Help!!  My repeater seems deaf.
By Kevin K. Custer  W3KKC

A gentleman emailed Repeater-Builder with these questions:

I was hoping you might be able to answer a few questions for me.  I have a GE MASTR II mobile UHF radio that was converted into a repeater.   This is the 40 watt version 450-470 and running on 444.825/449.825.   I am running a Decibel Products 3 can duplexer (1 receive 2 transmit cans) model 4072 (I think), controlled by an ACC RC-96 controller.  The problem I am having is the machine is putting out the power fine and can hear s9 or better over 60  miles away but it seems deaf,  I had it tuned up by a GE service tech,  but it is still deaf.  I can be @ the site talking simplex on my mobile to a person running low power on same freq. but when they go through repeater, all they do is kerchunk, kerchunk, kerchunk it.
Any help would be appreciated!

And my response to him:

In order to help you, I will need you to do a few things:

First, I would take a handheld to the site, connect it directly to the feedline going to your repeater antenna and see if you can talk to a distant user simplex. (over 10 miles or so)  The idea here is to see if there is an antenna problem.

Next, you will need to add a switch in the PTT line from the controller to the repeater.  This will allow you to instantaneously switch the transmitter off and on without monkeying with TouchTones. The idea here is to be able to quickly turn the repeater transmitter off and on. A one-step-better trick is to put a center-off SPDT switch into the PTT line with the armature to the transmitter, one side to ground, and the other side to the controller. With the switch in the middle, the transmitter is disabled. With the handle up (the armature grounded), the transmitter is on full time, forever. With the handle down (the armature connected to the controller) the system is normal.

Then, get a local speaker and plug it into the RC-96, if you don't already have one.  Reconnect the repeater and duplexer system to the antenna.  With the transmitter turned off, (with the switch you have added), have the same user key into the repeater and listen to them on the local speaker.  If your repeater receiver is working ok, you should hear them with the same quieting as you had when you talked with them on the handie connected to the repeater antennas feedline.  A good repeater receiver will hear about as well, or slightly better than a hand-held connected directly to the feedline.    The idea here is to see if the repeater receiver is working ok.

In the next step, it helps if the distant user is NOT full quieting so you can get an idea of comparison.  Tell your user helper to transmit for 30 seconds.  It don't matter if he has anything to say because usually just plain dead carrier is best.  After listening for several seconds, turn on the repeater transmitter.  A good working repeater will allow you to turn the repeater transmitter off and on and have very little or no change in receive quality from the user.  If the repeater is desensing itself,  the person may get really noisy or be blocked out completely when the tx is turned on. The idea here is to see if your repeater transmitter is wiping out the user.  If this is the case, several things could be wrong, but, I would suspect that duplexer tuning or duplexer inadequacy may be the case.

Do these things and let me know your results.

His reply after my suggestions:

Hi again Kevin,

I tried all that you suggested and found out the duplexer is not doing it's job. (Desense)
I hooked up two antennas and system works 100% better so I am in the market for a good duplexer.  What would you recommend?

My conclusion:

Very good.

I suspected this all the time but there would have been no learning experience for you to tell you what to look for.  Now when something else happens you will have a logical approach to solving your repeater trouble.

As for a duplexer, I have but one theory.... buy a damn good one.

Even if it is not in your budget, make it a priority to come up with the money and buy one.  You can certainly skimp or build something for a transmitter, receiver, control, etc.   But the antenna, feedline, and most importantly a good duplexer is a necessity.   I use WACOM duplexers almost exclusively.   I would recommend the Wacom BpBr series duplexer called the WP-678.   It is a 4 can set which I have 3 UHF machines of my own on these cans and I never have desense due to duplexer problems.   The last 678 I ordered cost $790.00 list minus the 25% discount was $592.50 plus shipping.   There is also a 6-cavity version that I recommend for use at high level RF sites, like at commercial 2-way sites with lots of transmitters. Either one is a lot of money,but if you do not have the budget for a good duplexer you probably should not be in the repeater business.

Another method would be to find a good used set, but that can be difficult. Be Careful not to buy another set of inadequate duplexers.  Maybe subscribe to my Repeater-Builder email list and ask if anyone has a set for sale.  Again, be careful...


Copyright 1999 Kevin Custer
All Rights Reserved.