Hello all.

Struggling with a problem that has introduced itself to my 486. This system used to work perfectly, with the following setup:

-TMC EISA VLB board (PET48PN), 16MB RAM, 486DX/50, 512KB cache
- QDI QD6580 VLB controller (IRQ14)
- Tseng ET4000 W32i
- Voyetra V-4000 (330h, IRQ3)
- Adaptec EISA SCSI interface (IRQ11)
- 3com Etherlink III 3C509B (280h, IRQ10)
- Gravis UltraSound Classic (240h, IRQ7, DMA3)
- Sound Blaster 16 CT1740 (220h, 300h, IRQ5, DMAs 1&5)

COM1 is configured with defaults (IRQ4, 3f8h), COM2 and parallel ports are disabled. Obviously the SCSI card is configured with the correct CFG file. It all worked perfectly and I was very happy.

I then embarked on a benchmarking project, which involved the installing and uninstalling of various cards and CPUs. After completing this I restored the system back to its original condition and... the mouse didn't work. Obviously I removed everything I could (keeping the graphics card and the controller) and the mouse worked again. I subsequently realised it happens consistently when I install *any* kind of sound card, modem or video capture card. Not the MIDI card, NICs, graphics cards or anything else. So what do these problem cards have in common? They have an audio DAC and they use -12V. There may be other similarities, but I can't for the life of me fix the problem now. I tested the -12V rail with and without a sound card installed and there is no change. I measure the expected -11V on pin 2 (TX) of the serial port.

I have tried VLB and ISA hard drive controllers. I have tried ones with integrated COM ports and I've disabled them and used standalone COM cards. The same effect is observed. Obviously I switched the mouse. The weirdest part is that mouse drivers (I tried Logitech and CuteMouse) load and recognise the mouse, but when I open any program that could use it (such as games, EDIT, MSD, even Windows 3.11) the cursor is static. I have at times observed the cursor 'juddering' when it's not working.

Why would it now not work when it worked fine before? It's the weirdest (and yet, seemingly, simplest) problem I've ever encountered and I can't crack it. Any ideas or help would be greatly appreciated.