Hello Dave Here is a quick way to test for duplexer internal intermod-----result of signal rectification resulting from lighting burn damage, tuning under power damage, corrosion, bad connectors etc.
Put a transmitter on the REC. and TRANS ports tuned to REC and TRANS pass frequencies of each------for ham work, a couple of similar power mobiles work well. Put sample "T" and an adequately big, known to be good, dummy load on the duplexer antenna port--- 40 db. or so of suitable power attenuator is another OK way to go here. Now, with a spectrum analyzer connected to the sample "T" or attenuator string, run both transmitters at the same time (max. power) and look at what comes out of the duplexer. antenna port. If all is well, you will see only the two transmitter frequencies------similar display heights if the transmitters are of similar power and the pass losses of the duplexer are similar. If all isn't well, you will see the two transmitters and mix products too----and if you do, this is bad news------you may have to look down a way ----you should not be able to see any mix products. I have used this method to identify a couple of "bad" duplexers that desensed badly in service that looked absolutely perfect in tune up and checkout using the usual tracking oscillator / service monitor and or high-grade network analyzer methods. Scott, N6NXI ----- Original Message ----- From: ve7ltd To: Repeater-Builder@yahoogroups.com Sent: Thursday, December 21, 2006 2:32 PM Subject: [Repeater-Builder] VHF Duplexer problem - help? I was helping a local group tune a 6 cavity Sinclair Q2330 VHF duplexer. With a spectrum analyzer, I was able to get the pass/notch looking really good into a dummy load. I did notice that one of the capacitors did not tune linearly. All of the other caps would tune the notch smoothly as it approached the correct notch frequency. This one cap would tune weird. As you rotated it on one direction, it would tune the notch closer, then farther, then closer, then father. All the other caps would only move linearly in one direction when rotated. When hooked up at the site, there is not a lot of loss either on the RX or the TX. But as soon as the transmitter comes on, no matter what the power output, there is a huge amount of RX noise generated, whether terminated with a load or the antenna. The noise is also intermittent - It comes and goes. If that suspect cap is turned under any transmit load, it crackles loudly. None of the other caps exhibit this behaviour. The noise desenses the receiver badly. I have not scoped the transmitter yet, but it was working fine when removed from service (GE MASTR II 40 W station). I also have used and tuned many of these in the past, and they all have worked fine. There was signs of lightning damage on an old antenna they removed from the site a few years ago (the cable harness in a Sinclair 210C- 4 was burned in one dipole. I am unsure if they replaced the feedline. There is no reflected power. The duplexer also was originally tuned in the low 160's and I am aware that the harness may need to be changed. Here are my questions: 1) Besides the weird behaviour, is there any way to test if the cap is bad? 2) If there is lightning damage, where/how may it be visible? Is it repairable? Where would I find a replacement cap? 3) Does the fact that the system works fine with a signal generator and sprectrum analyzer suggest that the problem is in the duplexer? I have set up many repeater systems, and never had a problem like this before. Any ideas? Thanks all. This group is a great resource for people like me that want to get into repeater building. I would not be able to be where I am without the help you all provide. Dave Cameron VE7LTD
