On Mon, 2008-07-07 at 11:14 +0800, Marinus Loewensteijn wrote:
> 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 oscilloscope and no parasitic oscillation is detected.
With no RF drive and no forward bias the driver collector current should
be ZERO. It takes more than 0.6 volt forward bias on the transistor base
to cause collector current. If you have (check with VTVM with an
isolation resistor in the probe tip) any positive base voltage with the
bias circuit turned down to zero, you have an open in the base circuit
accompanied by collector to base leakage in one of the driver
transistors. That might be from a leaky capacitor in series with a
feedback resistor if there is one in the circuit.
> I checked the emitter connection which has two 1 Ohm resistors in
> parallel and there is not a short across those (potentially increasing
> the idling current).
> Seems these transistors are running in somewhere between class A and
> class AB which on its own is not a big issue as far as power dissipation
> etc goes (imho) and which may give less IMD and is supposed to have
> better harmonic suppression.
> Are transistors from different manufacturers having large differences
> in spec's or are these transistors counterfeit as seems to become more
> and more prevalent?
Transistors have a wide range of gains, RF transistors probably more so.
But relabeling is possible but that should also show up as poor RF
bandwidth, e.g. much lower gain at 10m. Unless the subs are really
better transistors intended for use up through VHF which is possible.
The gain (beta) of the two transistors in an amplifier stage ought to be
fairly well matched, else the stage output won't be symmetrical.
> Thanks, 73, Marinus, ZL2ML
73, Jerry, K0CQ
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