FT-847 Roofing Filter Mod

NB! Don't attempt this mod unless you are very familiar with micro electronics, and are willing to run the risk of ruining your radio! I'll accept no responsibility for the outcome of your endevours.

The original designer of this mod was Charlie Mazoch Jr., W5VIN, who was kind enough to send me all the information he had developed at the time. Charlie's original article (with permission from the author) and Charlies Gain-control mod.

The mod, as implemented by me, requires an INRAD 45,705 MHz Crystal Roofing filter http://www.inrad.net/product.php?productid=235 and the IC-765 add-in Board (not sure if this is available any more) alternatively they have now developed an "Experimenters Kit":
http://www.inrad.net/product.php?productid=268&cat=2 .

Circuit of the IC-765 board and changes, Q1 & Q2 are 2SC3355 in my implementation.

The INRAD IC-765 board with control lead and coax attached. As can be seen I've used extremely thin (1.75mm) RG-178 teflon coax for the signal connections.

I choose to manufacture a screen for the rear side of the PCB, its soldered to the GND traces on the PCB, whether it makes any measurable difference I don't know, but it's an neat way of adding some spacing to the solderpoints.

I chose to insert the Roofing Filter between the two tranformers just after the first mixer, in order to have minimal impact on the rest of the IF circuit.
Remove capacitor C3371 and drill two 0.9 mm holes on each side of the solder pads.

Here seen on the circuit trace drawing from the FT-847 Technical Supplement

and the other side of the IF board, from same source.

Here the coax-cables are mounted to the board, a small area of the ground plane on this side is scraped clear and the screen of the coax soldered to it with the the inner lead and dielectric emerging on the reverse side, where they are soldered to the C3371 pads - tricky, I know!

These points on the AF/CNTL board changes status when the mode is changed.

Mode        TP1013    TP1014    TP1012
SSB        	L    	H    	H
CW        	L    	H    	H
NAR CW        	H    	H    	L
FM        	H    	H    	H
NAR FM        	H    	L    	H
AM        	H    	L    	H
NAR AM        	L    	H    	H
L = 0.4V;    H = 8V

We want the Roofing filter used for all modes but AM and FM

Roofing filter active in modes: SSB, CW Nar CW, Nar AM;  8R = H and 8T = L
Roofing filter by-pass in modes: AM and FM; 8R = L and 8T = H

This can be achieved using only TP1012 and TP1013 with this circuit. and most small signal PNP transistors (i used BC559) can be used!
While experimenting I constructed this circuit Dead-Bug fashion on top of the attachment points. Since it gave me no trouble at all, I never got around to change it.

The 8V supply is picked up at the radio's 8V regulator (Orange wire, see my Crystal Heater Mod for location).

Finally I twisted the leads together and routed them to the Roofing Filter Circuit board.

I had to relocate my #702 , 2.1 KHz Crystal INRAD SSB Rx filter, which found a new home next to the fan.

and the Roofing Filter circuit board fits neatly in the space cleared.

It's now been more than three years since I performed this mod, and it's been trouble-free ever since. I've not had the opportunity to perform the usual range of performance measurements on it (I have neither the requisite skills nor the necessary equipment at my disposal), but I can say that the mod as implemented by me, reduces the first IF gain by some 6-8 dB, which I don't mind!

Jarl, OZ9MO also modified his FT-847, but chose to do the implementation in a somewhat different way, although using the same filter and amplifier. With his permission I show his original measurements:

Outer trace is the FT-847 without the modification, and inner with INRAD Roofing-filter active.

Jarl also made some comparisons with other tranceivers (notations in Danish, but the meaning should be understandable)

I was so impressed with what the addition of a Roofing Filter did to this radio, that I went ahead and got myself a TT Orion 2! This is of course a much better radio in all respects, but my FT-847 still perform better than the vast majority of radios out there. It's been used by my ARC for Field-Day contests with very big antennas on both 40 and 80m, and outperformed anything else we had available (with the exception of my Orion!), including IC-781, IC-756 ProIII, and FT-950.


Back to homepage

Last modified  10.05.2012 Peter Frenning