Finally after a long wait new finals arrived: MRF 454 (Macom) replacing
blown MRF 458. I already had received new drivers, the MRF 476 (RFG).
When I was measuring what was wrong I measured a driver biasing
current of 15 mA which is on the high side but well within acceptable limits
(the manual specifies 5 - 8 mA). Suspecting that the drivers may have had a
knock too (some latent damage) I opted to replace those at the same time as the
finals.
The finals are biasing fine but the drivers are not. Even if I bring the
biasing voltage all the way to zero the drivers will still have an idling
current of 56 mA. When the RF amplifiers runs up all the way to 100 Watt output
then I have something like 220 - 250 mA going through
the drivers. I check with the input shorted and it is still present. I
checked with the osciloscope and no parasitic oscilation is detected.
I checked the emiter connection which has two 1 Ohm resistors in
parallel and there is not a short accross those (potentially increasing
the idling current).
Seems these transitors are running in somewhere between class A and
class AB which on its own is not a big issue as far as power disspation
etc goes (imho) and which may give less IMD and is supposed to have
better harmonic supression.
Are transsistors from different manufacturers having large differences
in spec's or are these transistors counterfeit as seems to become more
and more prevalent?
Thanks, 73, MArinus, ZL2ML
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