Batboard

I checked an MSF5K 800 repeater late Friday. On transmit, the power fluctuates up and down, and the TX Full and TX Key lights flash as well. It has TX and RX Lock. PA? I haven't looked at the manual yet.

There are two power resistors in the PA that somehow open their solder joints on the board and do cause a power problem. I noticed this in several ~35 watt MSF5000 800 MHz stations.
The resistors are easily seen when the cover is removed while the amp is hinged out. They resistors are located near the left bottom. Your problem may be very different. Perhaps it is best to check it out with a good dummy load first.

Nand.

My two bits worth:
I agree with Nand..The PA has metering sockets so the collector current for each stage can be measured. The mentioned resistors are the metering resistors for each PA module.
A bad resistor connection would give a full scale reading on the respective test position..Be careful you dont burn the test set.

It could also be a failed "Omega strip"
We have also seen burnt solder on the A+ distribution board under the deck, which is not too easy to get at.

The problem is that these are so reliable that no one has extensive experience in repair, such as with the MICOR PA.

I would also very carefully inspect the A+ connection on the power supply. This is not the best connector arraingement. I have seen these burn due to bad contact.

I have also seen a couple of times on 75Watt 800MHz units where the actual wattmeter-looking element for the sensor developed poor solder connections inside. First one drove me batty for about a week, as it was intermittant. I'd set the power to spec, and a little while later it would be 95W. Bring it back down & later on with would crap out & show about 40W. Worth checking, anyway.

Todd

My two bits worth:
Todd reminded me of some problems that I have seen:
The "Watt meter element" is for the "T STAT" function in trunking.
The RF connections could be burnt, as well as the possibility of having water inside.
The position of the element in relation to the vent on the side of the station makes for the likelyhood of water on top of the station getting inside the watt meter element.

Another possibility is the plug connection on the RF interconnect board and the grey cable that connects to the PA has become partially unplugged..(CXB)

This does not seem to be your problem, but I should mention that there can be problems in setting power.

If the power output is set out of spec, say to 100 watts the station seems to work OK, however you will have "mysterious" TIB fail, only to check out OK when you finally get to the site. The pa has gone into protective low power mode when it gets hot causing TIB fail.

Some stations seem to have two positions on the power set pot with the desired power.
Turn the control CCW, and work up to the desired power output.

Due to "KANGAN" which is what the sign on the assembly line at Schaumburg said="Just in time delivery" You have NO idea as to what type of alignment tool is required.. Be careful it is possible to break the control a very "cheap" part.

Thanks for the replies guys. I still haven't been able to check anything (had to leave house straight for a tower job 3 hours away). I might add that this is an 800 conventional repeater, not part of a trunked setup. When the power fluctuates, it is doing it rapidly and in time, and the lights flash at the same rate as well. Keying the repeater with a test radio, once the repeater is keyed it keeps keying and releasing over and over, 10-12 times before it stops on its own.

I just remember that I once had a station with a leaking feed through capacitor in the PA. I believe it was associated with a temperature sensor or the power measurement circuitry in a VHF station.

Nand.

My two bits worth:
Sure sounds like a very large RF "leak", or a very bad problem in the antenna system.

So much for "armchair troubleshooting"
Time to check out the station.
I am most curious as to what the problem actually is.

on the UHF's I've seen the interconnect copper straps between the modules open up before. Had one like this that would only push 40W out maximum before tripping out the station with PA FAIL. Went inside and found the bad strap. Re-Soldered it and now the station will do 100+W all day long without complaining (not through the filtering, straight out of the PA)

Kurt