alternator whine

[ka7wnf]

I have the following radios in my work truck, Motorola Spectra 800 Mhz trunking, TK-705, TK-730 and a dual band TK-630/TK-830 set. All get DC directly from the battery. The Spectra and the 30's also get ignition sense power from a tap in the fuse block. The TK-705 and the Spectra have nice clean TX audio, but the 30's have a horrible alternator whine with the 630 being the worst. Are the 30's more susceptible to alternator noise? I have regrounded them, added a noise filter, moved the cables and radios and only reduced it slightly. Any ideas? The 630 had nice audio when it was a single radio with the control head mounted right to it and did not need ignition sense. Perhaps it's getting in through the ignition sense but the Spectra doesn't have the problem.

[k2hz]

I have seen that problem in Kenwood "30" series dual band installations in some large trucks. The Kenwood KLF-2 line filter always fixed it.

What type of filter did you try?

[ka7wnf]

Some POS Radio filter I found in my junk drawer. I'll look for that Kenwood filter. Thanks for the info.

[ka7wnf]

PROBLEM SOLVED!!! After days of trouble shooting here is what it was. In the KRK4-DB adapter there is a 12 volt DPDT relay that switches the mic audio hot and ground from radio 1 to radio 2 as needed. Someone had soldered a jumper across the mic hot side contacts (came from Ebay so no idea who did it) so that when I transmitted on radio 2 it was actually picking up line noise from the wrong radio. Since only the hot side was jumped and the ground was open the cable acted like a huge noise antenna. I removed the jumper and found the relay was not switching. I tested it with an external 12 volt source and it came back to life. Put it all back together and HOLY COW it all worked. No more alternator whine and clean TX audio from both radios. If it stops working again I'll replace the relay. By the way does anyone have a schematic for the KRK4-DB?

Peter.

[ka7wnf]

Well today I figured out why the TK730 had alternator whine. I was testing a spare mic and discovered that the whine was gone when I tried it. Plug the original back in and it whines. So I took it apart. Someone had soldered a wire from the hangup button to mic ground. Probably in an attempt to enable off hook scan. Which in this mic didn't work anyway. So if all else fails look for someone elses dorked up "repairs". I removed the wire and it works great.