JU1 is only meant to be left in place for trouble-shooting, and it tells the CPU to ignore any output power errors that would normally shut it down. You should still be able to control the power with the pot. If not, trace that circuit.
There has to be some kind of forward power sensor, either built into the PA or externally, after the low-pass filter on the way out of the station. The PA's sensor comes out on the GREEN wire in the 6-pin Molex plug between the PA and RF tray. You should get 2-3VDC on that pin when transmitting. An external power sensor should feed the SCM via two RCA plugs. These were usually only present on trunking stations. Motorola usually under-rates their stations, so I'm not surprised that a 75w station is putting out 100 watts. It could even do more if you really pushed it. My 75w UHF station will easily exceed 100 watts, but I've never timed it to see how long! Bob M. ====== --- Steve Crumley <[EMAIL PROTECTED]> wrote: > --- In [email protected], "Bob M." > <[EMAIL PROTECTED]> > wrote: > > > > 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 > > > Thanks for the reply Bob. The model number is a > C65CLB. I have tried > cleaning the pot, but thats not to say its not bad. > I have tried > another IPA with the same results. This station came > without the > directional coupler, so nothing is connected to the > brown wire. > However it has been working for several months in > this configuration. > When I put JU1 in place it will go to 100 watts > without any power > control even though its a 75 watt station. > > Thanks for the help > Steve ____________________________________________________________________________________ 8:00? 8:25? 8:40? Find a flick in no time with the Yahoo! Search movie showtime shortcut. http://tools.search.yahoo.com/shortcuts/#news
