Noise in radio TX

[kf4sqb]

I'm trying to help a local volly FD with a radio problem in one of their trucks. Its an early 90's (IIRC) model ford chassis, with a diesel engine. When transmitting (M1225, VHF Hi), there is a strange sort of "squeal" in the transmitted audio. Receive is clean. Its not the alternator (we pulled the belt off of the engine, and it made no change). The problem only occurs with the engine running, doesn't matter if strobes, siren, etc. are on or off. We've tried a different radio (Maxon mobile), with the same results. Portables exhibit the same problem if used in close proximity to the truck. I've checked all I know to check with the limited test equipment I have, and am at a loss. Has anyone ran into a similar problem over the years? Any hints or ideas welcome and appreciated!

[mike m]

Sounds like fuel pump noise, I remember several years back that Ford had a SRB about something similar and the fix was ugly, it required adding a filter at the fuel pump which is mounted in the fuel tank.

The ARRL used to have an entire technical section on line pertaining to similar problems, you may check and see if it's still on their web page.

Mike

[Bill_G]

Interesting problem. Based on what you wrote, if proximity is all that is required, that it occurs in portable xmit audio, then the truck may have an intermod problem. Does this happen on all freqs or just one? If it is the fuel pump or some other electrical accessory, you can prove that by turning the key to RUN without starting the engine. If it occurs, start pulling fuses until the problem stops. If it doesn't occur, if it requires the engine be running, then I would check for a block grounding strap, a chassis strap, a grounded hood, etc.

Any idea how long the problem has been occuring and what measures have been taken already?

[Jim202]

You have made a statement here that just does not make any sense at all. I can understand maybe a mobile
connected to the electrical system of a vehicle having an issue with noise on transmit, but you need to explain
to me just how a portable can do the same thing. There is no way that you can have the same problem with
a portable.

Maybe your looking in the wrong direction. My guess is that you have some sort of audio issue that is being
picked up by the mic. How about trying an experiment. Take and disconnect the mic. Then wire up a
wire from the PTT connection and ground it. See if you still hear your noise.

I would be looking at maybe the sound that your hearing might be coming from the turbo on the engine. The
solution might be as simple as a noise canceling mic.

Jim

I'm trying to help a local volly FD with a radio problem in one of their trucks. Its an early 90's (IIRC) model ford chassis, with a diesel engine. When transmitting (M1225, VHF Hi), there is a strange sort of "squeal" in the transmitted audio. Receive is clean. Its not the alternator (we pulled the belt off of the engine, and it made no change). The problem only occurs with the engine running, doesn't matter if strobes, siren, etc. are on or off. We've tried a different radio (Maxon mobile), with the same results. Portables exhibit the same problem if used in close proximity to the truck. I've checked all I know to check with the limited test equipment I have, and am at a loss. Has anyone ran into a similar problem over the years? Any hints or ideas welcome and appreciated!

[Bill_G]

Excellent point Jim. It may not be a real problem, but a perceived problem. The truck may have a ground loop between the radio dash mount and/or the ant mount and/or the radio black wire connection point that does induce noise in the power line during xmit that can be heard in rcvrs. Portables with their mic gain set a bit hot might pick up engine roar that is unrelated to the original problem but called "noise" that sends techs chasing their tail looking for a single problem when in fact there are multiple issues. Some dispatchers cannot tell the difference between path loss, picket fencing, alternator whine, doubling (two guys talking at once), over deviation, mic housing creak, and a bad mic cord. It's all just noise, the system is broken, it's a life safety issue, and all the managers bear down on the tech to fix IT - the single problem. Woohoo. Good times.

OTOH, I have seen some very strange things happen in the older Ford tilt cabs when ground straps break, or are removed during service.

[kf4sqb]

Fuel pump? I'm not going to say the problem truck doesn't have an electric fuel pump, but I've personally yet to see a diesel vehicle with an electric fuel pump. Granted, I've only really dealt with older equipment, but, this is older equipment in question.

The problem seems to occur on several frequencies, but is worse on some than on others. As far as I know, the problem has been there since the VFD got the truck a few years ago.

I have thought about the possibility that the noise is being picked up by the mic and transmitted, but if that were the case, I would expect it to be the same on all frequencies, but it is not. Also, there are some older trucks in the county here that have a lot more ambient noise in the cab than this truck has, but they don't have a problem like this. The best way I can think to describe the noise in question is like its on the verge of starting audio feedback, with alternator whine mixed in, if that makes any sense. Its not, however, the alternator. As I said in my original post, we tried removing the drive belt from the alternator, and the noise persisted. The idea that it may be "turbo noise", picked up by the mic, would certainly describe the noise fairly well, but the truck doesn't have a turbo. As for this "not making any sense at all", because of the nature of the problem and it showing up in handhelds as well, I guess it also doesn't make any sense to you how a "squirrel-cage" type AC motor works, either?

[Bill_G]

Have you verified the engine has to be running, or does just the key have to be on?

[kf4sqb]

Have you verified the engine has to be running, or does just the key have to be on?

Actually, no, but I will see about it.

[Pj]

I've seen similar with the siren placed in radio repeat with the volume turned way down. It had something to do with a problem with the siren amp itself...I want to say it was a FS PA300 hooked up to the radio. The dept use to turn the knob all the way to the left and people just assumed it was "siren off" even though they passed the "manual" selection.

Just throwing it out there. Of course if audio was coming thru the siren speaker at any level..I would hope somone heard it!