The Collins 75A-4 Thread.

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

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  1. K4KYV

    K4KYV Subscriber QRZ Page

    You will note on the schematic that the C-numbers of the 510 pf caps (147 and 148) are out of sequence with the other components around them. The original receiver did not have them. Those were added in later versions. Clipping them out just brings the receiver back to its original design. Due to Collins' physical layout (deep chassis and layers on top of layers of wired components) I found it way too difficult and time consuming to remove the 0.01uf caps without damaging adjacent wiring and components with the hot soldering iron, so I simply bridged the new 0.1uf caps across the originals and left them in the circuit. If the originals are not shorted or leaky, their capacitance is so low (circa 10%) that it will have negligible effect on the new ones. Leave the old caps intact and use the wire leads on the old ones as terminals to solder the new ones to.
    AF6LJ likes this.
  2. N2EY

    N2EY Premium Subscriber QRZ Page

    What do you mean by "the homebrew"?

    Do you mean that 6BE6 mixer circuit?

    If's what's going on:

    The component values in that circuit are very different from the recommended ones. They're all very much greater, and the plate voltage is lower.

    What this does is to greatly decrease the conversion gain - and the noise. That gain has to come from somewhere else.

    There's also the question of what bands you use it on. 160, 80/75 and even 40 are a very different environment from, say, 20 or 15 meters.
  3. N6YW

    N6YW Ham Member Volunteer Moderator QRZ Page

    He is referring to his Homebrew Receiver, the one he built that he references throughout this thread.
    He is quite an outstanding builder too.
    N2EY and AF6LJ like this.
  4. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    I'd say so too, Billy!
    Just in case Jim N2EY missed this photo of Brett N2DTS in his lair...

    N2EY, AF6LJ and N6YW like this.
  5. N6YW

    N6YW Ham Member Volunteer Moderator QRZ Page

    Need we say more? N2DTS is one of the finest builders I have ever had the pleasure to greet and view here.
    His work is impeccable. Pride in craftsmanship is to say the least!
    He rocks.
    N2EY, KD2ACO and AF6LJ like this.
  6. N2DTS

    N2DTS Ham Member QRZ Page

    Yes, both my homebrew receivers.

    Page forward to see more of the circuits.

    I got the basic mixer design from someplace, and just stuck that LO circuit into it.
    There is not really much to it, what values are greater then what?
    I don't pretend to be an expert, I just tried different designs and different values within the designs till I got something that worked well.
    The receivers will pick up strong signals with no antenna hooked up, yet the audio is silent with no strong signals.
    Works great on 40 meters and I suspect it would work well on 20 with a decent 20 meter antenna.
    I suspect many receivers were designed to use poor antenna's and provide way more gain than is needed when a good antenna is used?

    I listen to weak signals with the antenna bridged to various receivers using a passive splitter.
    Normal operation has a 3 way passive splitter in line and I can also switch to direct.
    My Anan 10E is about the best sdr receiver I have had for a number of reasons, and there is no difference and sometimes the homebrew does better at audio recovery on weak signals.

    I have had a large number of receivers, new and old, sdr and not, tube and solid state, 75A4, qs1r, flex 1500, 3000, 5000, Icom 756 pro 1, 2 and 3, ten tec Argonaut 5, r390a, nc300, nc303, Scott SLRM, SX17, R1000, Elad fdm duo, KX2, ts440, icom 735, sdr-iq, and others.
    Nothing has topped the home brew receivers for AM reception on 80 and 40 meters where I operate.
    Almost all of them have much more background noise that covers up weak signals.
    The qs1r, the Anan, asnd the Elad have been the quietest of modern receivers.

    No rf amp (some radios had two!) single conversion, low plate voltage, low gain tubes makes it quiet I suppose, that is why I do not understand the need for special mixer designs using special rare tubes.
    What would happen if I made a low noise rf amp and put it into a converter to bring it down to 7 MHz into my receiver?
    Or just used a good RF amp into my mixer and changed the LO frequency?

    I could never build a receiver that does a good job from 1 to 30 MHz I am sure, but just how important are the mixer designs?

  7. N2DTS

    N2DTS Ham Member QRZ Page

    Receiver number 2:

    Many people build much better then I do, and not everything I build works, or works very well, but the receivers seem to.
    AF6LJ likes this.
  8. VE3AJM

    VE3AJM Ham Member QRZ Page

    Nice looking shack you have over there Brett. The only way these days to hear you and Russ WB3FAU and all on 40m is to listen in on a online receiver in Knoxville TN. The 40m band conditions for more local work has been terrible.

    Thanks for the link to Jim Tonnes critique of detector/agc issues with many boatanchor receivers. The 75A-4 certainly has those issues as well. The detectors for AM SSB and the AVC circuits have issues.

    Al VE3AJM
    N6YW and AF6LJ like this.
  9. N2EY

    N2EY Premium Subscriber QRZ Page

    Great stuff and excellent workmanship! Thanks so much!

    The cathode and screen resistors are much higher values than normally used, and the plate voltage much lower. What that does is to reduce the gain - and the noise.

    I'm no expert either, just another homebrewer. If it works for you, it works for me.

    Is there a way for you to try it out on 20? Would be fascinating to see the results.

    I don't think so. I think the real issue is that many receivers were designed to minimize the number of stages, with good or poor antennas, and so the mixers run at high gain levels.

    Most receivers designed after 1960 weren't optimized for AM, for one thing.

    All depends on what the receiver is used for. One big concern for the contester and DXer is the ability to hear a very weak signal right next to a very strong signal, (CW in particular).

    Try it and see!

    IMHO, it all depends on what you want the receiver to do - and in what environment. Consider the 75A-4 that started this's fundamentally a (dare I say it) single-sideband receiver. It does well on other modes, but it was meant to be an SSB receiver first and foremost - and that shows.

    73 de Jim, N2EY
  10. N2DTS

    N2DTS Ham Member QRZ Page

    Sorry about the thread hijack, I just wondered why such a fuss was made about mixers.
    Yes, my receivers are simple and only do AM on the low bands, an easy job.
    I wonder how long the 75A4 is going to pull down the prices it does.

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