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Friday, 22 June 2012

FT-847 PA Mod - there's a better way!


Following the mod of the PA filter in my FT-847 here:

I've been thinking some more about this; given that the PA is capable of delivering 100W on 6M or 50MHz, it wasn't clear to me why I Was struggling to get more than about 60W out on 4M or 70MHz. There had to be some very large inefficiencies in there; then I discovered this link:

And all became very clear, very quickly! I don't know what package Marc used, but he has done a fine job. Once again the FT-847 came apart and I kept removing bits until I had discovered the PA board:

Now, to make the mod described by PA1O, Marc, I need to lift the PA board out. This was a bit scary as I had to remove all the screws holding the MOSFETs onto the heatsync and then very gently flip the board over. Once upside down, I could clearly see the two caps described in the article:

Then with rather a lot of brute force and ignorance I managed to get the one cap removed; it's been stuck down rather firmly so it kind of came off in bits with no hope of ever putting it back again!

So, after carefully putting it all back together (including a dab or two of heatsync compound), the radio is back together. I've removed the ferrite I added when I did the previous mod:

And I now have a full 85 - 90 Watts out on 70MHz with about 16A current draw and still have 100W output n 50MHz with a reduced current draw of 17A (it was about 21A); so I think we can conclude that efficiency is improved.

I can't see any negative side effects of this modification to the PA, the output seems stable on all bands but I'll be sure to let you know if I find out differently.

I'm going to look at the output spectra a some point in the near future to make sure the signal is nice and clean and all harmonics are suitable suppressed.

Cat's not impressed:

Good though, egh?


  1. Did the same mod couple of months ago and worked fine.

    73's Ton pa1pxl

  2. Does the mod that gives 60w also help receive sensitivity and noise floor?Im not bothered about gaining another 30w but i have heard the reciever is deaf on 4m without a mod

    1. No change to RX; you need to fit a pre-amp