I've been playing around with one of my old PCs recently, an IBM PC 340, and at some stage in trying to get a compact flash adapter to play nice I hit a weird issue where the system BIOS (a Surepath BIOS with next to no settings) started reporting the HDD size incorrectly as ~350MB instead of 850. I decided to reset the BIOS, and since I had to take just about everything out of the machine to get to the jumper I also upgraded the CPU from a P100 to a P166. While doing that I neglected to move the reset CMOS jumper back to it's standard position and powered up the machine.

I realised something was up when aside from getting the "settings have been reset" message, which I expected, it also complained about the battery, which I know is good. It did correctly report the new CPU speed and HDD size though. I powered it off, moved the jumper, and fired it up, only to get no display. Now it seems I don't get a display or any POST beeps unless the clear CMOS jumper is in the clear position or not on the board at all, in it's usual place the machine is just seemingly dead.

Based on what I'm seeing I'm guessing I've done some damage, but where? Presumably the CMOS chip, but what are my options from here?

  • Well, did you go thru CMOS setup afterwards?
    – Raffzahn
    Commented Oct 25, 2023 at 22:13
  • I did, not that there's much to do - there's really very few options and it configures automatically for the most part. That said, I can only access it with the jumper in the clear position so not sure it actually saves when doing so.
    – Matt Lacey
    Commented Oct 25, 2023 at 22:14
  • 2
    Well, that's too many changes at once. Try figuring out one step at a time what makes it hang - is it the new CPU it cannot handle even if it detects the speed correctly, or is it the CF adapter that is causing trouble, or did the CMOS clear jumper being left too long to reset position managed to discharge the battery to too low voltage. I once managed to stop the RTC clock crystal oscillator from oscillating (by poking it with oscilloscope) and the motherboard did not even respond to power and reset buttons until the battery and mains were removed momentarily. Unplug and try a new battery.
    – Justme
    Commented Oct 25, 2023 at 23:59
  • @Justme yeah oscilloscope can occasionaly provide nice over-voltages if ground is not provided properly prior to measurement ...
    – Spektre
    Commented Oct 26, 2023 at 8:21
  • 1
    @Spektre It was unlikely something simple like overvoltages or missing grounding at any point. Simply the few picofarads of capacitance and 10 megaohm resistance of the probe, when poked to an extremely sensitive RTC crystal circuit, will disturb it enough to make it stop or oscillate at overtones. Which means, touching the crystal pins with a finger is also bad, and indeed, the best idea to recover is to unplug mains cord and CMOS RAM backup battery and wait for any stored energy to decay.
    – Justme
    Commented Oct 26, 2023 at 10:06

1 Answer 1


No, I'm just an idiot who didn't go back and check his assumptions. Maybe this will happen to somebody else, hopefully not. In the end I ordered a spare motherboard off eBay (relatively cheap, so not too painful in this story). When it arrived I looked and the CMOS jumper was in the clear position, or so I thought.

For some reason this prompted me to double check the label on the inside of the case. I knew from documents online etc. that pins 1-2 is normal, 2-3 is clear, but what I'd been assuming this entire time, was that the pins were numbered from left to right when viewed from the front of the case - the jumper pins go across the case. Turns out this assumption is wrong, and the pins are actually 3-2-1. I'd double checked which pins needed to be closed several times, but not sure I ever checked the order of the pins.

The reason I'd been seeing issues with it when I thought it was in the "clear" position was purely because of the changes I was making, causing the BIOS to flag warnings each time. Too many changes, as others stated. Still having issues with the CF adapter, but in the mean time I'll continue setting it all back up. Don't be like me. Triple check the basics.

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