Hi guys, I have this 286 motherboard:
https://theretroweb.com/motherboards/s/leadma … inc-lm-103fs-16

It came with quite some bad battery corrosion. After a few days of carefully fixing the traces, it came back to life again. Upon first attempts, it just showed a dreaded "CMOS Inoperational" message and refused to go further into POST. I suspected the RTC module (HM6818P, I believe this is a clone of https://theretroweb.com/chips/2644 ). It probed fine, but I noticed very weak signals on the data line between it and the keyboard controller (DAC-PCKB42).

I replaced the keyboard controller with another AMI (KB-BIOS-Ver-F, P8042AHP) and the motherboard booted up properly without any issues, and I was able to run DOS and run benchmarks without issues.

Now I wanted to slow down the motherboard via the turbo switch header. This is JP1 on the board, and it has 3 pins. If I short pins 2 and 3, the Turbo LED lights up. Otherwise it goes off. However, whether the turbo led is on or off, I see no effect on the CPU speed at all. It just stays at full speed (12MHz).
There is another pin header JP2 that also has three pins. If I short 2 and 3, the benchmarks jump to 16MHz (according to TopBench/Landmark/SysInfo).. I believe this is a wait state setting of some sort. But regardless of this setting, the turbo switch is still not making any effects.. I expect it to halve the CPU speed when disconnected.. but right now it just does nothing.

I traced the 25MHz oscillator to go into one of the pins of the GC101A chipset. I also traced the middle pin of the turbo switch header to go to another Headland chipset. This leaves communication between the main and the other chipsets, which is difficult to trace further because I have no datasheets for these chipsets.

If I probe the CLK signal on the CPU, I can see that it starts at 12.5MHz upon boot, until the POST starts loading MS-DOS, then CLK jumps to 25MHz and stays there. So the chipset must be controlling the speed according to some logic/settings. Some BIOS setup programs provide an option to choose the CPU speed at boot (slow vs fast), but the BIOS setup program is quite basic and only offers: time,HDD,FDD,monitor and scratch mem for BIOS. No other advanced options.

I tried the BIOSes from TRW's page, and they're both identical when it comes to exposed options. I also tried MR BIOS, but that one didn't offer any speed controls either.

I know some boards used a separate SETSPEED program to control the turbo functionality. But I always thought the hardware switch would take precedence. Anyways, I'm not sure if this board uses such a tool, so any idea if this is the case here? Or perhaps it is still some severed connection that is preventing the turbo switch signal from reaching its end point? Again, the turbo LED responds fine to the turbo switch..

I'm puzzled 😵‍💫