Have you run your duplexer and antenna system successfully with another radio---i.e.., no desense etc.?
If yes---probably radio trouble. If no----There are lots of possibilities Check the duplexer first------Put a dummy load on in place of the antenna and an iso-tee in the receiver input feed. Set up a weak (threshold) signal into the receiver and turn the transmitter on and off. Does the transmitter desense the receiver-----It should not up to the maximum power that you are going to operate at. If it doesn't, you probably have antenna or antenna feedline trouble. If it does, you have duplexer trouble---be sure that your jumper cables are all good.. Check the duplexer tuning first----common subject here. If you are satisfied that the unit is tuned properly and it's performance is adequate there is another possibility and it is bad news. Is there a possibility that the duplexer pass band adjuster has been moved with transmitter power applied or has it been out of service for years and possibly damp? What can happen is that the typically silver to silver connection between the fixed and movable parts of the center post have been burn't or corroded. The effect is the same----transmitter power results in rectification at the bad connection and wide band noise is produced and lots of it which puts enough energy on the receive frequency and desense occurs----in spite of the duplexer looking perfect on all fancy instrumentation. The more transmitter power, the more noise and desense. And sometimes desense quits below some low power and everything works properly. How do I know about this? I have two Phelps-Dodge VHF duplexers that test perfectly that are unusable in repeater service. Scott ----- Original Message ----- From: tomnevue To: [email protected] Sent: Sunday, April 22, 2007 2:25 PM Subject: [Repeater-Builder] Need VHF MSR2000 Repeater help I have 2 VHF MSR2000 Repeaters that I'm building up at the same time. Both units have the same problem and I'm hoping someone with MSR2000 experience can give me a suggestion on where to look next. I'm having a terrible time with receiver desensing, transmitter hanging on and audio oscillations (intermittent/random growling). I've been thru separate antennas, duplexer with better than 90dB notch, separate antenna with duplexer + extra cavity. If I run the duplexer antenna port into a dummy load, the problem is less but still there. If I run the transmitter port directly into a dummy load or if I turn the PA power down to about 4 watts (with duplexer/antenna connected)the problem is gone. I've replaced the repeater/duplexer jumpers with good double shielded coax and ground strapped the repeater/duplexer cabinet together. I've made sure all of the original repeater panels are in place. The problems exist with the controller in place and also with a test connector replacing it. One of these was a full working repeater in public service use and the other was a convertable base station. Both have the 110watt PA that I am running at 50 watts. The controllers are NHRC-2 connected to the Squelch Gate card using the procedure described by Skipp May. The 12 inch interconnecting cables use shielded audio in/out cables and COS/PTT wires are not shielded. The controllers are enclosed in aluminum boxes. HELP !!! Tom W2MN
