First thing I'd do is pull the Uniboard and squirt some contact cleaner into the power control pot, then turn it back and forth a dozen times to clean it. Then squirt some into all the connectors between the Uniboard and the pins in the RF tray. Put the Uniboard back and repeat the cleaning procedure on the cables going to the IPA/9.6V regulator card.
The power control is fairly simple but it does depend on the signal levels remaining constant. The forward power is detected at the output of the PA and becomes one input to the control circuit. The pot on the Uniboard is another input. These are balanced in an op-amp which controls the IPA. If the sampled output of the PA goes too high (either too much real power or the sense circuit goes bad), the controller is supposed to lower the IPA drive power. Similarly if you lose power coming out of the PA, the IPA will be told to push more power out. You're getting 10w out of the IPA tells me it either lost the forward power sense signal, or the PA really is putting out less power than it should, so the circuit compensates. One of the six wires on the Molex plug between the PA and RF tray carries the forward power sense. You should measure this with a digital DC voltmeter (it'll be in the 0-2V range) with the station operating at the proper power, and note that for reference. Then when it happens again, you can disable the power controller and see whether the PA is making all the power it should, or if it's low, then you can troubleshoot it further. The controlling transistor and it's components on the IPA could also be giving you a problem. An intermittently open solder joint, for example. Because the power control circuit is a closed loop, you'll be chasing your tail for a while trying to figure out what's causing it and what's being affected by it. You didn't say which station model you have. There are different ways of temporarily disabling the power control circuit depending on model. But you probably don't want to leave it disabled with the IPA putting out that much power. The fact that you only have to touch the pot makes me suspicious of a dirty pot. They're fairly cheap. Could also be a break in the foil on the Uniboard. Bob M. ====== --- Steve Crumley <[EMAIL PROTECTED]> wrote: > One of my 900Mhz MSF's is driving me nuts!. Every > once in awhile it > will shut the PA down. I have been thru the usual > checks for excessive > drive, SWR, connectors etc. It is a 75W PA, I will > set the output to > 75W and then a few days later it will shut down. If > I turn the power > control it will jump from zero to over 100 watts > outand thats all. I > check the IPA and it is putting out 10 watts with no > power control. > Then suddenly it will go back to normal. I can > connect just the IPA to > a dummy load and its still in shut down. > > Does anyone have a mod to bypass this shut down > feature. So I can just > set my power level where I want and let it run. > > Thanks ____________________________________________________________________________________ The fish are biting. Get more visitors on your site using Yahoo! Search Marketing. http://searchmarketing.yahoo.com/arp/sponsoredsearch_v2.php
