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+--- Thread: FT817 Bias tee modification (/thread-532.html)
FT817 Bias tee modification - VK3ALB - 18-03-2018
Hi All,
For those of us that use the FT817 as a transverter driver the question of PTT always arises. You can use the PTT line on the ACC socket or indeed use the PTT line and +13.8V line from the ACC socket to drive an external bias tee. All very nice but then you need to remember to pack your PTT cable or bias tee. A better way to do it is to build the bias tee into the radio.
VK5ZT determined that the DC path to ground at the output connector of the FT817 is 22K which means there's no need for a blocking capacitor. This mod uses the TX5V line from the PA as the source of the bias tee voltage. I used a 4K7 series resistor bypassed by a 0.01uf ceramic to feed the common terminal of RL3016 which is the antenna change-over relay to switch between front and back socket. The easiest place to inject is at the junction of L3077 and L3081. Ground the bypass capacitor at JP3003.
Work carefully because space is tight and you don't want to melt any adjacent components.
RE: FT817 Bias tee modification - VK3PY - 19-03-2018
I can attest to the usefulness of this mod, having done it to my FT-817. However, be aware that it only works for the VHF/UHF bands. It does not provide the PTT switching voltage on the HF bands. Dunno why. It isn't important enough for me to pursue.
Chas
VK3PY