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I have an FT-847 that I use almost exclusively for satellite work. With RS-44 now activated I'm using mode J (2m up, 70cm down) a lot and I've noticed that when I press the "RF AMP" button the transceiver goes deaf.

I've confirmed that it's not the switch as the internal preamp works fine when receiving on 2m. I've checked menu option 30 and it's set to "INT" so that's not the problem.

So, does anyone have any ideas?

If I'm reading the schematic correctly the RF AMP switch enables Q3009 for 70cm. On the diagram it's marked as IMD3(D3). Does anyone have another part number that it cross-references to?

That transistor "IMD3 T108" is a strange fellow. You can find info by Google it. Yaesu part # is G3070053. I'm sure they have it in stock. Can also be found elseware used by Panasonic and others.
Are you sure that the FET, 2SK2685ZT Q3004, isn't the problem? That is the UHF amp. Yaesu part # G3826857. Quite common easy to find.
Rgds
Nils SM7DVO

Excellent information, Nils. It's very helpful.

No, I'm not sure that Q3004 is good. I only know that the radio is completely deaf when the "RF AMP" button is pushed on 70 CM.?

Tack!