Jan. 2004 Tower Trip
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January 19, 2004 Repair Trip

Problem:  It was noticed that the audio for the 442.350 repeater would go away when the APRS Digi transmitted.  Some sort of de-sense was suspected.

We didn't have a third tower helper on this trip, so there are no photographs this time.  

WT4X and KD4BBM went up the tower with KU4GY as ground support.   Since the two frequencies were so far apart, it was unlikely that the it was an RF problem.   WT4X brought up a spare power supply.  Sure enough, when the APRS Digi transmitted, the 50 A Astron power supply went low voltage.   Regulation problem in the supply.   The APRS DIgi and the 442.350 share the Astron, the 147.000 has its own GE Power Supply.  Both are traditional heavy Iron power supplies, not Switchers.  

The power supply was temporarily replaced with 35 Amp rack mount.   This instantly cleared up the problem.   We suspect that the logic circuits in the controller, which run at 5 volts, continued to work when the PS voltage went low.   However, the audio circuits, which use a higher voltage, did fail.  Net result, the repeater stayed on, but no audio.   With the new supply everything worked.  Both transmitters on at the same time totaled 30 amps on the power supply.   Power levels were checked:

Repeater Forward Power Reflected
147.000 50 2.5
442.350 50 2.5
144.390 40 0

We also tightened the latches on the doors.   When closed, the doors rattled a bit.   They're nice and snug now.   The cabinet is looking good.  Just a couple small dots of rust where the paint was chipped.  

Now with stable power, users might notice a better signal level on the APRS Digi.   Time will tell if we have a good range.   If the power source had been intermittent, it could explain the loss of the last APRS radio.  It was found in a locked-up state on the last tower trip.   Poor power regulation could have been a cause of this symptom.   That's totally a theory.  

On the way down the tower, KU4GY reported no audio on the 147.000.  It would key up, but no repeat or controller audio.  We went back up to check things.   Nothing really obvious.  In the process of troubleshooting, we found the 802 fuse in the Mastr II receiver was blown.   This fuse runs the audio amp in the receiver, which was working, and the controller.   The troubleshooting could have blown it too.   But once we replaced it, the repeater started working.   On closer examination of the fuse, it appears the element was corroding.   White dust was coming off of it and some was laying at the bottom of the glass tube.  Could it be the fuse had become a resistor before blowing?  That would have caused low voltage to the controller, which would have caused the logic to work, but no audio.  Sound familiar?   We couldn't reproduce the problem so there's some doubt that this was the cause... but without it failing again, we couldn't trouble shoot any more.   I had heard another complaint of this carrier with no audio on the 147.000, so this could be a reoccurring problem.  Hopefully we got it.  


    South Florida FM Association, 2003   

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