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Squealing Sounds On Several Kenwoods

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kenwoodgeek

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This is just more of a question out of curiosity, since this isn't my problem to fix.

Several of the local school bus company radios transmit these weird squealing sounds while the drivers are talking. These sounds are usually quiet, but can sometimes be louder, and the pitch of the squeal sometimes changes as the radio continues to transmit.

The company uses a large mix of TK-8150, TK-8180, and NX-800 units on conventional, but I think it's mostly the older TK units that make this sound. Something's obviously not right with these radios, but I'm just curious as to what, exactly. Is it the radio itself? Microphone or microphone cable? What would be a solution to this problem?

While I was recording some transmissions in December, I caught one of the loudest examples I've ever heard of this sound. I just uploaded it to YouTube so you can hear what it sounds like.

Link: https://www.youtube.com/watch?v=kFTMt4w4xtg
 

WPXS472

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Heflin, AL
Could it be alternator whine? It has been a long time since I heard it and I thought the frequency would be lower. The fact that the tone went lower in frequency sounded like alternator whine of an engine slowing down. I have heard RF getting back into the microphone path cause squeals and the PLL reference frequency get in there, but it is always a stable frequency.
 

mmckenna

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Could be. So what would be a fix? Wiring the radio power to the battery instead?
Yep, probably alternator noise. You can identify it because the pitch lowers as the engine is slowing down. Likely, in that recording, the bus was coming to stop as she was transmitting.

The solution:
Run power off the battery. Proper grounding, Proper wire routing.

Depending on the bus, the engine can be in the rear. The batteries are usually towards the engine. Running long power feeds to the battery system takes a lot of work, and some installers will take the easy way out and tie into an existing power feed. Some larger buses have a large terminal compartment near the driver. The large buses we have at work have these, accessible from the outside on the driver side. This is usually where the fuse panel is, terminal strips for various connections, etc. It's a tempting place to hook up radios, especially for installers trying to crank out a fleet of buses in short time. Sometimes it works, if it's done right. Sometimes the installer will put in a filter on the power feed. Sometimes they just ignore it.

I had a GM pickup once that did that on an install. I had wired the power feed directly to the battery, both positive and negative. I had alternator whine like that, very faint, but it was there and annoyed me. I fixed it by moving the negative wire off the battery and grounding it near the battery to the sheet metal.

It comes down to installer skill, honesty, proper testing, and the customer caring enough or noticing when there is an issue.

For a bus fleet, this isn't surprising. The radio works and you can understand the driver, that's what most people care about.

I know our dispatchers would call me on an install like that. We've gone through the list of local installers and found the good ones and avoid the bad ones.
 

kenwoodgeek

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Toastycookies, not sure, exactly.

Thanks for that explanation, Matt. Yeah, there's several radios on the fleet that have that whine. If the person who programmed these radios is the same person that installed them, a small private company, then they didn't do an incredibly good job overall. Programming is terrible also, haha. I've seen many sloppy installs on several of the buses. Power wires and antenna coax hanging all over the place on some of them; it's pretty ugly.
 
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