Kostas’ Yaesu FRG-7 adjustment that improves opposite sideband rejection

Many thanks to SWLing Post contributor, Kostas (SV3ORA), who shares the following video and writes:

In this 7.8Mb video (attached) is my solution for “converting” the Yaesu FRG-7 for single signal reception on SSB:

Not a mod actually, no additional filters, no soldering of any kind. Just tune the BFO on USB and on LSB a bit far away from the 455KHz ceramic filter (using the transformer for LSB and the capacitor for USB, as the manual states). As the video shows, this provides the near to
carrier selectivity to cut off the unwanted sideband.

The price you pay is more high frequencies (but in the wanted sideband) and a bit attenuated low frequencies as the filter is effectively shifted to higher frequencies. Very high frequencies cut-off is helped by the tone control of the receiver to some point.

This is a cost-free mod and requiring even no soldering skills, neither any mod to the receiver. Now as you tune the bands in SSB and CW, you do not hear the same signal twice. On AM mode nothing changes, since the BFO is switched off in this mode.

Many thanks for sharing this, Kostas! This seems like a simple adjustment for one of my all-time favorite receivers!

Post readers: Check out Kostas’ website for more modifications, ideas and radio projects.

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15 thoughts on “Kostas’ Yaesu FRG-7 adjustment that improves opposite sideband rejection

    1. Kostas

      Davide, neither me. I do not like modifying these vintage radios. That is why I propose this alignment, because no modification to the radio is required. True SSB is achieved with it and this is to be appreciated for an FRG-7.

      Reply
  1. Ron F

    Yeah, this was a ‘mod’ people commonly asked for back when I was servicing the FRG-7. Never really saw the point, since it doesn’t change selectivity at all. It helps with adjacent-channel interference from the opposite side, but makes it worse from the *same* side.

    My boss/mentor at the time came up with a small add-on board that auto-switched in a narrower filter when sideband was selected; basically, an auto-version of what’s now known as the “Kiwa mod” (though I at least didn’t know of that then). From 35+ year old memory, only a couple of customer radios received that mod. My own had an extended version that switched in different filters for LSB & USB (for RTTY vs FAX) , but that involved some modification around the mode switch.

    I wonder if any of those customer radios – or even mine, which I sold to buy a FRG-7700 – are still floating around?

    Reply
    1. Kostas

      Ron, if you suppress the opposite sideband, this IS “single signal reception”, which the FRG-7 alone doesn’t have. With this mod, the wanted sideband though gets wider, but this can be fixed by AF filtering if you find it annoying, whereas the opposite sideband suppression has to be done in the IF.
      Saying so, I do not find the wider bandwidth annoying, what I find annoying is to hear the same signal twice in the dial and in the case of HAM bands to hear scrambled signals because of the opposite sideband. This makes things really messy.
      And yes, it is easy to add a mechanical filter and in fact I could do another video for it. But the no-cost, no-solder approach seems very attractive to me and it’s something everyone can do. See my new video.

      Reply
      1. Ron F

        > Ron, if you suppress the opposite sideband, this IS “single signal reception” …

        Never said it wasn’t.

        > … which the FRG-7 alone doesn’t have.

        Oh, it does (hence the mode switch on the front panel having “USB/CW” and “LSB” – unless, like fine tune, that too was missing from the very early ones?). What it lacks is sufficiently narrow filters to make it worthwhile, so it’s not very good 😉

        What your alignment of the BFO does is tune it (even further) off to one side. To get correct audio (SSB voice) or tone (SSB RTTY/FAX/etc) recovery, the signal itself has to be similarly tuned off to one side. But the IF bandwidth remains the same – so in doing that, while on one side you’re putting the unwanted sideband down the filter slope, on the other side you’re putting the more of the next adjacent channel into the filter’s passband.

        I’ll even say that that’s a reasonable way to align it in some cases; it’s essentially how you use vintage comms receivers radio with a variable BFO/clarifier & fixed filters. But a variable BFO gives you the option of balancing noise/interference from above/below to suit the conditions/interference; with a fixed BFO & filter, the choice of which way to go is always going to be a compromise – done as per the manual its open to interference from around & on the other side of the nominal (“carrier”) frequency; done your way its open to off-channel interference (i.e. from the other side of the sideband you’re after).

        I’m not saying it’s useless, and I’m not saying don’t do it. As I indicated, it was a common enough request 35~40 years ago when the FRG-7 was still fairly new. If someone wants to do try it, then they’ll no doubt learn something. But in my experience, while it’s probably OK for casual broadcast / sparsely-populated ham band listening, when it comes to difficult conditions it’s no solution to the actual problem of ‘too wide an IF filter’.

        Reply
        1. Kostas

          Ron, the FRG-7 is not a single signal receptor as it comes from the factory, I have tested that, unless mine was very misalligned on initial testing. And this is because like you say it does not have narrow filter. Watch the video and you will see why.
          I agree, you do not limit the filter bandwidth in any way, no need to explain that. With this alignment you cut off the unwanted sideband and you broaden the wanted one, this is clear.
          What I am saying is that to cut off the unwanted sideband this is the only way to do it (in the IF), if you don’t change the filter. But, having performed this alignment, then you CAN limit the bandwidth of the wanted sideband in the audio domain if you like.
          If you do so, even on crowded conditions, you will cut off the very high pitched interference from the wanted sideband, but in the audio domain now. What we are talking is a simple audio low pass filter. The FRG-7 has already a simple one, just set the tone switch to LOW.
          By combining the alignment and the tone settings, the total effect is similar to a true sideband filter. You have heard that in practice in the past, so you know what I am talking about.

          Reply
  2. Arthur Smith

    I have the FRG-7, and watched the video. I’m still unclear as to what was done, and what to do, in so far as improving SSB reception.

    Reply
    1. Kostas

      A simple crystal oscillator can be used at any frequency the radio can cover. Connect it to the ANT input and tune T406 and TC404 for an opposite sideband suppression as you tune the dial to the signal. Both T406 and TC404 had to be turned anti-clockwise in mine. If you see at the manual you will find this info at the BFO section. But the tune frequencies change with this alignment. You do not need a frequency counter. Just tune in the RX to your oscillator signal. As you tune in the receiver close to your oscillator signal, you should hear a tone changing from high to low and then this should not continue from low to high again. That indicates sideband suppression.

      Reply
  3. Kostas

    Yes that was the point. Not to modify the radio and not to spend a single Euro on a mechanical or crystal filter (these are expensive).
    I knew this radio was DSB before bought it because the ceramic filter does never have enough selectivity. That was too bad because otherwise it is a very nice sounding and sensitive receiver. So looking at the circuit, it gave me the idea to try this simple adjustment and make the LSB/USB BFO further “away” from the filter center passband. If the ceramic filter was steep enough, I could reject the unwanted sideband.

    It worked like a charm! This does not require a single soldering, or any lab equipment. A simple crystal oscillator can be used at any frequency the radio can cover. Connect it to the ANT input and tune T406 and TC404 for an opposite sideband suppression as you tune the dial to the signal. Both T406 and TC404 had to be turned anti-clockwise in mine. The video shows the results, so no more words to say. Note, I have currently terrible +20db noise in my location.

    The good thing also, is that the AM reception quality is NOT affected at all, which would be the case if the ceramic filter was replaced by a mechanical or crystal one, unless you installed a separate filter and switched between them, which might require a separate switch to the receiver, which is a thing I did not like.

    There is no magic here, the mechanical filter is just steep enough to allow this setting and the opposite sideband suppression possible without loosing too much clarity of the wanted sideband. Lesson was, before trying any mod you found on the net on your precious radios, consider the theory and the operation of their circuits FIRST.
    Now I have a SSB FRG-7, you can have that too at no risk of damaging the radio!

    73
    Kostas sv3ora
    http://qrp.gr

    Reply
  4. Mike S

    Sounds like a primitive way of implementing a fixed passband shift; something which was a standard (and adjustable) feature in later, more sophisticated communications receivers.

    Reply
    1. Michael Black

      Actually, it sounds more like adjusting the BFO properly.

      For SSB (and CW) you want the BFO on the slope of the selectivity, so the audio image is down the slope. If the BFO is in the middle of the passband, then the audio image gets through. Any SSB transmitter, you want to get the BFO crystals

      I’ve never had an FRG-7 so I’m not sure why it has a control for upper and lower sideband but this is needed. Maybe bad adjustment at the factory.

      Passband tuning is something different since it lets you shift things off the slope without changing the beat note. It’s not just shifting the BFO frequency.

      Reply
  5. Dave Schmarder

    I had a FRG-7 radio over 40 years ago. I bought a new filter from Murata and soldered it in. I had to do a little tinkering to get it in, but the results were great. I did lose some of the highs on AM reception, but it wasn’t bad.

    Reply
    1. Andrew

      Dave, I believe that Kostas’ mod is really smart, since you won’t modify the circuit at all, and reversing it is easy and leaves no traces

      Reply

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