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The Collins 75A-4 Thread.

Discussion in 'Amplitude Modulation' started by N6YW, May 16, 2017.

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  1. N6YW

    N6YW Ham Member Volunteer Moderator QRZ Page

    The NC-240D...
    Long on looks, short on performance.
    AF6LJ likes this.
  2. VE3AJM

    VE3AJM Ham Member QRZ Page

    According to the Datong manual, the incoming HF signal is first up converted to the 144-145 MCs range, and it is then down converted to the 28-30 MCs range for use with a receiver that can copy the 10m band. If you have a 2m receiver, one can access the 144-145 MCs first conversion output as well. The unit may be used as a 2m converter in addition. You can select any 1 Mcs band from 0-30 Mcs on the front panel of the unit. 7 ICs, 10 FETs, 5 transistors, 11 diodes.

    The unit has a built in attenuator and preselector circuits. I am pleased with the how the UC-1 performs with the 75A4 receiver overall. I've noticed some overload issues on the 19m band, but that had more to do with the crappy distorted audio coming out of Radio Havana Cuba messing with the BBC broadcast, than with the Datong unit.

    Datong made high quality stuff. I had one of their FL-3 audio filters with the automatic notch function.

    Al VE3AJM
  3. N2DTS

    N2DTS Ham Member QRZ Page


    The above circuit seems to work amazingly well at least up to 40 meters and I suppose it would be fine on 20 meters as well.
    With the normal antenna, it has low noise and enough gain to work as good as the best SDR I have tried for recovering the audio of an AM signal, and does better then most due to its very low noise levels (of the entire receiver).

    The passive tuned input is a high Q B+W coil stock and air variable.
    Nothing special or even good about the design other then its easy to get working, but the lack of many stages and the low voltage and low gain keeps noise very low.
    I have never had any issues with other out of passband signals causing any problems, no birdies or spurs or anything else.
    It was MUCH quieter then the R390a, the sdr-iq and the flex 3000.
    The 5000 was the same as are other high end sdr's on the lower bands.
    It does not seem to overload, although there is some difference between detector circuits I have used.
    The original circuit was greast at pulling weak audio out of the noise level but did not like super strong signals or very high modulation percentages (over 150%).
    The current detector (all american 5 copy) does not distort on really strong signals but may not be quite as good at signals on the edge of noise.

    It does not do .1 to 60 MHz, it might not do well with a 5 foot antenna (I never tried that).
    L.O operates at 455 KHz above the listening frequency, the digital display has a 455 KHz offset so it reads the exact frequency.
    The LO drifts VERY little even from a cold start.
    It also uses B+W coil stock.
    N6YW and AF6LJ like this.
  4. N2EY

    N2EY Premium Subscriber QRZ Page

    Exactly. But....a good mixer can be better in all those ways than a mediocre RF stage.

    When you look at classic WW2 designs such as the BC-342, the two tuned RF stages are there largely to insure that the mixer noise is overridden.

    G2DAF came up with a number of 1960s-era receivers using a low-gain RF stage (to make up for the tuned-circuit losses) and a really good triode mixer. Excellent performance was reported in the later versions.

    I was lucky to obtain a number of 7360s at reasonable prices over the years. My current homebrew receiver uses a 6EH7 RF stage (with its own gain control) feeding a 7360 mixer, then into an 8 pole crystal filter, another 6EH7 as first IF, then another 8 pole crystal filter.

    The RF stage was necessary to overcome tuned circuit losses and to reduce IF feedthrough - which can be a real problem with no RF stage.

    Way back in Feb 1972 QST, W1KLK described "An Experimental Receiver for 75 Meter DX Work". It had four tuned circuits at the signal frequency (!) and a low-gain grounded grid 7044 triode RF stage. The mixer was a 7360 and the IF filter was from an R-390A. This receiver reportedly outperformed "a $700 commercial receiver" in the dynamic range department (may be a reference to the 75A-4?)

    G4OEP built a receiver very close to what you describe, except he used a 6JH8. Here's a link:

    Note the use of the 6JH8 as a balanced mixer (balanced LO drive and output).

    There's also the Pullen mixer. There's a mod for the 75A-4 which uses the 6ES8 as the first mixer in the Pullen circuit.

    Well, there was one big change: the use of double conversion in the later models to improve image rejection. But, yes - the basic design philosophy and hardware didn't change.

    All good info - thanks!

    I once had an NC-173 and thought it to be quite good for its time and price range. Not an HRO, but good. I'd also owned an SX-99, and IMHO the NC-173 was an all-around better receiver.

    73 de Jim, N2EY
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  5. N2DTS

    N2DTS Ham Member QRZ Page

    Is all that fancy mixer stuff something that only matters on higher frequencies?
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  6. N2EY

    N2EY Premium Subscriber QRZ Page


    On the bands below about 10 MHz, things like receiver noise figure are generally not as important as on the higher bands, because, in most typical locations with typical amateur antennas, there's so much noise coming down the feedline that a super-low noise figure isn't important.

    But that isn't ALWAYS true. In a quiet location, or with a small/directional antenna, there may be a need for a super-low noise figure even on 160, let alone 80 and 40.

    The usual test is to connect the antenna, tune the receiver to an unoccupied spot on the band in question, turn up the gain until you hear the background noise roaring away, and then disconnect the antenna. If the noise drops dramatically, it means the noise coming down the feedline is far more than the noise figure of the receiver.

    The various "dynamic range" considerations are important on all HF bands, particularly if you like to operate in situations where there are a lot of strong signals on the band and you want to hear the weaker ones.

    Pentagrids make nice mixers but they are quite noisy.

    73 de Jim, N2EY
  7. N2DTS

    N2DTS Ham Member QRZ Page

    That is the thing I do not understand.
    The homebrew is almost totally silent with no antenna connected, I have to turn the volume way up to hear anything.
    If I connect the antenna, there is loads of noise, and I seem to live in a fairly quiet location most of the time.
    I can hear signals with it hooked up to a dummy load (all shielded and grounded).

    For weak signal work, the homebrew is always as good or better then anything else.
    So how much can mixer noise really be an issue?
    Would I have more noise if I ran it with 450 volts on the plate or something?
    I do not see how its much of an issue on HF...
    AF6LJ likes this.
  8. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page


    The National NC-2-40DT and NC-2-40DR (the "T" is the table mount version and the "R" is the rack mount version) were, for their time period (basically a pre-World War II / World War II design), excellent receivers. Now, there was a LOT learned, regarding receiver design, etc., during the war and after, that allowed significant improvements that, with the exception of the Collins 75A-1, did not come into play in the civilian market until the very late 1940s and, usually, into the 1950s.

    The comparison between some of the best receivers of the 1910s, such as the Marconi 106, Marconi CA 294, the U.S. military IP-500, and the U.S. military IP-501 with even the Hallicrafters S-38, in terms of performance, might serve as an example. Today, the S-38 is considered to be a relatively poor receiver. But, had the S-38 been around in the 1910s, the S-38 would be considered "light years" ahead of the competition.

    Basically, everything is relative and one has to temper their rankings of equipment taking into consideration the "state of the art" when the receiver was designed.


    I definitely agree that the owner, of the equipment, can decide as to if his / her unit is for "show", to be used, or whatever!

    A while back, there was a local antique / vintage radio collector who was "restoring" his wood cabinet receivers with a highly polished, "hand rubbed", lacquer finish that, basically, sparkled. His work received praise from a number of other, especially less experienced, collectors. However, as the local collectors' group grew into a national organization and started having a national convention with judged contests, things changed.

    Instead of his radios being judged highly by the finish, they were downgraded because of being "over-restored"! Yes, most of the manufacturers used lacquer on their radios. However, "hand rubbing" to a gleaming finish was not normally done. Lacquer was used because it gave a fairly hard finish, was easy to apply, air dried very fast, and, especially, because it was cheap!

    There are a relatively few people who, when a radio is repaired, want the original "look" to be retained no matter what. That means re-stuffing old components, hiding "modern" components so that they cannot be seen, and so forth. But, most people want the unit to actually work and really don't care about how the underside of the radio "looks"!

    When I restore an antique / vintage radio for entry into competitions, I definitely do things like re-stuff capacitors. But, when I am repairing a unit for someone else, and especially when I am working on one of my pieces of equipment, I replace defective parts with modern parts and I really don't care how the underside "looks"!

    Glen, K9STH
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  9. N6YW

    N6YW Ham Member Volunteer Moderator QRZ Page

    To your point, and an excellent one at that, I must make known that I have owned just about every receiver made by National, Hammarlund, Hallicrafters
    and Collins. In my experience, the Hammarlund Super Pro series up to SP-400, and National 100's & HRO series up to 5TA-1 series, walked circles around
    the 75A-4 in favor of copying AM. However, regarding the A-4, I find it pleasing that so many have some great ideas about making it a better receiver not just for AM but for SSB as well, thus the focus on this thread... not being terse here.
    My comment about the 2-40D is based on experience and while it's a great looking unit, it's performance falls short... way short. My 5TA-1 blew it out of
    the water in sensitivity and listening pleasure. In fact, of any receiver I have ever owned where If I had to park myself for hours with "cans" on, it would be
    the HRO & SP-100/200's. The SP-600 is a joy but only for SWL on the lower bands, which I do all of the time.
    However, seeing how the thread is regarding the A-4, and is about making it rock and roll like it should, I am going to steer the thread back on topic.
    Thanks for your invaluable input and I hope you chime in on some more of the acceptable mods making it perform better.
    AF6LJ likes this.
  10. VE3AJM

    VE3AJM Ham Member QRZ Page

    I had a Sherwood SE-3 synch detector given to me last month. Looking forward to getting it latched up with a 75A-4. Don K4KYV says its a huge improvement for AM and sideband along with using an external audio amplifier.

    Al VE3AJM
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