[IRCA] Vast selectivity improvement for my SRF-M37V

["Mike Westfall, Lost Almost NM" <mwdx@xxxxxxxxxx>]

This is for you experimenters out there that are complaining about the
poor selectivity of the Sony SRF-M37V Walkman mighty-mite ultralight.

It turns out that the SRF-M37V does not use the same chip as the
SRF-59. I don't know where I ever got the idea that it did.
I took my SRF-M37V apart and there is a TA737A5 at its heart. Of
course, I couldn't find a datasheet anywhere for it. I guess it's a
Toshiba part, though.

Anyway, there is a small 450 kHz ceramic filter on the circuit card
next to the main chip, which I unsoldered and replaced with a SFP450I
ceramic filter from a dead DX-440 that I had laying around.

The results were simply staggering! I can now receive the very weak
KABR on 1500 kHz, despite the fact that my local KRSN is right next
door on 1490!  Impossible to do on my car radio, even!.

For even more selectivity, I might have tried to use the narrower
SFR450K from the same DX-440, but it's a little bigger, and wasn't
sure it would fit, but the filter I used gives great selectivity.

Taking the radio apart isn't too hard. After removing the four screws,
the case pops right apart, without using fingernails, etc. There are
two circuit boards inside, one for the keyboard/microcontroller, and
the other for all the analog stuff. Yoiu have to unsolder on red wire
to get the two to unfold from each other. Then you have to unsolder a
couple spots on a copper foil shield which is covering the TA737A5 in
order to get to the solder pads for the ceramic filter, which is
marked "50J". Remove the the filter and clean out the holes. The new
filter has to be laid on its side, and small wires run from it's pins
to the holes where the old fiter was  (no, it's not a drop-in
replacement!). AFter you fold the two circuit boards back together,
it's in there nice and cozy, and won;t be going anywhere. If you're
worried about movement weakening and breaking your wires, I guess you
could dab some RTV under the new filter to hold it in place, but I saw
no need for that.


NOW I have a sensitive, selective little DX monster that can switch to
9kHz tuning!
I can't wait to try it with my RS Loop later!

-- 
Mesa Mike
LA de NM
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