Batboard

Ok so here's an odd problem...

I have recently installed a low-band Maratrac in my 1967 Crown Firecoach fire engine. The Maratrac replaced a Mocom having the same issue as I'm about to describe.

The Maratrac receives audio great even at distance, but when I go to transmit on it, the user listening can't hear any audio. What they do hear is what sounds like an open mic, but no audio. When I stop transmitting they get a squelch sound and then the channel releases. The radio's TX light illuminates and it is obviously transmitting something because listeners are able to hear the channel open up on their end, but the radio won't pass any audio.

The antenna seems to be mounted fine. It is a full whip on a side mount ball and is grounded out well. I even ran all new cabling to it when I was attempting to troubleshoot. Ran the radio in RSS and it is programmed fine. Swapped out mic's and still the same issue. Again, it receives really really good, but it is having issues transmitting.

The odd part is the Mocom that was installed previously, had the same issue! I thought it was just the Mocom going bad, so decided to upgrade to the Maratrac, plus the head matches my A9 Syntor VHF setup.

Any ideas guys?

Are you saying that you are using a different Mic, Control Cable and Control Head?

Did you bench check these radios outside of the vehicle?

Sounds kind of fishy that 2 radios would do the same thing. I would try both of them on the test bench and see
just what they will do. Good voltage, good service monitor, good dummy load. Eliminate anything outside the
radio that could effect the testing.

ps: I never put a radio in a vehicle without first testing it. Too much work goes into an install to end up with a
problem and you don't know where to start looking. At least you knew it worked before all the effort was expended.

Jim

Had a similer problem on lowband radios(syntor-x) and found the problem to be a perticular DC noise filterthat fed the back of ther control head(not Mot. ) so removed the filter and that fixed the transmit problem. Also have a friend that worked in a Mot. shop that had a whole fleet of fire trucks with the same problem, he removed the filter and the problem was fixed. Just a thought. good luck. Davie n 3 c h u at atlanticbb dot net the filter has a toriod in it.

I know this is way late for a reply, but I had the same problem on a fire engine. Turned out the cable to the pump panel microphone got moisture / road salt in it, pulling both mikes audio signal down to nothing.

Another late reply:

Check all your power connections. I've seen a 60 watt Mocom 70 powered by a wall-wart (yes, a wall-wart ), that worked fine for RX, and would actually TX and key the county fire repeater, but had no audio. Hooked it to a better power supply (car battery), and everything worked fine. Your problem could be as simple as a corroded or loose power connection.

This is going to call for getting inside the control head mic connector. Measure the mic high pin and see if the DC
bias voltage is present. If not, you need to find out why it is not there. The mic will not have any audio with
this voltage missing. Measure from the pin to ground. Should be someplace around 5 to 9 volts.

The bias voltage comes from a resistor inside the radio in the mic circuit area.

Jim

Step one: check the VSWR, and check the transmit into a dummy load instead of the antenna without any other changes.
Stem two: open the mic and check to voltage on the red and black wires. That will tell you if the mic bias is getting to the mic. 9 volts RX, 4-6 volts keyed.

Step three: make sure the power for the main Red lead and the small Green lead going to the control head come from the SAME battery point. ( from the Maratrac manual)

Sounds like a basic troubleshooting 101 problem to me too. More likely the control cable, mic, or head than the radio based on what you said.