January 2015 report from Albemarle Amateur Radio Club technical support (N0WP and K4DND)
January has been a very busy, and currently appears to have been, a successful month for AARC members working on our repeaters.
Our latest round of work actually began on December 11 2014 when we addressed some hum at 146.760 by replacing the power supply on Marshall Manor with a Motorola supply. This resolved the hum, but we still were experiencing some repeater noise that has been perceived as “desense†and a crackling noise on received user audio. This noise or a variant of it has been present for some time.
January 7 2015
146.760 had gone partially silent even though it is being keyed with user activity. No audio from users is being retransmitted. Weather was very cold, and since we diagnosed the problem to either CTCSS decoder or controller audio path, we took spares for both items to site, and replaced both, then set all audio levels to specification. We also moved some power supply connections in an attempt to minimize a slight hum still present.
January 9 2015
In an attempt to create the very best isolation between transmitter and receiver at 146.760 to address the perceived desense. we had decided to remove the 6 cavity duplexer from 146.925, re-tune it for 146.760, then install it at 146.760 following any necessary work. On 1/9/15, we travelled to Heard Mt. to remove the old 6 can duplexer set. N0WP took them home to analyze, repair, and retune them. We found two bad piston variable capacitors during the verification of the duplexers. Replacements were ordered, and after repairs, the duplexers were tuned for 146.760/146.160. Total pass frequency loss on both channels result was about 1.5 to 2.0 Dbm, and reject frequency loss was between -95 and -105 Dbm.
January 20 2015
Returned to the 146.760 site with the repaired and retuned 6 can duplexer set. First returned the power connections to the original configuration which did resolve the recent “motorboatingâ€.
Next, we removed the 4 can Wacom duplexer set and installed the repaired and retuned 6 can set originally from 146.925. The initial results are very good, with noticeable improvement in desense, crackling noise, and receive sensitivity.
Wacom duplexer set was returned to N0WP’s home for analysis and re-tuning to 146.925. Main issue found was a “greasy†coating on one internal piston that caused erratic pass and reject response on the transmit side.
January 28 2015
Back to 146.925 where we installed the newly cleaned (repaired) and re-tuned duplexers originally at Marshall Manor now at Heard Mt. We then adjusted all audio levels to spec and tested. All operation now appears normal. Hopefully the issues found and resolved with this duplexer set will explain part of the ongoing noise we had been suffering at 146.760.
February 1 2015
N0WP