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Author Topic: kwm-2 main vfo stability  (Read 3255 times)
4X1KS
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« on: October 17, 2017, 12:13:48 PM »

Hello all.. I'm working on my kwm-2 winged emblem vfo.  my external vfo 399c-1 works like a charm.  today I did the VFO dial centering on the 399c-1 and it came out perfect.  When connected to my kwm-2 it is stable and pretty accurate.  However, the main vfo is another story.  It is very unstable when the rig is first turned on and does not produce a nice clean calibration tone on 14.300 lsb like my other kwm-2a.  It does become stable after it warms up for a good while.  But even after it is warmed up, it still doesn't have the nice clean perfect tone that the external VFO does...and it seems a little drifty whereas the external vfo is right on and doesn't move.  Any ideas on what to look at?  Clearly this is something specific to the main VFO circuit.

73 and thanks
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W9GB
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« Reply #1 on: October 17, 2017, 07:14:38 PM »

Collins 70-K2 PTO (used in KWM-2 and S-Line)
SERVICE ADVENTURES
by Jim Miller, N4BE
http://www.wb4hfn.com/COLLINS/UserArticles/PTO_Servicing/70K2-PTO_Service.htm

Collins 70K-2 PTO Repair Story
by Ernst F. Schroeder, DJ7HS
http://www.qsl.net/dj7hs/70K-2_PTO_repair.htm

Service Line II - Taming 70K-2 PTO Frequency Shift vs. Voltage
by Dick Weber, K5IU
The Signal, Collins Collectors Association, #67, 3rd Quarter 2012
http://collinsradio.org/Signal/newsletters/Issue%2067%203rd%20Quarter%20of%2012.pdf

Collins Radio Service Bulletin (SB) #1011, dated 6-26-63
http://www.collinsradio.org/wp-content/uploads/2015/05/SB1011-Assy-Retype-pdf.pdf

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4X1KS
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« Reply #2 on: October 18, 2017, 05:43:54 AM »

thanks so much for the links.  Strangely enough, I turned on the rig today and the main vfo is solid.  I am getting a hunch that the jim miller's article is on target with my potentially drifty vfo.  However today it is stable...I guess we call that intermittent.  Not sure what to do next.  However one issue that bugs me is the VFO overtravel.  According to the book, I should loosen the 2 set screws on that are on the stopper collar.  However, the chassis plate is in the way...only 1/2 of the set screw heads are visible and the Bristol wrench can't get in there.  On possible solution is to drill a hole in the edge of the plate above the set screws so that I can have access.  I noticed in Dennis Brothers video, he has not plate obstructing his access to the radios collar set screws.  It looks like on his radio someone cut away a bit of the plate to allow access.  What do you think?

73

Mark 4x1ks
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4X1KS
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« Reply #3 on: October 19, 2017, 11:07:40 AM »

With help from Jim Miller, I was able to disassemble the KWM-2 PTO from the chassis.  Waiting on caps to do the PTO rebuild.  Based on the symptoms, the replaced caps in the PTO should solve the drift and off frequency issues.  Thanks for the help. Kudos to Jim N4BE
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W9GB
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« Reply #4 on: October 26, 2017, 02:07:29 PM »

Mark -
Great to hear.  Look forward to the update.

gb
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