Hello
I have a old Yaesu FL2000B amp that has been giving me fits and thought I would
see if group has some thoughts that might help out here. I have had this amp
many years and used it with very little problems even though it has always
showed reflected power on the input side. Back when I used a Drake T4X to drive
it no problems with the reflected. I now have a FT1000mp and its built in ant
tuner takes care of the reflected. About 3 months ago one of the tubes shorted
grid to plate and burned up a grid resistor and trashed the electronic bias
circuit I had put in many years ago. I thought this was opportunity to work on
the reflected problem. I fixed the aforementioned problems and I have a pair of
tubes in that put out resonable power now so I beleive the tubes are good. That
leaves only the amps input ckt reflected power to deal with.In my searching I
found a article detailing a method using a resistor to simulate the tubes input
impedance and use something like a MFJ 259B for tuning to match with the amp
off. I tried this method and used a external supply to activate the changeover
relay. I was able to change capacitance values to obtain a very good match on
those bands that were bad not all needed this but those that did turned out
very good this way. However when firing up the amp my new match is now worse
than before. The manual for the amp recommends disconnecting the HV leads from
the transformer and tuning the input I also tried this with the same poor
results. Most of my efforts have been spent on 10M as this is one of the worse
bands with at least a 2.1 swr and I noticed that the plate and load adjustment
greatly affects the SWR reading max out is not the lowest SWR point. I have
tested the changeover relay and the band switch to make sure they are properly
making contact and swiching as they should. I have correct idle current and
decent output power.I just need to get the reflected power down but nothing I
have tried seems to work I was hoping someone might have a new
suggestion.Thanks for your time73 Morris wa4mit
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|