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G6KUI > UHF 04.10.07 03:08l 55 Lines 2518 Bytes #999 (0) @ WW
BID : 19008_G6KUI
Read: GUEST
Subj: Re: MM 432/100 Amplifier Bias?
Path: DB0FHN<DB0MRW<DF0ANN<DB0ZDF<DB0LJ<DB0RES<ON0AR<GB7CIP<GB7COV<GB7DBY<
GB7DBY
Sent: 071004/0036z @:GB7DBY.#23.GBR.EU NPF2.54c [G6KUI PMS Alvaston Derby]
My 432/100 amp is the one with the MRF646 driver and 2x CM50-12a outputs.
It draws about 2 amps ( as read by the power supply meter ) when put into
TX but with zero drive.
My MML 432/50 which uses a single CM50 is set at 1 amp quiescent current
with zero drive ( as per the specification sheet ).
I hope the above info helps.
73, Pete G6KUI
}
}
} Hi all,
}
} I'm trying to revive a Microwave Modules 432/100 Amplifier. This is a 100W
} o/p solid state amp using 3x CM50-12a devices, 1 as a driver and 2 in
} parallel for output. I have obtained a circuit diagram for a slightly later
} revision which uses an MRF646 as a driver with 2x CM50-12a for the output.
} Apart from that difference it seems to be very close to what is actually in
} my example.
}
} The amp has failed and there is evidence that someone has "been at it"
} judging by the 20A fuse being replaced by solid wire and the numerous
} melted bits where the soldering has been wielded. However, I'm slowly
} making progress. The dead short across the rails was a shorted 22uF tant
} decoupler on the driver. The RF switching is dead and I have no idea if the
} VSWR protection and thermal shutdown works. But when keying the amp, bias
} appears at the bases of all devices. I suspect that all 3 RF devices are
} working to some degree. I know that RF devices can fail such that they no
} longer work correctly at RF but exhibit DC characteristics that look OK.
}
} What I don't know (it's not on the circuit) and I haven't found reference
} to it on the net, is just what the bias currents should be set to. When
} keyed, the amp draws some 3.5A which seems a rather high standing current.
} So does anyone know what the current should be? What would be cool would be
} for someone with such an amp to measure the standing current when the amp
} is keyed but with no input applied. That would help me no end. I dont want
} to do too much more with it till I'm sure the bias is OK. There's no gain
} at present, but judging by the "friendly fire" soldering iron damage to the
} input circuit, I wouldn't be surprised to find there's no drive getting to
} the base of the 1st transistor. I'm hoping that it will be a faulty
} component on the PCB I can change without having to remove the PCB, that
} looks like a major undertaking.
}
} So if you have a MM 432/100 amp, can you measure the current drawn when the
} amp is keyed but with no input applied and let me know. Thanks in advance.
}
} 73 de Andy GM7HUD
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