I have a strange problem with a Turbo XT clone motherboard. Originally the motherboard runs at 8 MHz, and shorting two pins switches to 4.77 MHz (there are another two pins to connect a Turbo LED indicator).
This afternoon I installed the motherboard in a case with a VGA card and a floppy disk controller, and run some software. Some minutes after, the board started to lock randomly, so I removed it from the case and started to track the problem, removing all cards and all ram and started to put it again. The motherboard booted, but after POST the turbo led was turned off and it switched to 4.77 MHz. It continues to do it, no matter if the turbo jumper is shorted or not. During memory RAM test I can switch turbo on and off and the speed changes (ram count speed is affected and also does the Turbo LED), but after POST beep the speed is locked at 4.77 MHz and the turbo LED doesn't lit again. A Ctrl-Alt-Del sequence make the turbo switch mechanism to work again just before the BEEP, then it is disabled again.
I have tried to:
- Replace the 8088-2 with a V20
- Replace the power supply
- Install only some banks of RAM
- Remove all cards
- Replace the original BIOS with another different (AMI 8088 BIOS)
and the behaviour remains the same. What can be causing this problem?
Here is a picture of the board:
Click to enlarge
This afternoon I installed the motherboard in a case with a VGA card and a floppy disk controller, and run some software. Some minutes after, the board started to lock randomly, so I removed it from the case and started to track the problem, removing all cards and all ram and started to put it again. The motherboard booted, but after POST the turbo led was turned off and it switched to 4.77 MHz. It continues to do it, no matter if the turbo jumper is shorted or not. During memory RAM test I can switch turbo on and off and the speed changes (ram count speed is affected and also does the Turbo LED), but after POST beep the speed is locked at 4.77 MHz and the turbo LED doesn't lit again. A Ctrl-Alt-Del sequence make the turbo switch mechanism to work again just before the BEEP, then it is disabled again.
I have tried to:
- Replace the 8088-2 with a V20
- Replace the power supply
- Install only some banks of RAM
- Remove all cards
- Replace the original BIOS with another different (AMI 8088 BIOS)
and the behaviour remains the same. What can be causing this problem?
Here is a picture of the board:
Click to enlarge