Using ALC to run FT-847 QRP ...




Notes:
  • External power, 7v to 15v, meter lamp intensity will vary with supply voltage.
  • External power cuts internal battery via battery positive lead.
  • Only LED power indicator lights with battery power, meter lights only on external power.
  • ON/OFF switch kills both external and internal power sources.
  • Minimum negative voltage is one diode drop, typically 0.6 volts.
  • Maximum negative voltage is approximately 1.80 volts due to meter loading. <
  • Maximum negative voltage is higher on ecternal power.
  • Meter has full scale reading of about 1.8 volts and is nonlinear due to diodes.
  • Battery current drain controlled mostly by LED current demands.
  • FT-847 is typically down to 10 watts at -1.2 volts and completely shutdown at -1.5 volts.

    Some of you might want to use the FT-847 with an amplifier or transverter. Many amplifiers provide ALC feedback to prevent the transmitter from overdriving the amp. Transverters don't often have such protection, but you can provide ALC control to your rig to keep the RF power out within the limits of the transverter.

    Rick VE3CVG recently reported the results of tests he did by adjusting the negative ALC voltage and measuring the resulting power out. Rick's report is reproduced below. Keep in mind that, if you lose your negative ALC voltage, then your FT-847 power output will rise to whatever level you have set with your RF Power knob.

    "I performed some simple tests on the ALC circuit of the FT-847 with a view towards limiting the power output so that the radio may be connected to a Down East Microwave transverter. I was really pleased with my findings. Bias(V) RF out max.

    Notes:
  • -1.190 10w RF Out control reduces output pwr when set between min and 9 o'clock
  • -1.155 5w RF Out control reduces output pwr when set between min and 9 o'clock
  • -1.213 2w RF Out control has barely perceptible effect on output pwr below 9 o'clock
  • -1.250 1w RF Out has no visible effect
  • -1.286 .50w RF Out has no visible effect
  • -1.313 .25w RF Out has no visible effect
  • -1.410 few mw RF Out has no visible effect
  • -1.510 not visible RF Out has no visible effect

    "Readings were taken with the FT-847 set for 21.100 MHz with mode set for FM. Bias measured at RCA-type ALC jack on rear of FT-847 using a digital multimeter set on 2000mv. RF level approximated from MFJ949E tuner/swr bridge set for 30w (full scale)."

    "Power supply made from 9v wall wart into an 8v 3-lead type regulator. 2k, 10-turn, variable resistor across output. +V lead to gnd of FT847, tap on variable resistor to ALC connector. When the radio is keyed via the PTT button on mic, the bias voltage drops 3 mv. All bias voltage readings above were measured while radio PTT was keyed."

    "PLEASE NOTE - this is for info only, your radio may give different results."

    Please let us know what you find. I still want to measure the actual current drawn by the ALC circuit, but I know that its small. I will repeat the tests at 144.200 MHz.





    One sad note is that I never got to see how this device performed on field day as it was stolen out of my truck along with all my other field day stuff including the FT-847 on a Thursday before field day week end, from a church parking lot no less, a couple of years ago. Nothing was ever recovered. Later I replaced the FT-847 but have not rebuild the ALC unit.