![\"\"](\"https:\/\/serg.org.au\/wp-content\/uploads\/2024\/07\/VCO-testing-work-in-progress.jpg\")
<\/figure>\n\n\n\nOne important test point (apart from the supply DC) is the DC control voltage to the VCO<\/strong>. Normally its recommended to be about 50% of the DC rail that supplies it. This one was very high at 18V, and then I saw the “out of lock LED” glowing faintly! Yet reinstalled back in the frame it was operational.<\/p>\n\n\n\n Checking the pcb number against the manual it became clear that this was (quite cleverly) a Low Band VHF module pressed into service. As the Low Band VCO is run higher than the RX frequency (ie high side injection) the free running frequency was almost enough for high band amateur usage. Just not quite!<\/p>\n\n\n\n The DC being high tells me, the OSC is too low in frequency to get proper lock. As the DC increases on the varicap diode, the internal plates “virtually” widen, thus causing a lower value of capacitance, thus causing the increase in frequency of the VCO. However if the DC is near maximum is shows the free running frequency is not high enough and even with a high DC control voltage cannot reach the lofty frequency required.<\/p>\n\n\n\n Two options. Reduce fixed capacitance on the VCO, or reduce the inductance of the coil. Dismantling and checking the coil construction showed it was not as easy task so I decided to reduce the VCO fixed capacitance instead.<\/p>\n\n\n\n After that it a simple matter of removing the metal shield surrounding the pcb for access, replacing C10 and C11 fixed capacitance of 82pf with 47pf to increase the free running frequency and testing. I cheated and looked at the VHF Hi Band VCO components!<\/p>\n\n\n\n Now the DC control could be adjusted to about 12V allowing control above and below normal operation. And as a bonus it works outside<\/strong> on the bench, and installed <\/strong>into the frame.<\/p>\n\n\n\n Now back to the problem I was working on!<\/p>\n\n\n\n Oh, by the way, there were some signs of dry joints in various stages, showing on the PCB. One was on a\u00a0SMB RF<\/strong>\u00a0socket that was looking like a problem in the near future. Judicious application of a hot iron fixed those.<\/p>\n \u00a0<\/p>\n\n\n\n A short story today! While working on SERG Club repeaters, ie a Philips PRF1520, I struck an RX module that worked inside the unit, yet it would not work outside on the bench. Others worked ok. These are self contained modules normally working with the connection of only a DC supply and input to the RF socket at the … Continue reading “PRF1520 – VCO repairs. July 2024”<\/span><\/a><\/p>\n","protected":false},"author":103,"featured_media":0,"parent":0,"menu_order":0,"comment_status":"closed","ping_status":"closed","template":"","meta":{"footnotes":""},"class_list":["post-6534","page","type-page","status-publish","hentry"],"_links":{"self":[{"href":"https:\/\/serg.org.au\/index.php\/wp-json\/wp\/v2\/pages\/6534","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/serg.org.au\/index.php\/wp-json\/wp\/v2\/pages"}],"about":[{"href":"https:\/\/serg.org.au\/index.php\/wp-json\/wp\/v2\/types\/page"}],"author":[{"embeddable":true,"href":"https:\/\/serg.org.au\/index.php\/wp-json\/wp\/v2\/users\/103"}],"replies":[{"embeddable":true,"href":"https:\/\/serg.org.au\/index.php\/wp-json\/wp\/v2\/comments?post=6534"}],"version-history":[{"count":3,"href":"https:\/\/serg.org.au\/index.php\/wp-json\/wp\/v2\/pages\/6534\/revisions"}],"predecessor-version":[{"id":6657,"href":"https:\/\/serg.org.au\/index.php\/wp-json\/wp\/v2\/pages\/6534\/revisions\/6657"}],"wp:attachment":[{"href":"https:\/\/serg.org.au\/index.php\/wp-json\/wp\/v2\/media?parent=6534"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}![\"\"](\"https:\/\/serg.org.au\/wp-content\/uploads\/2024\/07\/452706720_26011053148538384_7410398348002624874_n-1.jpg\")
<\/figure>\n\n\n\n![\"\"](\"https:\/\/serg.org.au\/wp-content\/uploads\/2024\/07\/dry-joint-1.jpg\")
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