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Topic: 85watts on 4m from the FT-847, A mod to sort out the FT-847's 4m PA< Next Oldest | Next Newest >
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PostIcon Posted on: May 01 2009,00:36   Ignore posts   QUOTE

In standard UK spec the FT-847 gives out just 10 watts on 4m but at a rediculously heavy power consumption of around 20 amps. Clearly a lot more RF is being generated than is being radiated due to very poor filtering of the 6m IF and therefore the PA is actually amplifying numerous spurii on either side of the desired frequency with the spurii at around 45MHZ being only a couple of DB down on the strength on the desired frequency on 4m!!!
This manifests itself as the radio getting very hot when you have qso with a few long overs on 4m.
Fortunately there is an extremely simple mod that can be done to the UK spec Yaesu FT-847 that addresses the very poor 4m PA performance.
The mod was discovered by Dutch Ham, and electronics genious, Marc Vlemmings (PA10) and it basically involves removing a single 470pf SMD capacitor (C5051) and then snipping one little wire on the other side of the radio (thats goes to the ALC transistor)...and thats about it!...Well there is actually one difficult bit...You have to dissasemble a lot of the FT-847 to get to the capacitor in question, but as long as you make notes on paper of which screws and panels go where as you go it should'nt take more than half an hour or so to do.
The results are a maximum output of around 85 watts on 4m but at a much reduced power consumption of around 16Amps AND the power consumption at 100 watts on 6m also goes down from 21Amps to around 17Amps at the same time! The PA efficency on 6m and 4m is therefore greatly improved, you get a lot more power to play with and the FT-847 no longer gets hot when you have a QSO with a few long overs on 4m.
Interested?...See here for info on the mod and a few others that Marc came up with here:
http://www.vlemmings.com/PA1O/
73
Alf (G7VGG)
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