Modifica allo Yaesu FRG-7000: filtro stretto anche in AM

Yaesu FRG-7000: a modification to use the narrow filter in AM mode
(by Michele D’Amico IZ2EAS)

Introduction

The Yaesu FRG 7000 is a colorful, vintage receiver that is still fun to use. The main drawback when used in AM mode is the availability of only one IF bandwidth, that is way too wide (9kHz at –6dB, measured) in crowded band conditions (What ? “Crowded bands” in 2017 ?). The narrow filter (2.8 kHz at – 6dB) is accessible only in USB/LSB modes.

However, this limitation can be overcome with a very simple modification, that is absolutely not-invasive and completely reversible. With this modification when you select the AM mode the receiver uses the Wide Filter, while when you select the AM-ANL mode the receiver uses the Narrow Filter.

But first let me state it very clearly: do it at your own risk. I will accept no liability for any damage you will do to your radio, to yourself or to this sector of the Universe by following the procedure outlined in this document.

Please feel free to contact me if you want to exchange ideas at: iz2eas(AT)gmail.com

The mod

Since the filters are selected electronically by activating a diode, the modification is very simple: just “cut” a connection on the “mode” switch and rewire it. This will allow you to select the Narrow Filter in AM-ANL mode, while the standard AM mode uses the Wide Filter.

The mode switch is easily accessible from underneath the radio, there is no need to dismount anything apart removing the receiver’s cover; the photo below shows the position of the mode switch (circled in red).

In the schematic below the modification is shown: just “cut” a connection (red cross) on S2(e) switch and rewire it with the blue wire. The section (e) of switch S2 select the desired IF filter (CF01 –wide- or CF02 –narrow-) by grounding a diode (and hence activating it). 

As standard, CF01 is selected for AM and AM-ANL modes, and CF02 for USB and LSB modes. By rewiring section (e), we activate CF01 only for AM, and CF02 for USB/LSB/AM-ANL; as simple as that !

Please note that I kept the ANL circuit operative, since I could hardly notice its effect; if you want, you can disable it but then the modification will be more invasive and you will need to rewire also section (a) of S2.

On the left the band switch before the mod. The connection that need to be interrupted is the one circled in blue, easily accessible, where the white-and-blue wire is connected.
There is no need to dismount the switch.

The two pins of the switch are soldered together; I cut the wire and then used a de-soldering pump to remove the solder and separate them. See:

On the right the band switch after the mod. Note the added green wire that jumpers the fourth (and last) pin of the switch (AM-ANL position) to the first and second pins (USB/LSB position), i.e. to the white-and-brown wire.

The white-and-blue wire is now connected only to the third pin (AM position). See: