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Monday, 18 June 2012

FT-847 on 4M or 70MHz

The FT-847 that I have here includes the 70MHz band when you sequence through using the band up and down button; it also excludes the more recently allocated 7.1MHz to 7.2MHz at the top end of 40M. The latter doesn't bother me too much.

The RF out on 70MHz is a mere 10W, and I have recently been reading about a mod to increase this safely.

The details of the mod have been published by G4FUF here:

The first step is to remove the outer casing by taking off the carry handle and other associated screws, then we need to confirm a number of jumper settings and then measure some test points:

this is to confirm that the on-board configuration is as it needs to be. The test points are in this area of the upper board:

Then the lower board needs to be moved out of the way, this is a simple task by removing 4 gold screws and flipping the board towards the front of the radio:

This provides access to the PA, but first the shield has to be removed:

 Then, finally, we have access to the PA:

Then a piece of ferrite needs introducing into the centre of L5006; this will bring the resonance down to below the 70MHz band.

****** IMPORTANT ******

It is essential that the correct type of core is used otherwise excessive heating may occur; you cant just use any piece of ferrite! The core needs to have a relatively low mu and should be a VHF/UHF type. The core from a TOKO S18, MC120 or MC122 are usable and are currently available from JabDog:

****** EVEN MORE IMPORTANT *******

There's a better way:


You can see the modified component right in the centre of the photo here:

Once this is complete, it will have initially made no difference at all!
We also need to remove the ALC mod that is still restricting the output on 70MHz, this is completed by snipping the green wire in the image below (this is the top board in the radio):

Now I have the desired 65W (ish) out on 70MHz. I've ordered a pre-amp kit to improve the radio on RX, I'll post more when it arrives.

Good though, egh?

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