i86time
Well-known member
A "You're not as smart as you thought you were" moment (Discs of TRON audio hum)
I purcahsed 30FathomDave's (and a few other's here) UDOT with the speech mod and I've thoroughly been enjoying it. The only problem was the audio. There's a sweet spot, about 30% volume, where there is no hum, but the audio is too quiet for me to enjoy. If I turn it up, I get annoying audio hum. But if I also turn it down to play quietly the hum also increases in volume and is louder than the audio. There was a switching PS in it to drive the -5V needed for the Squawk n Talk. After reading all the posts here about how switcher + MCR boards= hum, I figured I'd be a genius and pull the switcher and use an original -5V auxillary board. So I wire it all up and now there's a constant hum at all levels. I pulled the -5V board out of circuit to check and when I fired it up, guess what I found. The loud hum was gone but the sweet spot in the audio was still there. It wasn't caused by the switcher at all but by something else in the system. If I had just pulled the connectors from the S&T earlier, I'd have probably realized that.
So, how do I get rid of this hum? I did notice that if I only pull the +5 unreg from the power chassis that goes into the -5V board, the hum does subsude a bit. Could that be anything? I did have to use piggyback connectors on the fuse clips, but everything spec'd out.
I purcahsed 30FathomDave's (and a few other's here) UDOT with the speech mod and I've thoroughly been enjoying it. The only problem was the audio. There's a sweet spot, about 30% volume, where there is no hum, but the audio is too quiet for me to enjoy. If I turn it up, I get annoying audio hum. But if I also turn it down to play quietly the hum also increases in volume and is louder than the audio. There was a switching PS in it to drive the -5V needed for the Squawk n Talk. After reading all the posts here about how switcher + MCR boards= hum, I figured I'd be a genius and pull the switcher and use an original -5V auxillary board. So I wire it all up and now there's a constant hum at all levels. I pulled the -5V board out of circuit to check and when I fired it up, guess what I found. The loud hum was gone but the sweet spot in the audio was still there. It wasn't caused by the switcher at all but by something else in the system. If I had just pulled the connectors from the S&T earlier, I'd have probably realized that.
So, how do I get rid of this hum? I did notice that if I only pull the +5 unreg from the power chassis that goes into the -5V board, the hum does subsude a bit. Could that be anything? I did have to use piggyback connectors on the fuse clips, but everything spec'd out.

