front to back contact. Didn't find anything that seemed bad. Each board is about 0.070" thick. That should give plenty of wipe.

    After reinstalling the mother board. I rechecked the power supply voltage. I replaced the filter caps and diodes about 5 years ago, when I first dragged this out. Figuring that they would not hold up. I have a little over 8 volts at the motherboard connection and I'm drawing just about 2.5 amperes. I checked the regulators and there is no noticeable oscillation or voltage dips during operations.

    I still believe that there is an intermittent some where, because I can not repeat the fault exactly. Some times I'll load 2 sectors and then sometimes many more, prior to the timeout waiting for the interrupt.

    I think I'll take a few days away from this and maybe something will come to me. Thanks for the help Mike.

  2. #132

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    Quote Originally Posted by Mike_Z View Post
    I still believe that there is an intermittent some where, because I can not repeat the fault exactly. Some times I'll load 2 sectors and then sometimes many more, prior to the timeout waiting for the interrupt.

    I think I'll take a few days away from this and maybe something will come to me. Thanks for the help Mike.

    When you re-built/wired the new controller board from scratch, did you populate it with a fresh lot of IC's from elsewhere, or did you transfer some or all of the original chips to it ?

    The reason I ask is that I struggled for over a week with an intermittent fault on my SOL-20's keyboard. Mostly it was working, randomly it stopped and was non responsive, but that might only last for minutes then working again, very frustrating too. It was not affected by mechanical forces, vibrations or temperature of any of the components, the power supplies were perfect.

    I finally found, by having the scope permanently connected to it, that some of the keyscan signals, at the time of the fault vanished. Again having to wait to find out why I traced it to a faulty counter IC where one of the FF outputs just was randomly disappearing. Again, not affected by temperature or anything else. So these 74 TTL chips can do odd things like this. I don't fully understand what the mechanism of this would be inside the actual IC, especially the random nature of it and it not being affected by heating or cooling, but it can happen for sure.

    So it might be somewhere there is an IC doing this, either on your board or elsewhere. Its much easier if an IC just dies.

    Though that probably doesn't qualify as the most interesting TTL IC fault I have seen. There are two others I had to tackle with my vintage computers.

    In one case a NAND gate in the sync generator chain on one of my Matrox video cards had failed, but it was working too. It was totally ignoring one of its input pins and acting as though that pin was high always. So the output was responding just to the input on the other pin, and a reset pulse was getting generated when it shouldn't have been. It was almost as if, inside the IC, that pin connection had gone O/C and assumed a logic high.

    In another case, a 74 series IC on one of my NOS YE-DAT 5.25" disk drives was putting out random square wave pulses, like clipped noise, onto one of the drive cable connections, inhibiting the other drive on the cable, but on its own this drive was still working apparently normally, which is probably how it slipped out of the factory.

    So what I am getting at is there does seem to be a very large range of failure modes for 74 series and other IC's and they can be intermittent too. These are the hardest to track down. But never give up, you will nail it in the end.

  3. #133
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    I used the same chips. Although I have replaced the 8272, FDC, 8237, DMA, and 8259 PIC, with no effect. The other logic chips got mixed up, meaning that they were not placed back in the same place.

    Since the trouble seems to be in the interrupts, not getting them during the execution phase, I could look at changing those. I'll have to make a list and try it, Thanks Mike

  4. #134
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    Well..... I just could not keep away or stop thinking about this problem. While watching football on TV, I thought that maybe the problem really is with my S-100 sockets. I had cleaned them and checked continuity. They appeared OK. Since I was having trouble with the Interrupts and the fact that I only use IR2, maybe if I jumper it out that would prove the contacts where either good or bad. I have the DMA controller on my FDC board and the Interrupt controller on the CPU board, so the IR2 signal must take the bus to travel between the two. I had made a circuit to test the PIC and those interrupts worked over and over for an hour or better with no errors, but this external signal went directly to the CPU board and did not have to maneuver across the bus. So I made a short jumper wire that directly connected the 8272 INT pin directly to the 8259 IR2 pin. To my surprise CP/M booted up and ran with no errors. I ran some memory check programs and my memory is OK. WordStar ran and I could read edit and save documents. This morning CP/M fired up again without a problem. So I'm mildly confident that my S-100 sockets on the motherboard are the problem. Now, I am a bit cautious, I've been led down this path before. CP/M did in the past start and stop for long periods of time, but this time I think I actually found a problem and bypassed it. Time will tell.

    Next, I'll have to find some new or better S-100 sockets. Anyone have any ideas where a guy could purchase some new ones? Thanks for all the help and suggestions. Let you know if my machine fails again, most old equipment does sooner or later. Mike

  5. #135
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    I've used these guys for oddball or obsolete connectors. Looking, they have several 50 pos/100 pin 3.18 mm spacing possibilities for pretty good prices. All name-brand NOS. Just be careful that you grab with 3.18mm/0.125 spacing ones and not the 2.54/0.100 ones!

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