Engine noise on QuickBox dual head kit

[HumHead]

I just put a dual head QuickBox kit on the VHF CDM1550 in one of our ambulances, and it is having some engine noise problems.

Here's the overview:
-The noise is only present with the engine running, and changes pitch with engine speed.
-The noise only shows up on the TX audio. RX is fine.
-The noise only affects the radio when the QuickBox is attached. When the head is put on the radio locally, there is no noise.
-The noise is still present with only one head connected to the QuickBox kit, so it doesn't appear to be picked up by either of the head cables.
-There is also a remote mounted CDM1550 low band in the rig (Motorola remote head kit). The heads, cables, and drawers are all in essentially the same locations. The low band radio is fine.
-The ambulance in question is a LifeLine type III, built on a 2000 Ford E350 Power Stroke Diesel V8.

Before I spend an afternoon filled with assorted filters and chokes, has anyone else ever run into this?

Thanks!!

[thebigphish]

fuel pump. every 2k in our fleet did this until they put a choke on the fuel pump wiring. it showed up in different rigs on different radios, and not on all of them at the same time...

[jim]

Do a search on fuel injection driver circuit noise on the Powerstrokes. It was just a few weeks ago.

Try to ground everything at one single point also.

[HumHead]

Hmmm....

Thanks for the insights.

I was tempted to call it ignition noise, but I just couldn't bring myself to do that with a diesel

Interestingly, as noted above, the problem is only on TX audio. RX is fine. To make things even more strange, the low band radio has an SVR200 VHF repeater on it. The antenna for the SVR is on an "L" bracket on the driver's fender, and both the low band and the SVR200 are perfectly clean.

My current working suspicion is that the noise is being picked up either through the power leads on the QuickBox adapter, or through RF by either the control box itself, or radio interface cable from the control box.

Maybe for my next test I'll try powering the QuickBox off of a seperate power supply and see if the noise stays or goes. If it goes, I'll try a power filter on the input. If it stays, I guess it's time to look at RF pickup on the TX audio lines.

The good times roll on...

[thebigphish]

We had the same problem, the hum was only on TX. ..the RX unit wouldn't pick it up...it got to the pt. that you could tell if the engine was shifting because the noise would throttle down when the truck shifted...quite the amusing whine.

[HumHead]

Well, I spent some quality time on the phone this afternoon with some helpfull people, and learned some interesting things:
1) Power Joke is just about a perfect fit as a name.
2) The Quick Box power leads pretty much only power the audio amplifiers for the speakers. The rest of the interface and control head circuitry is powered by the filtered B+ line off of the radio.
3) Motorola's idea of filtered B+ in the CDMs is apparently a single ferrite bead on the power line.
4) As a result, line noise on the radio's B+ line is passed onto the "filtered" B+ line on the control head.
5) The combination of the interface board and the cable capacitance on the Quick Box is an almost perfect recipie for transfering B+ noise to the TX Audio line.

The solution?
1) I tried a shorter jumper between the chasis and interface box. No diffference.
2) I stopped by the local "You've got Questions We've got Cell Phones", picked up a 20A noise filter, and put it in-line with the radio's power leads. Problem solved!! All of the noise went away.

The only bad news is that I now have a filter with a big ol' Radio Shack logo in the electrical cabinet of my shiny new $100K rig.

Good thing I'm planning to do a Tessco order on Monday...