msf5000 cxb 800mhz pa troubles

[mtxman]

i have a 150 watt msf5000 cxb model 800hhz repeater that the power output keeps dropping out. i have swapped the circulator, low pass filter and the forward, reflected output sensor and both pa's. nothing seems to make a difference. the pa lights drop out when the power drops and fluctuates. the power supply voltages are staying where they should be when the station is keyed. the uniboard has been replaced 3 times also and i have checked the connections on the board underneath it. when these stations start this crap it can be hell to get them stable again. i can turn the power as low as 80 watts and it still trips sometimes. i have set the trip points and overdrive control by the book several times, still no change. any ideas? thanks.

[Al]

I've had trouble with the IPA transistor heating up when the station is keyed for 2-5 seconds and then going low gain. You didn't say whether you've checked the PA drive level or not, but if you haven't, get out your can of CO2 while keying the station from the front panel. Just a very short blast to cool off the driver chain transistors will verify whether or not they're failing.

[mtxman]

i have already swapped the pa's on this station. when they start doing this i have not had any luck on fixing them. i have swapped the uniboard also. it seems to be a weakness of this repeater. when they work they are great, but when they start dropping power like this you might as well unplug it and forget it. the power supplies are holding good voltage. i am out of ideas. i would like to just replace it if i could find a good price on a used unit. thanks.

[Doug]

mtxman,
What Al is refering to is different from your pa. The IPA circuit or board if you will is located to the rear of your rf tray. Its located between your vco's. You'll notice a driver transistor which acts as an exciter
Doug

[Big BOB]

If you have done the usual capacitor change to stop the controller from shutting down or going into reset. I had one of my 900 MSF's shutting the PA down and what I have been finding ( and had a UHF do this) is on the regulator board in the power supply. It seems that because of the age of the MSF there appears to be like a dissimilar metal issue in the connectors. Check the smaller of the plugs on that board. I had the same thing could not find any reason the PA was going into shut down, SWR, PA, IPA, Voltage etc.