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Author Topic: Commander VHF-2000 6M amp I was working on  (Read 1541 times)
W1QJ
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Posts: 1444




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« on: October 09, 2011, 01:12:36 PM »

A month or so ago I had a Commander II 2 meter single 3cx800 amp in here for repair.  That amp was finally fixed up after we found several different problems.  We then moved on the Commander 2000 6M amp with 2 3cx800's.  That amp was fixed today.  It was a very interesting problem indeed and very well may happen to others like it?  Maybe?  If anyone might be intersted in the somewhat intersting problem let me know.  I like the Commander amps they have very nice Rf sections.  I find though (as we know from another thread) that some of the mechanics leave a bit to be desired.  Repairs that might be simple with other amps may be a feat of contortionism for some Cheesy
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K1ZJH
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« Reply #1 on: October 09, 2011, 01:59:05 PM »

Sure,  I'd love to read the gory details of what you found and how you approached the problem. Please do
so!

That's what I expect to see here, instead of the degenerative personal attacks that are developing in other threads.

Pete
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W1QJ
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« Reply #2 on: October 10, 2011, 04:29:39 AM »

Sure Pete, It appears the gentleman who owns this VHF 2000 does a lot of EME work.  As you know EME is a 100% duty cycle mode and I do believe he ran it at full legal limit of 1500  watts.  At about a 30 second or 45 second transmission every other minute, there seems to be a lot of heat in the tank circuit.  In the case of this amplifier, it appears that the insulation material which looks to be either Teflon or Delrin?  Really not sure what it is, but it is not ceramic (which it probably should be).  There are 2 of these "stand offs" that support a silvered strip of copper strap that sandwich the 2 500pf blocking caps.  The bottom strip extends just under this support which is tapped to accept a SS screw which holds down the 2 parasitic suppressors, the HV tap to the plate choke and the strap itself that picks up the one side of the blocking caps.  The SS screw passes through all these connections down into the tapped insulation material.  The top side of the 2 blocking caps picks up the tank coil to the RF section.  The problem with the amp was no output.  With any amount of drive there was no indication of power with any adjustments of load or plate controls.  It appeared as if the blocking caps were open.  I checked them and they were fine.  The amp drew plate current as drive was increased.  Make a long story short here is what happened.  I removed the SS screw which passed through a lug from the plate choke for HV supply, then down through both parasitic suppressors, when the screw came loose and was removed I then noticed that the lower silvered strap which was the bottom connection to the blocking caps was embedded into the insulation material.  Apparently the connection got so hot it melted the insulation material which liquefied and migrated up through the threads of the screw and under the connection of the flatstock and the next connection.  Then it solidified when it cooled and made a nice insulated spacer between the flatstock and the rest of the connections.  Even the threads on the screw were insulated at that point.  This whole melting and solidifying of the material actually disconnected the tank circuit from the anode.  The fix was to clean off the threads of the screw and with my Dremel tool I shaved down the insulation material flat and reassembled the whole thing.  Problem solved.
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