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The AM Forum Receiver Suite: Post your photos and stories here!

Discussion in 'Amplitude Modulation' started by N6YW, Aug 15, 2016.

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  1. KD2ANN

    KD2ANN Ham Member QRZ Page

    I don't have as many receivers as I used to but I use a WWll navy National RBL-2 for vlf listening ( 15 to 600 khz) and an SW-3 early version awaiting restoration.
     
    KM1H, N6YW and (deleted member) like this.
  2. KA0KA

    KA0KA Ham Member QRZ Page

    I just use slope tuning if the band is congested, I never listen center tuned, it is a waste of bandwidth, better to listen on one sideband or the other, so my war mode is an 8 pole 4kHz filter, I can get 4kHz of audio not 2kHz. It is cheap, I know but that is the point it works great. Most OM's I speak to dont seem to grasp the concept. Next up is the narrow 8 pole 6kHz filter, same thing I get 6kHz of audio not 3kHz. But I prefer to listen in 20kHz. RXer is flat down to 20Hz as well.
     
    Last edited: May 10, 2017
    KD2ACO likes this.
  3. K4KYV

    K4KYV Subscriber QRZ Page

    That's OK when band is congested, but as you have observed with the 20 kHz filter,when the full channel width is uncluttered, better to receive with both sidebands, with mid-point tuning. The vector addition of USB and LSB gives you twice the audio voltage at the detector, which means 4 times the effective power, or 6 dB. But doubling the received bandwidth increases the total background noise by 3 dB, but you still get a 3 dB advantage. Of course, if there is QRM on one sideband, you can narrow the bandwidth and tuned to one side copy only the uncluttered sideband. That's actually a form of diversity reception, since you can switch to the other sideband instantly if QRM changes. But I don't find that mode of reception to be any advantage unless QRM is present.

    When the full AM channel width is uncluttered, even in the presence of atmospheric noise, I find I get better intelligibity with the full bandwidth listening to both sidebands. I usually copy with the 8 kHz mechanical filter (an R-390A filter shoe-horned into the 75A-4). If the band is congested, I switch to the stock 6 or even to the 4 (another R-390A filter). I rarely use the stock 3.1. I also have a 16 kHz filter out of an R-390A, but I use it only rarely. Not many amateur AM signals have enough undistorted high frequency response (up to 8 kHz) to make it advantageous over the 8. It works well for broadcast signals, though (both on MW and SW), on rare occasions when I find one with a high-fidelity signal and anything interesting to listen to.

    The synchronous detector is a great advantage. You still hear all the noise and QRM that fall inside the pass-band, but the synchronised product detector makes the interference practically transparent, so you can copy the desired signal right through it, kind of like when talking to someone in person at a noisy party. You hear all the background chatter, but the brain is able to tune it out so that you hear the person you are talking to perfectly through all the noise.
     
    N6YW likes this.
  4. KA0KA

    KA0KA Ham Member QRZ Page

    I have been in contact with Bob W0YVA for some time, he is finishing up a great CE 100V for me along with the MM2 for kicks. It is a real shame that 100-R never made it to full production. I have thought about making one, it would be fake, but I would try my best to duplicate it. I think there is one 100-R left I have seen. Thanks to http://www.ce-multiphase.com for providing these images for the fans!

    My current TXer is 20 to 20kHz and I use massive pre-emphasis followed by aggressive super fast zero over-shoot limiting. This was all needed to keep the highs from clipping the TXer and keep my talk power avg as though I was at 200% symmetrical. It sounds great in my RF Demodulator when I pull up 12-18kHz in the parametric EQ, but I pull the highs down in the audio processing after 7kHz because most will not hear it anyway.

    The 100V will come with plug-ins to bypass the audio limiter and filter thus providing the same low-level hifi as can be achieved with any rice box feeding the balanced modulator and bypassing the filter on TX as is the case currently.

    Again, no 100-R to be had :(

    100-R.PNG
    100-R_.PNG
     
  5. W2VW

    W2VW Ham Member QRZ Page

    And the easy way to tell if the selectivity is good quality is the ability to take an AM signal and slice off one sideband along with the carrier. If the filter or filtering system can slice an AM signal and make it sound just like a slopbucket then it's ready for use on a congested band.
     
  6. KA0KA

    KA0KA Ham Member QRZ Page

    If you move too far, the slop of the filter will attenuate the carrier signal and you will only have the sideband energy. Any crystal filter works this way, the better the shape-factor the steeper the skirts. My filters are 1.6 shape-factor.
     
  7. K4KYV

    K4KYV Subscriber QRZ Page

    With the Sherwood Engineering analogue synchronous detector, you can tune to one side and still recover the audio with full fidelity. Even if the carrier is down on the slope at the edge of the passband, enough is left to give the local oscillator something to lock onto. I still prefer to receive both sidebands whenever QRM permits, and the detector, being a product detector, makes what QRM there is in the passband much more transparent than it would be with an envelope (diode) detector.
     
  8. KM1H

    KM1H Ham Member QRZ Page

    Why not just use a R-390 and do away with mechanical filters and their phase distortion? Or do the R-725 conversion to a 390A.
    I run my almost stock but fully rebuilt 390A into an external audio amp for AM and it sure sounds better than my 75A3 or 75A4. So does my TS-950SD with the 12kHz filter.
     
  9. K4KYV

    K4KYV Subscriber QRZ Page

    I have an R-390, but it has minor issues I need to fix. I found the L-C tuned circuits to be almost as good as the mechanical filters, and the audio sounds better. The SE-3 would work well with it too. Also have a R-392, which has the same types of L-C i.f. filter, except it lacks the 16 kHz option. One drawback with the 390 series is the choice of selectivities, 2, 4, 8 and 16. They really need a 6; sometimes 4 is too narrow for comfortable AM reception and 8 is too wide.

    I use the outboard sync detector with my 75A-4, along with an external audio amp feeding a BC studio monitor speaker. The audio section of the 75A series receivers is an afterthought at best. I made some improvements in mine, but the stock detectors, both product and diode, introduce a lot of distortion. With the SE-3, the internal detector and audio section of the receiver are by-passed altogether, but with the A-4 I still get the phase distortion from the mechanical filters. The stock 75A-4 doesn't have enough mechanical filter slots; 0.6, 3.1 and 6 aren't enough. I use an outboard box with additional filters, giving me a total choice of 0.3, 3.1, 4, 6, 8 and 16. I use the 8 most of the time for AM.

    Interestingly, the R-390-A wasn't built to improve performance over the non-A model. It was a cost-cutting measure; mechanical filters are less expensive to manufacture than an L-C i.f. strip with satisfactory performance. I believe there is a version of the 390-A that uses the 390 i.f. strip, for some kind of digital mode the military wanted to use, but for which the phase distortion with mechanical filters resulted in unsatisfactory performance.
     
    AF6LJ likes this.
  10. KM1H

    KM1H Ham Member QRZ Page

    The 390 was also a bear to service except at a depot. That special RX was the R-725 I mentioned above and many have built clones.

    My A4 is no longer an AM radio and uses cascaded 500, 800, 2100kc mechanicals with a 6BA6 IF under the chassis between them to provide just enough gain to compensate the added insertion loss.

    I also made changes in the product detector plus the AGC for SSB/CW....Collins never made a decent SSB/CW radio specifically for ham use until the 75S3B/C.
     

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