Thanks Ron and Jim for the feedback. Ron, what you said about changing the impedance after the repair is what I was thinking. I did check the output with the spectrum analyzer and did not see any problems there. I assume though, there can be small enough changes in the impedance to affect the overall tuning of the duplexer reject freq that probably cannot be seen on the spectrum analyzer.
I also used the solder wick trick on the output with the little expansion bump. I was thinking that could change things as well. I am going to just leave it at the 25 watt output and see how it goes. We are entering into a $90k RF engineering contract with Motorola right now to replace our entire system. So I don't really want to start playing with it too much. -Tim -----Original Message----- From: Rogers, Ron [mailto:[EMAIL PROTECTED] Sent: Wednesday, April 21, 2004 10:49 AM To: '[email protected]' Subject: RE: [Repeater-Builder] To adjust or not.... I wouldn't touch it either. Reducing the TX power is probably the best thing at this time. However, read on........ If you recently rebuilt the final amp stage and changed components, chances are you have slightly changed the characteristic impedance of the amp output and this is affecting the match between the amp and the duplexer. (tuning an amp for maximum RF output at the antenna port of a duplexer after replacing the PA component DOES NOT ensure that you have tuned the amp to match the impedance of the duplexer) This could cause a number of issues including the duplexer's reject freq. notch setting on the transmit side of the duplexer. This could allow more white noise from the transmitter to get into the receiver if the notch is not set right on the RX frequency. This mismatch can also cause intermodulation or spurious components to be generated inside the PA section itself. Now, I assume you are using a band pass, band reject duplexer. If you would feel better by adjusting something and if there are separate RX reject notch tuning adjustments in the TX section of the duplexer (like Sinclair or TX-RX uses), I would only try adjusting them for best RX quieting while the TX is on the air and there is a weak signal being received by the RX. Start with the reject notch adjustment in the can closest to the amplifier stage. I certainly would not try to adjust the main Pass frequency plungers on any of the duplexer cans at this time. Ron -WW8RR- (formerly WB8ERB) -----Original Message----- From: Jim B. [mailto:[EMAIL PROTECTED] Sent: Wednesday, April 21, 2004 1:15 PM To: [email protected] Subject: Re: [Repeater-Builder] To adjust or not.... Tim Shephard wrote: > I service a public safety repeater. It's a master II. > > About 6-8 months ago I rebuilt the PA, replacing the two output transistors, > caps, etc... I even added a couple of fans and it's been working great at > around 40 watts. > > Lately we have been having some trouble which has been nicely termed the > whale noise. After dispatch would transmit via a wire line, very > infrequently we would have a noise come through or cover their > transmissions. > > Yesterday I checked everything and found I was getting some desense, around > 8db. > > I looked at the duplexer and it looks like it has not been touched in around > 25 years. The shafts are rusty, cables look very fragile. > > So, I decided not to try and retune it because I feel something inside may > snap and I'd be worse off then I am now. So I adjusted the transmit power > down to 25 watts and no desense. > > We are planning to replace the radio system in the next year, so I don't' > really want to invest in a new duplexer. > > What do you guys think? > > -Tim > I think you did the right thing. If it works ok at 25W, leave it alone and hope it lasts until you get the new box. Replacing the cables between the duplexer and station is easy enough, since length isn't important, if you suspect they may be a problem. The cables between cans in the duplexer are a different matter-length is critical, and unless one goes completely, I wouldn't mess with it, again since it's all gonna get replaced soon. -- Jim Barbour WD8CHL Yahoo! Groups Links Yahoo! Groups Links Yahoo! Groups Links <*> To visit your group on the web, go to: http://groups.yahoo.com/group/Repeater-Builder/ <*> To unsubscribe from this group, send an email to: [EMAIL PROTECTED] <*> Your use of Yahoo! Groups is subject to: http://docs.yahoo.com/info/terms/
