Somebody who previously owned my Commander HF-2500 replaced the carbon resistors in the plate parasitic chokes with a different type of resistor.? This worked fine on low frequencies 160M, 80M, and 40M.? ?When I used the amplifier on 10M, it became very unstable, most likely with a parasitic oscillation and caused the plate blocking capacitor to explode and catch on fire. The main fuses blew, as well as the one on the board that contains the bias and control circuitry.? After replacing these fuses and installing carbon resistors in the parasitic suppressors, I found that the amplifier would no longer work on 80M.? Eventually, I discovered some discolored places on the plate RF choke where it had evidently arced through its insulation causing the needed 10M high impedance to shift.? The choke was replaced? with one I had from another Commander HF-2500 that has been a source of spare parts.? While trying to do this, the windings (there's more than one - rather complex winding - some over the top of others) became loose, and I unsuccessfully spent several hours trying to get the windings back like they were and tight on the form they were wound on.? Even though not satisfied with the replacement choke with loose and sloppy windings, I tried the amplifier on all the usual bands starting at 160M and up to 10M.? It seems to work fine up through 15M, but the 10M band doesn't tune properly - most likely because the plate choke was still a problem.
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Does anybody have an original Commander HF-2500 plate choke that they would like to sell or know where I might find one?? Another possibility is to substitute a different plate choke that works well in another amplifier with similar plate parameters. Probably the big challenge in using a different choke would be to mechanically fit it into the rather crowded place in the Commander. Another possibility would be to add a second plate RF choke with a high impedance on 10M in series with the other choke, but this still might be problematical due to the tendency of the big choke to break down as before. Suggestions, especially from anyone who has successfully used a different choke in the Commander, would be very much appreciated.
73, Riki K7NJ
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