Quantar PA Problem

[ldanna]

Been working on quantars for our Amateur radio repeaters. I am having a problem with 2 Quantar PA modules. One of my Repeaters showed no power out. Replaced the PA it works fine now. I took the bad PA home and fired it up in my test frame. Sure enough no output. On the RSS metering page it showed high internal SWR. HMMM I had the PA on a good 50 Ohm dummy load. I checked the load with another PA . so load is fine . The PA shows 10 to 1 SWR. So I bypassed everything from the PA board to the output connector then put PA in PA test mode Still no output. this is a UHF R2 PA. Then I had similar no output on a 900 Mhz repeater. same result brought that PA home same internal SWR failure. Both PA's look very nice cosmetically no sign of overheating anywhere on any board. I even went and re soldered all the inter board straps . Still no joy. I swapped PAs with working ones and they were fine indicating the Exciters were OK Has anyone had this failure and what was found to be the problem>?? Any trouble shooting suggestions??

Thanks

Larry

[jackhackett]

How exactly did you bypass them?

Circulators are a common failure point, I haven't worked on Quantar PAs but I've had a number of bad circulators in older models such as MSF5000.

[ldanna]

Thanks for the reply . I un soldered the circulator pins and placed a jumper from the PA output board to the N connector. I will try and replace the circulator from a good unit to test to see if the circulator is defective.. what fails in the circulator I am curious as I have not seen that before.

Regards
Larry

[desperado]

always a possibility the SWR bridge is faulty.... you may consider bypassing it, or removing the output line just for testing.
If you put the repeater in test mode and key it, it will run against a high SWR as well. if it thinks it's got a 10 to 1 VSWR then it's gonna turn off the output to protect it self

[ldanna]

OK here is an update to boggle the minds out there. I swapped the SWR and circulator from a good unit. Same hi internal SWR error. I then put the Circulator and SWR board from the bad PA into the good PA . The good PA still works fine, concluding the circulator and the SWR boards are not the problem.
Any further suggestions to follow up on??
Larry

[jackhackett]

The circulators that I've seem bad were burnt up inside, probably by lightning strikes. Guess that's ruled out.

Can you tell if it's trying to make any power? Exactly which PA model is it?

[ldanna]

No indication of it even trying to make power , even with PA test set to on .
More testing I thought I would check the Analog Mux U4104 which gets the swr info. the forward voltage to it is fine and the reflected voltage is identical with the good and bad PA modules so I was looking to see if u4104 was bad. I swapped out U4104 between the 2 modules and the problem stayed with the bad PA
So I am more perplexed than before. Is there any other place where this internal SWR error can be generated
Does anyone have a detailed description of the theory of operation of the PA? In the past Motorola Manuals did a nice job documenting each stage and the over all theory of operation. I guess those days are history.
Larry

[ldanna]

Voila we have lift off. After spending way too long down the rabbit hole. I took another path. The PA not making any power should have clued me in but being an old dog it's hard to learn new tricks. So I started checking the driver PA and found it had a bad transistor. I simply swapped Driver boards and the bad PA came back to life. SO if you have a PA that comes up as PA fail and the message says there is a SWR problem the driver is a good place to look further.
Thanks again for the group responses they helped me think a bit harder.

Regards
Larry

[jackhackett]

Yeah, that's why I asked if you were getting any power. If you've got no power being generated the SWR circuit is pretty much useless. I figured it was probably a bad amplifier stage somewhere, can be hard to narrow it down, especially with class C amps.
Glad you got it going.