Batboard

Are the 110w UHF (mid split) PA's interchangeable between digital and analog MSF 5000 repeaters?

We've got a digital station that's been running along fine, but the PA has gone out. We have several spares, but they're all from analog stations. When we swap the "digital" PA with an "analog" PA, the station behaves the same way as it does with the bad "digital" PA. There's no power out at all - the front lights just flicker when the PTT switch is activated, and the Bird wattmeter doesn't even wiggle.

I can fill in part numbers after I'm back down to the tower site, I was just wondering if there was an obvious solution that could be had without them.

Thanks for the help,

Matt
AK4MK

Everything is interchangeable between CLB and CXB stations. If your station won't make power even at the lowest setting on the power control pot then something is wrong.

Weak transistors in the PA not producing power at the "expected" level versus input power can trip the PA Fail alarm... the feedback loop used for power control is *very* sensitive. You should also check the power control cable connection at the bottom of the RF tray, and pull the RF uniboard out and check the passthrough pins for corrosion... it looks like black crud at the base of the pins.

+1 on Otaku's comment on power control. If it does the same thing with another amp I'm suspecting something on the outside of the amp in the control loop. Does the station have a internal circulator with a thermal wire which is brown in color? If so you might try checking that for shorts. Also, the CLB station has a jumper on the control board that will bypass the power control circuit completely. Cant remember if CXB had that too. I believe it's ju 1. If you pull that, you can key the station up and if it makes full power and I mean balls-out power, the power control Loop has a problem. Whatever you do don't run the station with that jumper out in service. It would be very helpful if you had the service manual. It will explain this procedure for checking your PA. I'll also suggest checking the power supply to make sure your DC voltage isn't sagging when it tries to key up.

Well.... embarrassingly enough, we found the problem.

As was said, the "analog" amps work fine in our "digital" station. The key is to install it not looking at the color bands on the in and out leads, but the lead length and/or position. Our original "digital" amp had the input labeled with a red band, whereas the OUTPUT was labeled with a red band on the "analog" amp.

Thank goodness for the built in circulators that I'm convinced kept the replacement amp from frying when we installed it with the input and output crossed up!

Thanks for the comments and suggestions.

Matt
AK4MK

You can thank Motorola for being so consistent in their cable labelling color codes. That's bitten more than one user.

Bob M.

I love those fixes! I'd much rather feel stupid and laugh at myself than beat my head against the wall because I can't fix something.

Agreed!

Thanks again, y'all for your suggestions.

Have a wonderful Christmas season this year!

73

Matt
AK4MK