Morris, This is a very interesting thread, and it prompts me to make a general comment about duplexer tuning. Please do not take any of the following to be a criticism of your friend's knowledge, equipment, or procedures.
Whenever the subject of duplexer tuning comes up, there are always responses from folks who use one or two handheld radios as signal sources and detectors, and they claim that they have had repeated success with such basic methods. Then, we frequently hear from folks who claim that one must have a spectrum analyzer equipped with a tracking generator in order to perform a "proper" duplexer alignment. The folks in the second category usually claim to have vast experience in this area. Finally, we occasionally hear from a few folks who airily claim that only a network analyzer can perform a "proper" duplexer tuning, although a spectrum analyzer with a return-loss bridge may suffice. Sometimes these posters have an elitist attitude, and that can offend the "lesser folk" who don't have such equipment. The fact is that any method that works, is okay! The difference between the methods often is a fraction of a dB. But, sometimes, that fraction of a dB is important- Public Safety is one example. It does little good to use a GPS-disciplined network analyzer to fine-tune a duplexer to within a gnat's eyelash, if the repeater owner simply plugs a crystal from Billy Bob's Discount Crystal Emporium into an existing channel element, and then wonders why his repeater doesn't seem to be on frequency. Those of you who have tuned a BpBr duplexer using only a spectrum analyzer or an RF millivoltmeter know that the bandpass display is very broad. Although you may be able to tweak the notch exactly, the peak of the bandpass response is difficult to tune. Ah, but look at what you see when return loss instead of transmission is viewed! You can nail that bandpass peak with extreme accuracy, and that is important for maximizing sensitivity and minimizing desense from other emitters. I guess the bottom line is that sometimes, an interference problem can be solved by tightening tolerances and fine-tuning each component. Make certain your own house is in order before blaming others. 73, Eric Lemmon WB6FLY -----Original Message----- From: Repeater-Builder@yahoogroups.com [mailto:repeater-buil...@yahoogroups.com] On Behalf Of Morris Dillingham Sent: Tuesday, March 17, 2009 3:54 PM To: k1ike_m...@snet.net Cc: Repeater-Builder@yahoogroups.com Subject: RE: [Repeater-Builder] SCR-200(A) Continuing saga of the SCR-1000 which received signals that keyed COR but with no received audio: We tested again today with the transmitter inhibited but using the duplexer. Receiver performed fine up to 5 or more miles away. Apparently the duplexer is questionably aligned and is causing desense. We go on Saturday to the original tuner who is expressing doubts about how he originally tuned the duplexer. He has spectrum analyzer and other equipment which will allow checking for spurs, etc. so it's not all that bad that this happened, especially since I learned so much about the importance of separation of signals. 73 de Morris KI4IUA > -----Original Message----- > From: Morris Dillingham [mailto:mdi...@nnwifi.com <mailto:mdilli%40nnwifi.com> ] > Sent: Friday, March 13, 2009 10:34 PM > To: 'k1ike_m...@snet.net <mailto:%27k1ike_mail%40snet.net> ' > Cc: 'Repeater-Builder@yahoogroups.com <mailto:%27Repeater-Builder%40yahoogroups.com> ' > Subject: RE: [Repeater-Builder] SCR-200(A) > > Joe, the duplexer had been tuned by someone else. It did appear to work > ok in another lashup with other equipment. I haven't been back to the > test site to try it without the transmitter but it could have been desense > as the COR worked. I'll have to get my cohort to try it as he is close to > the equipment in fact it's at his QTH. You may have guessed by now that > I'm new to this facet of radio. I'm beginning to realize how important > this desense topic is. Thanks for the great question you pose. We plan > another test tomorrow with the transmitter off. > > > 73 de > Morris KI4IUA > > > > > -----Original Message----- > > From: Repeater-Builder@yahoogroups.com <mailto:Repeater-Builder%40yahoogroups.com> [mailto:Repeater- > > buil...@yahoogroups.com <mailto:Builder%40yahoogroups.com> ] On Behalf Of Joe > > Sent: Friday, March 13, 2009 9:39 PM > > To: Repeater-Builder@yahoogroups.com <mailto:Repeater-Builder%40yahoogroups.com> > > Subject: Re: [Repeater-Builder] SCR-200(A) > > > > Hello Morris, > > > > I'm glad to heard the that you got the receiver working! Just one > > question, was the test duplexer tuned to the proper frequencies? Was it > > desense or just loss of sensitivity when using the duplexer? In other > > words, were you able to hear distant signals with the transmitter turned > > off? > > > > 73, Joe, K1ike > > > > > > Morris Dillingham wrote: > > > Well, guys, I promised to follow up with the results of my attempt to > > repair > > > the old Spectrum 1000 with the later model receiver. Thanks to Larry > > and > > > Joe, I have determined that my receiver is not an SCR200A but probably > a > > > later model of the SCR-200. I obtained at least three versions of the > > > schematic from this series receiver thanks to these two gentlemen. > Of > > > utmost importance was finding out the transistor part number since my > > > schematic was the wrong version. I was able to replace the front RF > > amp > > > and get the receiver working again. Before doing that, however, I > noted > > the > > > inadequate wiring, IMHO, in the power supply. The SCR-1000 upgraded > PS > > has > > > plenty of safety factor designed in as far as series pass transistors, > > etc. > > > but the size of the wires was amazingly small. I completely rewired > the > > > high current circuits to match the rest of the design. I am wondering > > if > > > during the period of this antique repeater there was a shortage of > > copper > > > and the price was high. After rewiring, the power output of the > > transmitter > > > was noticeably increased to about 78 watts while it's specified as a > 70 > > watt > > > PA. I took the repeater to a staging area where we had another > duplexer > > to > > > test it on the air. (The actual installation is in the county jail in > a > > > jail cell) Distant signals would develop a COR but there was no audio > > when > > > using the duplexer. When the antenna is directly attached to the > > receiver > > > with the transmitter on dummy load, it works fine. At this point we > > were > > > too tired to go further so will get back on it again soon. I suppose > > the > > > front end may need touching up with that duplexer which has been > checked > > out > > > on another setup. > > > Thanks again to Joe, Larry and the list for their excellent help. > > > > > > 73 de > > > Morris KI4IUA > > > > > > > > > > > > > > > > > > ------------------------------------ > > > > > > > > > > > > Yahoo! Groups Links > > > > > > > > > > > > > > > > > > > > ------------------------------------ > > > > > > > > Yahoo! Groups Links > > > > > >
