First post, by Beegle
I received recently an AdLib clone that has a ton of interference.
Can't really figure out why though, as the usual suspects don't seem guilty this time.
Here's the link so you can listen for yourself :
1. I tried checking if that card uses -5v, something that had given me much trouble in the past on a CT1350 when I didn't know about it. It does not (pin 5 on the underside not present)
2. I tried checking for bent pins on the underside, that could be touching other metal traces. A few were unbent manually. Still didn't do the trick.
3. I tried checking capacitors, but they are very small and I can't see any leaks or "expanded" tops.
4. I tried checking the 3.5mm connector, plugging fully, halfway, forcing sideways. Hum is always present.
5. I tried turning the potentiometer, and the hum increases/decreases accordingly. I'm assuming that the interference is present before the sound gets amplified.
6. I tried the card in another ISA slot, same result.
One small thing I noticed in the video, by the different sounds produced by the initialization of the floppy drives, is that one gives little to no interference (3.5" drive) and the other is clearly audible (5.25" drive).
Since only the 5.25" drive is connected to the 12v power, I'm assuming that the hum we're hearing is a 12v hum.
Curious, I looked at the 12v pins (both +12v and -12v) and noticed that the -12v pin is present (pin 7, image attached), but not connected to any trace leading to the board.
Anyone have ideas of where the hum could come from?