Micor UHF Base/Repeater Station Issue

[railtrailbiker]

Our ham radio club is having a tx power output problem with our 75 watt UHF Micor base/repeater station.

Exciter power output is 1.8 watts, but the power control board keeps shutting down the first stage of the PA.

We've replaced the power control board, circulator, and PA with spares known to be in working order.

Still unable to get any output from the PA.

We've bypassed the PA and are running the station now just on the exciter, but would certainly like to get back up to full power.

Suggestions?

Thanks.

[PETNRDX]

Is this a repeater that has been in service, or one you are building?
I find the usual problem when the power control board is shutting the PA down is that the(switched A minus) SW A- for that board is not being passed all thru the backplane, etc. from the Station Control module.
Next likely culprit is the Station Control module itself.
Not making or switching the SW A- in the correct order. Bad component on the SCM somewhere.
If you try to TX does the PA put out power for about half a second?
And after that, while still keyed, is the center of the three pins on the lower edge of the power control board at 14 V or so, because it should go to pretty much 0 V on TX, and float at 14 or so on RX.
I have seen bad contact pins between all the interconnect boards loose that SW A- on its way to the power control board.
And, you could have a bad pass transistor on the side of the power control chassis (M9701) if the SW A- is getting to that power control board but it is not passing control voltage to the PA.
I have only seen that pass transistor bad a few times in nearly 40 years of working on the Micor. But it does happen.
Also, make sure the PA really puts out power by MOMENTARILY connecting + 12 V to the CTL pin on the PA itself while keyed. Caution, the PA will put out FULL power of over 100 watts if it is a TLE1713.
If this is a station that has not been in service, and you are modifying it to use as a repeater, make sure that there is a circuit path for SW A- from the SCM to the power control board. Sometimes things get modified, and people never get them to work, they sit, and someone else "inherits" the radio and gets to play "go fish".
Bypass of the T/R relay, etc.

[railtrailbiker]

This is a repeater that has been in service.

Thanks for your suggestions; we will revisit the site and check things out again with your notes in hand.

[PETNRDX]

If it has been working, then my suggestions are:
Check the PA by the temporary touching a jumper from 12 V+ to the CTL right on the PA.
If that is OK, check to see that the center pin on the lower edge of the Power Control board goes low on TX.
If you read a "low" at the power control board on TX then it kinda leads to the power control board or its pass transistor.
If you don't get that "low" at the power control board, then it leads to the Station Control module or interconnections.

[SlimBob]

1.8W sounds high.

[bernie]

My two bits worth:
There is a transistor on the chassis next to the power control board.
That can cause your problem.