'Yaesu FT-840 AM-filter.'


The vast majority of Yaesu FT-840 radios out there do not have the optional XF112A (XF-122A) 6kHz wide band AM filter installed.
Purchasing this filter option still can set you back $150 on the second hand market.





While this factory filter indeed does work very well , its additional cost is often hard to justify for the casual listening on the shortwave bands.


Using the radio on AM mode without this filter invokes the SSB filter , and at half the bandwidth the audio quality of the AM stations is of course quite terrible.

While I can afford to buy a factory filter I decided to give a go to an idea that I have seen floating around on the net for some time ,,, that of a LC filter , instead of a X-tal filter.

After 2 failed design layout mistakes that created bad boards I got it right on the 3rd board.
The accessory filter board must slide down onto header pins on the radios main board exactly , and this precise fit was a little 'fussy' to get right.

Once the right board was etched up I installed the several components as shown on the schematic.

As the design on the net had little technical information to go on I decided that I would have to work that out too.
The toroidal core used was a FT37-43 with 7 turns of 0.3mm wire and that gave an inductance of 21.35uH for each winding.

Each winding is the same , and it has in parallel with it a trimcap of 1-10pF along with a NPO 15pF capacitor.
A 0.01uF capacitor goes to ground.

Together this would allow a resonant frequency of 8.2MHz which is the radios IF.
Adjustment of the trimmer is not critical as the bandwidth is very wide.

It is important to note that the output side of the filter is flipped in relation to the input.
The 4 pin connections on the radios board are really ---


1                     4
2                     3
3                     2
4                     1
front






To provide correct orientation my PCB artwork has a mark to indicate the front (fascia) side of the radio.
It is the small indent in the circle.

Note too that I have used 5 pin sockets , which are a little more common to find , but if you can find 4 pin ones it is better.







In operation the filter does as expected , it does offer wide band reception on AM mode.
The audio quality is very nice , especially on strong signals where you can really appreciate the sound with minimal noise.

Unfortunately the filter is VERY wide ..... !!!!!!!!!!!!!!
When the wide filter is being used the radio is quite useless at digging out of hard to hear signals that are alongside strong signals .... but then , perhaps you would not be using the wide filter in this situation anyway ?

The effect that the wide bandwidth gives is like this...
To the ear it sounds like the frequency pulling that you can get with FM signals on the crowded FM BCB , but of course it is not the same.

What is happening here is when you are listening to a station and its signal strength drops away the AGC will increase.
Then any other signal that is heard within the wide filters response becomes louder , even if this station is off frequency.
So it sounds as if the radio has tuned away , but it hasn't.

This however is really the only negative aspect to this 'wide as a barn door' LC filter.

As stated earlier it is no comparison to a real X-tal filter , but for the cost of say $5 , it is pretty hard to beat , and quite usable as it is.....



PCB files.
filter1.ps
filter1.pdf
filter1.brd
As usual , use of these files is free to all Hams , but use by anyone , in any way to make profit is prohibited.
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