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Additional notes on the Yaesu FT847 modifications for 70MHz operation

by Keith Naylor G4FUF

This further information should help to identify the "current" modification state of an FT847. It appears to be valid for UK imported models from s/n "8J"onwards.

The original manufacturer's modification has added a logic-level switching transistor on the RF-UNIT board (see the picture below). This device is switched on when the radio is within the frequency range 54-76MHz.

The output of this inverting transistor goes to the AF-CTRL-UNIT. This transistor applies a logic '0' into the power output control circuit at device Q1019. It has a similar effect as when a bad VSWR is detected, or AM mode is selected: this results in the power output on 4m being limited to around 10-15W.

Removing the wire between the RF and AF-CTRL units permits maximum output on 4m. The wire should not be removed until the PA inductor mod has been carried out otherwise excessive current will be drawn on TX! The extra transistor fitted must be retained as it will be used to switch the 4m pre-amp in or out.

If D3083 was fitted on the RF-UNIT then the power output on 4m would then be under the control of the HD-FWD pot (VR3005). As this diode is not fitted on any of the models seen so far, there will be no ALC on 4m (all the models checked so far had less drive on 4m than 6m or HF, so this is not a problem).

On all models seen so far, D3076 is fitted, to reduce the power output on 160m. For UK models they are set for 100W so VR3005 is actually redundant and could be used on 4m at a later date (subject to ongoing investigations to increse the drive power on 70MHz). The coupling of the RF monitor sampling the FWD and REV voltages appears pretty flat past 70MHz, so the LCD power output displayed is nominally correct.

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