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The Mobile Repair

In 1974 when I was working for another radio shop, our company installed a new radio in an El Camino for use by a AAA contractor to be used as a service vehicle to provide jump starts, tire changes and other minor problems.  Immediately after the radio was installed, the customer started complaining about the radio breaking up when in transmit.  The radio operated properly when receiving a radio signal.

At the time, I was driving a service vehicle which was a pickup truck with a camper shell.  The camper shell was outfitted with a desk and chair with sufficient test equipment and parts to perform most repairs of the radio at the customer location.  In those days, the radios were built entirely out of discrete parts making the radio easily serviceable for many maladies.  More serious and intensive repairs were handled at the office.  Since this was a new radio, there was no anticipation that there would be anything seriously wrong with the radio.  In fact, since the radio was brand new, we had an expectation that the problem may have been with the installation or that the problem with the radio would be minor.

After driving for 35 minutes through traffic, I arrived at the customer’s location.  The owner for whom I have repaired numerous radios in the past directed me over to the vehicle that had the problem.  I spoke to him for a couple of minutes to determine the symptoms he was experiencing so that I could locate the trouble with the least time and effort.  He explained that the radio would break up on transmit, but the receive signal was fine.

I proceeded to look at the installation and run the standard tests to determine that the radio installation was done properly.  The installer for our company was known as J.T. and he was a true professional and rarely made a mistake during an installation.  I checked the power connection and the antenna connection.  Both seemed solid, so I removed the radio and brought it to the service truck to check out the radio.  I plugged in the service truck with an extension cord to power my test bench and proceeded to run numerous tests on the radio.  None of the tests revealed any trouble with the radio, so I started inspecting various aspects of the radio for loose connections, taking apart assemblies, re-soldering connections, tightening screws and doing anything I could think of doing to find the trouble.  I returned the radio to the vehicle unsure that I was able to repair it or that there was any trouble with the radio at all.  I then looked at the power and antenna connections again to see if I missed something, but to no avail.  I informed the customer of what I found or more correctly, what I did not find.  We both agreed to give it another try to see if the radio would perform correctly.

Two days later, I was back at the customer location checking out the radio again.  The customer claimed that nothing had changed and it still was having the same problem.  I repeated all the tests again and still could not find a problem with the radio.  I tried to find out from the customer if the radio signal was dropping out or if the audio itself was intermittent, but the customer could not tell me which one was happening to the radio.  I swapped the microphone with another radio on the theory that the microphone may have been intermittent causing the breakup on transmit.  I left the customer’s location unsure again as to whether or not I had fixed the problem.

The next day I was back at the customer’s location looking into the problem again.  This time the customer informed me that the radio worked fine when the vehicle was not moving, but would break up when the vehicle was moving faster than about 40 miles per hour.  This is the 1st clue to the source of the problem.  This was the first time that I felt that we were getting somewhere in finding the problem.  My main piece of test equipment would not work in the car, so I could not take it with me on a test drive.  However, one of our other servicemen was up at the repeater site while I was at the customer’s location which gave me the opportunity to determine if the radio was intermittent with the transmit power or if the audio was breaking up from the microphone.  After running several test transmissions, he informed me that the radio signal was solid, but tone signaling was not holding the tone decoder open in the repeater during the transmission.  This was the 2nd clue as to the source of the problem.

We returned to the customer’s office and I took apart the radio to check out the tone board assembly which sends the tone signal that identifies this radio from other companies on the same frequency and allows this radio to talk to other radios.  I cleaned all the connections to the tone board, tightened the contacts, reseated the board and secured it so that there would not be any intermittent connection.

The next week I was back at the customer’s location checking out the same radio for the same problem.  I brought a whole new tone board assembly with me and swapped the tone board.  This should take care of the problem.  I put the radio back in the vehicle, but the person with the car keys was gone on an errand, so I could not take the vehicle for a test drive.  However, I felt confident that this should take care of the problem.

Again I was dispatched to the customer to check out the same radio.  I installed a power line filter to the radio in case there was something in the power system of the car causing the problem.  Also, I opened the radio and installed a power filter to the tone board so that it would also get clean power.  I left their offices and they called again later the same day that it was still intermittent. I came back and tried swapping the radio with another vehicle.  It became clear that the problem was with the vehicle because the new radio from the other vehicle exhibited the same symptom and the radio from the El Camino worked fine in the other vehicle.  However, I had checked out the installation at least two time previously and did not find anything wrong with the installation.

There were only two connections to the radio in the vehicle.  One was the antenna and the other was the power connection.  The antenna had to be working because the radio signal did not drop out.  The tone signal was the item that was dropping out and that relied on the power connection and not on the antenna connection.  Therefore, I proceeded to check out the power connection that I had checked out several times before.  I still did not see anything wrong with the power connection.  The customer had a replacement car battery, so I had them change out the battery in the vehicle.  The radio was still intermittent when driving over 40 mph.  I then rewired the radio to work from the battery that was not connected to the vehicle and the radio operated normally while at high speed.  It was clear that something in the vehicle electrical system was causing the problem, but what was it?

I decided to check out all the battery wiring in the vehicle.  I followed the positive wires through all the connections I could find and everything seemed to be solid.  I then tried following the negative battery lead and the large lead (#0 wire) went to the engine block which was completely normal.  Typically, there is a 2nd negative lead that goes from the battery to chassis ground, but I could not find the wire.  I searched for it and finally found what was left of the wire.  I did not know why at the time the small wire (#10 wire) to the chassis was important, so I decided that I should repair it even though I did not know why at the time.  After fixing the connection, we took another test drive and now the radio performed correctly.  Upon discussing the problem with the customer, we determined that the engine was sitting on rubber motor mounts and that the ground connection to the engine block got its ground connection through the transmission which did not make good connection once the vehicle reached 40 MPH.

I learned a valuable lesson about automotive electronics and so did the customer.  I repaired the mobile (vehicle, not radio).

 

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