First Time tuning a MSF

[Bigred]

OK, so what I'm reading here is you're looking at the spec a hooked to the receive of the duplexer, filters etc. and not really seeing anything popping up on your input which is good. Now my old IFR 1500 won't see much past -100 or so it's missing quite a lot. Success there is how low your spec a can pull a signal out of the weeds.

Now let me extract some quotes:

The key symptom is that PL opens but more often than not there is no audio. Sometimes there's picket fencing.
Here is the symptom. A user will key the repeater but no audio or broken audio is present.
But the repeater just keys on and off.
One can hear the repeater coming up but the signal is not passing.

If the repeater comes up, there's no audio, it drops and then repeats as long as the user remains keyed (third quote), this indicates there is indeed a densense situation and your transmitter is a factor in this. Think this way. Receiver is OK, user keys up, PL is detected, rpt PTT is asserted, and transmit ramps up to full. Now with full power TX your receiver is desensed somehow someway, looses the weak signal user, mutes the audio, runs out the hang time an drops TX. Now the process starts all over again. Classic.

Quit burning your eyes out staring at the spectrum analyzer. Get an iso tee (if you don't already have one) and start running desense checks. On the MSF use M3 as a relative RX indicator. Start with the antenna off and dummy load. This will confirm your duplexer, cables etc. are all playing nice. Suspect they are. Next check the floor noise, first with the dummy load and then with the antenna connected. With a busy site up high it's not unheard of to see 5-6dB of broad grunge on VHF your users will have to overcome (bet it's a lot quieter at your house). Sit on it for a while and see if it changes and if it does try to determine what equipment is transmitting if you can. Not much to do there except bite your lip and take it unless everyone gets together for a general site clean up. Sometimes the noise comes from near and far, that's why I say let 'er be unless you can pin point something. If an FM broadcaster is lighting up the site, what are you going to do?

Lastly do a desense check with the antenna attached and your transmitter. It should be zero. Some people let 1 or 2dB slide but that makes me nervous as it is just over the ragged edge and no buffer and could change when your back is turned. I think you will see that this is your problem as long as you're certain the repeater is up to snuff.

Now what do you do? Play detective. Does it happen all the time or occasionally? Can you see a spike on your input on the spec a? Maybe run the site freqs through an intermod program to see if there is a likely suspect. Bad antennas can arc and spark throwing out wide band noise as well as crusty connectors.

To be honest, I feel your repeater is doing fine, it's just that you are suffering from external IMD or noise generated at the site. If you have GM-300s and whatnot there, probably without benefit of filters or circulators, I cringe. It will take some resolve to hunt this down but first comfirm where the problem lies. Best of luck.

[krazybob]

Thank you for your reply. You've given me something to chew on. As for desense it isn't cycling and kerchunking and starting over again without a carrier. One the user drops so does the repeater. In fact, now that the repeater has been moved back to its original home it isn't exhibiting the symptoms. In fact it is working darn well again.

I will still check it for desense and in fact I drilled out the center pin of a F/F/F T connector just for this task. I used a Dremel tool with a rounded bit. Worked great.

Namaste

[krazybob]

This just in!

One of our users discovered and repeated this. If yelling or even talking too loud the MSF over deviates and the audio shuts down. I have never experienced this before. I don't understand it because I checked the OTA deviation at 3.99KHz. Using my Motorola MTS2000 HT set for 4KHz I was at 4.3KHz deviation. We aren't at 5KHz because our frequency coordinator, TASMA, has set a maximum deviation of 4.2KHz. We are on a pair that was reversed some time ago and the effect was that a repeater located 120 miles away but with excellent receive (like us) can hear our output on their input 15KHz away and VICE VERSA. We hear them. Even though I have an Angle Linear dual band pass cavity setup they are far too close in for it to help.

With this new information known it seems like a different problem than the first because I listen to the input and I don't hear over-deviation. I also don't see it on the spec analyzer.

[krazybob]

this is been a long road of trial and error. But the missing page of the receiver tuning manual, 4 - 44, indicates that the repeater is set for full clockwise squelch. that normally suffice is for public safety situations. Hams, as it indicates, are more willing to accept lower signal input levels.

tuning the squelch is based on the SINAD level. It indicates to tune the repeater squelch level for 15 DB SINAD but it doesn't indicate it what input level to do this. I am assuming that I adjust the input level so that it reaches 15 DB. But if further states that the receiver signat level should be set for 12 DB. how can I set one from the other?

I am anxious to get to the repeater vault! we solve the initial problem by moving to our backup location, but it seems that the problem may have been the squelch setting all along. That means I can move it further up on the mountain and coverage will become a figure 8 pattern of the high desert and the coastal areas all from one site.

[krazybob]

I must be an idiot. I have a second MSF that has poor receive and I cannot for the life of me see how to remove the preselector to blow it out. My past Elmer had it out and back in in 5 minutes. What am I doing wrong?

[krazybob]

I fixed it by simply spraying compressed air into the holes. I can now hear a 1KHz tone at 3KHz deviation down to 131dBm. Unusable for sure, but the best of the three that I own.