New Ambulance radio noise help

[nick223]

A local ambulance dept just took delivery of a new ambulance on the front cdm 1250 radio on TX is a horrible noise and whine. Behind the diver seat is a 2way radio power box for the radio so i added an inline noise filter to the radio at that point and it helped alot but the noise/whine was still present. Today i wake up to this email...Sounds like a grounding issue but like any ideas thanks



1. On about 75% of transmissions today we had no audio at all. There is no external speaker and the speaker on the radio was not playing. The radio was still receiving because I could see the red light blink when it would receive traffic for us. If I had a pager or portable on and open I could here the traffic. I had no problems transmitting at this time.

2. On one call when we were able to transmit on scene fine but not hear anything in return. While inside on the call we left the ambulance run when we went in. When we came out our transmissions were unreadable they had a lot of background noise and dispatch had trouble understanding us. On arrival at the ER we were unable to relay our ending mileage with the front radio or portables. The back radio was able to transmit and receive transmissions perfectly fine from both hospital and dispatch I used that to give our ending mileage.

 

[cmdrwill]

The 'power' for the radio should be completely separate from the power feed for all the noisy lights and other wig wags. Also the radio should ground near where it is mounted directly to the body.

Also the antenna may not be installed correctly, high SWR, no or not enough groundplane.

A lot of new police vehicles and the blankity blank installs are having noise problems, especially with the new 'take-down' strobes making the tx audio cut out, klickity klick.. The power for the radio is from the same wire from the battery as the 'lights' and siren use. Bad idea. Also the radios need a separate ground.

 

[n0nhp]

Last new modular ambi I installed had a fiberglass shell and very poor wiring harness to the module. I ended up going to the local metal fabrication house and getting a stainless steel ground plane made that I silicone seal and screwed to the roof of the module for the three radio antennas. I ignored the factory wiring and took a 8 ga, with an old Motorola fuse block directly to the battery switch and made my own distribution fuse block for the radios. A ground distribution was 8 ga to the closest chassis point.
I still had some siren whine on TX but it was acceptable just noticeable.

Good luck chasing the emergency vehicle radio demons, been there done that and have the scars and glass fibers still working their way out to prove it.

Bruce

 

[a417]

Diagnosis by substitution.

1) swap front and rear mics.
2) check for shorts to ground in the antenna conductor
3) swap front and rear radio units.
4) see if the problem is engine speed dependent
5) see if the problem is vehicle speed dependent.
6) power radio from isolated battery while vehicle is running.
7) use separate antenna (like a mag mount)
...

this is what I would start doing, this is highly truncated though, but it's a start.

I've seen some AWFUL installs by upfitters, including one guy who "wanted to keep the RF out of the power leads" so the bundle of wiring into the console was wrapped up in tin foil and then wrapped with electrical tape.

You're not exactly talking about a Rolls Royce there for a radio, so don't rule out a dud. It happens.

 

[Project25_MASTR]

So the RX without any sound issue could also be from an error in programming. Wrong tone or using a tone where dispatch is not transmitting one, the latter being more unlikely. Simple test is to force monitor and see what happens.

Trace everything backwards though. Try a mag mount on the roof of the cab (assuming it's a Dodge/Ford/GM chassis or other with a non-fiberglass cab) and everything else off and work from there.

 

[jim202]

Don't forget that you also have the ignition feed wire going to the radio. This is a common source of noise. You may need a filter in this line also.

Don't forget to check out where the ground wire from the power cable is going. Make sure it is a true vehicle ground and not some clip on the plastic dash. If that is the case, your relying on the coax cable ground to have the transmitter current going through it.

This will take some true Sherlock Holmes work to look to see just how the radio go wired. Don't forget to check on what the antenna has for a ground plane to work with. An SWR meter may be a big help here on the antenna. Use of a temp mag mount on the hood should walk you through the antenna problem if you have one.

Good luck on you hunting. Come back and let us know what you find.

 

[nick223]

Thanks everyone for the information. I'll let y'all know on the findings

 

[a417]

if you can say, who is the upfitter?

Some big players have known issues.

 

[cmdrwill]

if you can say, who is the upfitter?

Some big players have known issues.

That IS an understatement.