Radio Shack PRO-2042 10.7 MHz I.F. Filter Modifications

Overview

One of the wasted features on the Radio Shack PRO-2004/5/6/2035/2042 series of scanners is the "wideband FM" reception mode.  The original idea for this circuitry was to allow the scanner to receive commerical FM broadcasts - and who the fuck would want to listen to that crap?  Thankfully, we can change out a single component in the scanner's wideband FM demodulator circuit and tweak the wideband FM mode into something useful.

If you may not know, Radio Shack PRO-2004/5/6/2035/2042 scanners DO use a standard 10.7 MHz Intermediate Frequency (IF).  These scanners have a separate IF chain which mixes the (unfiltered) 48.5 MHz IF with a crystal local oscillator of 37.8 MHz to produce a final IF of 10.7 MHz, which then drives a Samsung KA2243 (or Hitachi HA12413) wideband FM demodulator.  One problem though, this only happens if wideband FM reception mode is enabled in the scanner or on that particular frequency.  I mention this because, back in the early 1990s or so, several companies offered scanner-based spectrum analyzers and other neat demodulation schemes (including video) that only worked if you could tap the receiver's IF chain somewhere for a 10.7 MHz output.  The magazines articles and authors would often mention that Radio Shack scanners would not work with these neat toys.  We'll guess what - they do!  You just had to enable the wideband FM reception mode, and the scanner would switch over to a demodulation circuit which uses a final IF of 10.7 MHz, instead of the normal 48.5 MHz and 455 kHz IFs used for narrowband FM and AM demodulation.  Those companies could have made millions more, if they would have just looked at the scanner's schematic.  Oh well...

What we'll want to do is replace the stock 10.7 MHz IF filter, which has a 6 dB bandwidth of around 280 kHz, with a similar filter which is much "sharper."  Thankfully, Digi-Key offers an exact drop-in replacement filter which has a bandwidth of only 13 kHz.  This reduced bandwidth greatly increases the scanner's sensitivity during wideband FM reception mode, while still allowing over modulated narrowband FM signals to be heard properly.  You'll find this "new" mode handy for receving analog cellular or cordless phone transmissions, and can also help to improve your wireless data or pager decoding schemes.

The replacement filter we'll be using is a Murata SFVLF10M7F00-B0, which is available from Digi-Key (Part Number 490-4713-ND) for less than $3.  The Samsung KA2243 actually uses two 10.7 MHz IF filters, but you only really have to replace one.  Replacing both wouldn't hurt though, as that will also help to increases the scanner's selectivity a bit.  Some analog cordless phones may contain a similar 10.7 MHz IF filter, if you want to try and salvage one.  The filter's input/output impedance should be around 300 ohms.

It should also be possible to construct a high-dynamic range 10.7 MHz IF chain and FM demodulator using narrowband 10.7 MHz crystal IF filters and other high-quality parts.  This will greatly improve the scanner's narrowband FM reception mode.









Pictures & Construction

Internal view of a Radio Shack PRO-2042 scanner showing the 10.7 MHz IF strip for receving wideband FM signals.

The 37.8 MHz crystal local oscillator is on the left, marked "X1."  The 10.7 MHz IF filters are the two little, three-legged orangeish things (usually marked "CF1" and "CF2") next to the Samsung KA2243.  The filter between pins 4 & 6 on the KA2243 controls the final selectivity, so that's the one you'll want to replace.

The part number for the stock 10.7 MHz filters in a Radio Shack PRO-2006 scanner is SFE10.7MA5W-A and they are also made by Murata.

Remove the old filter.

Turn the scanner over, and locate the pins for the Samsung KA2243.  Locate the filter (labeled "CF2") to be replace by following the traces from pins 4 & 6.  Unsolder and clean the pads.

Replace the filter.

You can solder the filter on the bottom of the board to allow easier access if you want to swap out different filters for experimenting.

An amplified 10.7 MHz IF signal, with an output impedance of around 300 ohms, is available at pin 4 on the KA2243, if you wish to further experiment with other demodulation circuits.



















Old Murata 10.7 MHz IF Filter Catalog Pages

Radio Shack PRO-2006 Wideband FM IF Schematic