First post, by rpocc
Hi to everyone.
I have one PCChips M396F V2.7 motherboard, based on SARC RC2016A5 chipset for 386SX. The battery is removed. I test it as well as my other MBs on an open stand without enclosure if that counts. Being otherwise clean It has one annoying malfunction:
Rarely it may coldstart normally but most of the time it requires holding reset shorted for 1-2 seconds (sometimes couple of times, or few short presses), and even after that sometimes it passes the VGA BIOS entry point and proceeds to boot, but sometimes it types few characters or lines from the Trident 9000 boot screen and then just hangs up. Without holding reset it doesn't even start outputting any POST codes. My card shows presence of all voltages, inactive reset, inactive IRDY, active CLK and FRAME.
This board clocks from a standard 14.31818 crystal and achieves 40 MHz with PLL chip.
So far I've tried: another crystal, another crystal overtones bypass capacitors of 22, 30 and 47 pf (original are disk 30 pf), another PLL, another keyboard BIOS (it reboots just before memory test with not-6042 keyboard BIOS but the instable boot remains the same), putting the video card and POST card into different ISA slots, testing continuity and voltage between CPU reset pin and the chipset, continuity between reset jumper and ground, chipset, continuity between powergood and chipset. Found absolutely nothing.
Didn't recapped the MB though it shows total of 100 uF at 5V and 3x10 uF at other power rails, having total 13x 10uF capacitors, so at least the initial values are normal and all of them appear to have no visible signs of failure.
In theory I could put some MCU like ATMEGA13A onto reset pin and put a simple timer-driven forced reset as a workaround but there's no simple pattern providing algorithm for guaranteed start without handing at one or another moment.As usual there's a simple choice: to forget and tear it down for parts (Not so much of universally useful parts) for other MBs or keep trying searching the cause of the problem but I'm out of ideas. Don't have another SARC chipset and really wouldn't liek the idea of replacing the CPU.
Any ideas, why motherboard could act like that?
