[Icom] Compaq Presario monitor and sound volume control.

[icom@mailman.qth.net]

I have done some experiments with the SVGA signal lines concerning why certain monitors
may kill the sound system On Compaq Presario 5200 series PCs. This problem prevented me
from transmitting RTTY and PSK31 modes.

On the SVGA socket:
 pin 11 = Sense 0
 pin 12 = Sense 1
 pin 4   = Sense 2
 pin 15 = Sense 3

The bit pattern of these sense bits indicate the type of monitor being connected.

I found (for the Compaq Presario) that if pin 4 (Sense 2) is not shorted to ground the BIOS or the 
Win98 sound card driver (Compaq's version) will shut 
off the volume control. This computer was originally designed to be packaged with a monitor with integrated
speakers and a volume control knob. The older monitors did not use the Sense bits and like my
older monitor pin 4 was omitted. I had to insert a small wire into pin 4 on the PC's SVGA socket
and run it to the socket's screw terminal to ground the pin. Now I can use an older monitor and
still have the volume control working.

I hope this will help someone in the future with a weird Compaq product.

Regards,
John Bielefeld
K1JEB