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Thread: PC6300 no longer POSTs

  1. #1
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    Default PC6300 no longer POSTs

    I had my AT&T PC6300 working and booting before I put in storage last October. It was a fully punctioning system.
    Before I put the system away I double checked that the battery was good. It was reading 2.8v and showed no signs of leaks. The drive was then parked and put away for storage on its side (drives closest to the floor).
    I pulled it out the other day and tried to boot it and knew something was wrong. When you power on a 6300 the keyboard LEDs will go from solid to flickering, then they will go out and the system will show something on the screen (POST data). This time however the lamps remained solid and nothing else happened.
    I opened the system and discovered that in five months the battery had gone dead and leaked. There wasn't any visible trace damage but the area around C10 to C0 (board is arranged into a grid) all showed light signs of corrosion. I pulled the system apart, cleaned the pins on socketed chips and edge connectors, washed the mainboard and left it to dry over a heating register and cleaned the video card. In the process I found the corrosion had gummed up the reset button and it was stuck on. Some contact cleaner and TLC fixed it.
    After letting the boards dry (completely) I reassembled the system and tried again with my POST card installed.
    The POST card constantly reported FF (wrong because that means the ssytem is booting) but had the right voltages and blinked the CLOCK light (and in turn the keyboard controller started flickering the LEDs again so that was working again too) but otherwise nothing else happened. It's now showing the same symptoms I saw on my Olivetti M24: Power is fine and visually it's okay even though the battery had leaked however it was otherwise doing nothing else (though in this case I actually had a keyboard and monitor).
    Components that show corrosion on their pins are a 0N74LS08N, two M5M4256P memory chips soldered to the board, two socketed MB8264A-15 memory chips, and a diode. Other damaged parts include the power LED no longer works (but receives +5v) and the speaker got hit pretty hard (no clue if it's still any good and if I'm missing a error beep or something)
    I can't confirm if the monitor is getting power. This particular monitor never gave any high voltage squeal when it was on though visually it was working.
    = Excellent space heater

  2. #2
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    Odd--I've liked the 6300 setup because the battery is normally upside-down, so you'd think that any goo would just drop onto the bottom cover plate. Guess not. On the other hand, I've had the speaker drop off its sticky tape and break its leads--I just replaced it with a small piezo unit.

    Does the flickering stop? If so, I'd hook up another speaker to see if it's trying to tell you something.

  3. #3
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    Flickering never stops. It should though as the POST tests progresses. The flickering however isn't because the BIOS is testing the keyboard controller I think. It's probably the clock signal. You can have the EPROMs removed and it will still do that so I assume it stops because the BIOS is chattering to the keyboard controller.
    I attached another PC speaker and got not a chirp out of the thing.

    Like I said, the only reason I got battery damage was because I stored the system on its side. If I had known this was going to happen it would of been stored the regular way.
    = Excellent space heater

  4. #4
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    Let me guess--you stored it so that the battery was on the highest end? :facepalm: I'd probably start by taking the mobo out and inspecting traces under each IC where the electrolyte leaked, even if it means removing the IC.

    Do you need a schematic?

    Lotsa fun...

  5. #5
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    Stop reading my mind. I facepalmed too when I realized who unwise my decision was but like I said, I swear that battery was fine before I stored it. If it was the other way around it would of dropped right off the board. I got no schematics but chip removal should be okay after I borrow my friends reflow station.
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  6. #6
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    Got the chips desoldered and removed. Same with the speaker. Corrosion was cleaned and there was no damaged traces. The speaker fell apart though from a lot of corrosion.
    Soldered the components back down and the problem persists.
    All I know for absolute certain that there is a clock signal. I can't say what else might be wrong.
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  7. #7
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    Well, there is a maintenance manual on the web, along with schematics. Time to get busy, I guess...

  8. #8
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    Damn.
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  9. #9
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    NecroBUMP.
    I've come back to this machine again.
    To catch anyone up with my situation The machine was working when placed in storage. Several months later it was taken out of storage and found that the battery had leaked and dripped down the front 2" of the mainboard. The machine now no longer POSTs. The PCB is visually undamaged from the leak but the power LED has died and the speaker has been ruined. Both have now been replaced. Corrosion was found on two sockets in ram bank 1 and were replaced with new ones. Several chips were desoldered so I could inspect for damage under them and found nothing. I reinstalled the chips with sockets.
    I have two POST cards, one tells me voltages, clock presence and a two digit POST code. The other card displays DMA, interrupts and two digit POST codes as well. +5 and +12 are good, a clock is present however only one card reports a FE code (or 00, or FF randomly when you turn the power on) and the other reports nothing. After a period the INT15 lamp will come on and if left for a long period you will get DMA 7 lit as well. The keyboard controller and POST cards respond to the reset button so that isn't stuck.


    Looking at part of the service manual I can tell that we are not reaching step 3 and I can't tell where in step 2 we might be hanging, if we're even reaching step 2 at all. I do now have a logic analyzer but in this case I am unsure what I should be looking for or at.

    Edited: As a sanity check, I compared my EPROMs to Chuck(G)'s image files and they are okay so the BIOS is not corrupted either.
    Last edited by NeXT; February 10th, 2014 at 12:48 AM.
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  10. #10
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    Quote Originally Posted by NeXT View Post
    I have two POST cards, one tells me voltages, clock presence and a two digit POST code. The other card displays DMA, interrupts and two digit POST codes as well. +5 and +12 are good, a clock is present however only one card reports a FE code (or 00, or FF randomly when you turn the power on) and the other reports nothing.
    I cannot find anything that indicates that PC6300 machines output POST codes.

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