MSF5000 PA Problem

[mtxman]

I have a 75 watt 800mhz msf5000 CLB model at one my trunking sites that has a power output problem. when i try to adjust the power output pot the transmit power jumps all over the place and will not stay at the 75 watt level where it suppose to. i have swapped the uniboard, but it did not fix it. Is the problem most likely in the PA or somewhere else? any help greatly appreciated.

[MSS-Dave]

Check the power supply first and make sure you have about 15.5 volts when transmitting with no AC ripple. More than likely the problem is in the amp itself. Test it this way. Open the SCB tray, look for JU 1 on the SCB. It's on the same end as the 5 volt supply toward the rear of the tray. Move this jumper to the opposite position and the red test light will come on. Pull the gray power control connector off of the PA. Key up and test using a spectrum analyzer and watt meter. This will run the PA at full power without cutback from the power control circuit. Chances are the amp may be spurious and screwing with the circulator built into the amp. If you have a external circulator, there is a chance that the sensor on the load is intermittant, you can pull the brown wire off of this to test (if installed). You didn't mention if when the power fluctuates the PA full light goes off, this will help to know. PA full will go out due to cutback by the power control circuit because of a problem in the amp or overtemp.

[wavetar]

Dave's advice is excellent. I have had to resolder the center pin connection inside the forward/reflected power sensor, located just before where the N-Female TX connector is mounted on the chassis. A hairline crack in the solder played havoc with a couple of units for us. One time the meter would read 95watts, I'd align it to 75, and the next day I'd read 50watts. Not to mention the central controller would keep disabling the unit because it would report high VSWR. Worth a look, anyway.

Todd

[mtxman]

Thanks fellows. the site is about 60 miles away so it may be saturday before i can get back to it. you definitely have got me headed in the right direction. i will make these tests and see what happens. the power at times was jumping as high as 150 watts. it was very strange. i did not get a chance to check the pa voltages, because it was late at night and i was trying to get home before midnight. will do that and hopefully get to the bottom of it. thanks again.

[bernie]

My two bits worth:

The forward power control is a loop that uses an analog voltage from a directional coupler on the PA.
The reflected power is detected by the thermistor on the output circulator, as well as PA heat sink temp.

The reflected power causes heat, which then causes power reduction.

The CLB can have a problem if the power control cable gets accidently un-plugged from the PA, or the other end on the interface board under the RF casting. ( This line is soldered on Digital stations)

When this happens the power control becomes very erratic because the loop is broken.
It is possible to get the station to make power, but the adjustment won't stay, the PA full lamp may not light.

Another problem is if the power set is set way too high, it will get erratic.
Turn the power set all the way down and work your way up to desired power.
On some stations there seems to be two places on the pot where the power can be set to the correct power.

If the power is set too high the PA will shut down when it gets really hot,
causing a service call.
TIB fail on trunking, but misteriously clears when you visit the site when it is cool.