Homebrew BA receiver design...

Discussion in '"Boat Anchor" & Classic Equipment' started by N2EY, Apr 16, 2018.

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  1. N2EY

    N2EY Ham Member QRZ Page

    The HBR thread got me thinking....

    The first HBR receiver design appeared in 1957. It was a big hit and followed by several others over the next 8 years or so.

    Which got me thinking (dangerous game!).....

    If it were 1957 again, what kind of receiver would I design?

    Here's what I've come up with....

    Start with a band-imaging 80/40 receiver similar to the SimpleX Super. However, this receiver would be much more advanced - it would have an RF stage ahead of the mixer, would convert first to 1750 kc. and then to 50 kc, and would have variable selectivity in the 50 kc. IF strip (like the DCS-500).

    The VFO would tune 5250 to 5750 kc. and would use the tuning capacitor from either an ARC-5 transmitter or LM/BC-221 frequency meter.

    20, 15 and 10 would be covered by a crystal-controlled converter switched in ahead of the 80/40 section.

    Construction would be modular - the converter would be a module, the VFO would be a module, the 80/40 section would be a module, and the power supply would be separate.

    That's just the first idea.....

    73 de Jim, N2EY
     
  2. K9STH

    K9STH Ham Member Staff Member QRZ Page

    How about "cheating" and using one of the surplus Collins PTOs for the tunable oscillator? That would produce very close calibration over the entire tuning range.

    In fact, I do have a PTO from a T-195 that I have been considering to use with such a home-brew receiver design.

    Glen, K9STH
     
  3. N2EY

    N2EY Ham Member QRZ Page

    Two problems with that:

    1) Most folks don't have a "surplus Collins PTO". Even fewer back-when. Plus it's not 100% homebrew.

    2) You're stuck with the range of the PTO, which may not be optimum for a ham-band receiver.

    ----

    Another idea (G2DAF did this - I'm just passing it along):

    Dual conversion design using a tunable first IF of 5.0 to 5.5 MHz and a second IF of 455 kHz or so. The use of ~455 kHz means common parts can be used (crystal filters, mechanical filters, Q multiplier, IFTs, etc.)

    For a tuning capacitor, dial and drive, the tuning cap from a BC-455 is ideal. Even has built-in trimmers. If the coil pack from a BC-455 can be had, there's the basis for the tunable IF.

    To convert the amateur bands to the tunable IF, the following crystals can be used:

    160: 7 MHz (tunes 2.0 to 1.5 MHz)
    80/75: 9 MHz (tunes 4.0 to 3.5 MHz)
    40: 12.5 MHz (tunes 7.5 to 7.0 MHz)
    30: 15.5 MHz (tunes 10.5 to 10.0 MHz)
    20: 9 MHz (tunes 14.0 to 14.5 MHz)
    17: 23.5 MHz (tunes 18.5 to 18.0 MHz)
    15: 16 MHz (tunes 21 to 21.5 MHz)
    12: 19.5 MHz (tunes 24.5 to 25 MHz)
    10: 23 MHz (tunes 28.0 to 28.5 MHz),23.5 MHz (tunes 28.5 to 29.0 MHz), 24 MHz (tunes 29 to 29.5 MHz), 24.5 MHz (tunes 29.5 to 30.0 MHz)

    Note that some of the bands "tune backwards" with this scheme, and that two crystals are used on more than one band.

    If it is desired to have all bands tune the same way, the following scheme can be used:

    160: 7 MHz (tunes 2.0 to 1.5 MHz)
    80/75: 9 MHz (tunes 4.0 to 3.5 MHz)
    40: 12.5 MHz (tunes 7.5 to 7.0 MHz)
    30: 15.5 MHz (tunes 10.5 to 10.0 MHz)
    20: 19.5 MHz (tunes 14.5 to 14.0 MHz)
    17: 23.5 MHz (tunes 18.5 to 18.0 MHz)
    15: 26.5 MHz (tunes 21.5 to 21.0 MHz)
    12: 30 MHz (tunes 25.0 to 24.5 MHz)
    10: 33.5 MHz (tunes 28.5 to 28.0 MHz), 34 MHz (tunes 29.0 to 28.5 MHz), 34.5 MHz (tunes 29.5 to 29.0 MHz), 35 MHz (tunes 30.0 to 29.5 MHz)

    Crystal harmonics and overtones can be used if needed.

    Plug-in coils could be used if a bandswitch is deemed too complex or expensive. There would be three coils: Input, interstage, and crystal oscillator (with the crystal inside). With a bit of care, the input and interstage coils could be shared on certain bands (40 and 30, 20 and 17, 15 and 12).



    73 de Jim, N2EY
     
  4. KK6IYM

    KK6IYM Ham Member QRZ Page

    I am currently building a homebrew BA receiver based on a design--- Deluxe Receiver for the DX Operator, in William Orr's Radio Handbook, Sixteenth Addition, from 1962. I have elected to build much of it on 5" by 7" by 3" chassis'. I have broken it into modules for the RF and first mixer; the first variable IF; the filter, second IF and BFO; the "hang AGC"; and the audio. I have been winding my own transformers, RF coils, and oscillator coils--except for the second IF where I have used a set of transformers, BFO, and crystal filter from a National NC 173. A few weeks ago, I had it to the point where I heard an SSB station from South Cook Islands. The modular design allows for testing and modifying different stages without having to work with a whole chassis radio every time. One of the main lessons I have learned is that a person needs to start with the audio and work toward the RF for testing purposes because otherwise it is really hard to test a stand alone RF, or first IF stage. Mixers need the following stage to tell a person whether they are working properly.

    This radio uses a crystal oscillator to mix down the RF to the first IF where the tuning takes place over a 500 KHz segment. This feeds the filter and second IF. I have changed the design to substitute the NC 173 crystal filter where the original design used a set of Collins mechanical filters. The NC 173 BFO is rewired to match the design for a 6BJ6 but still retains an injection potentiometer and feeds a 6BE6 2nd detector / product detector that can work for AM, CW, ad SSB.

    It has been and continues to be a great learning process.

    Norm
     

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  5. KK6IYM

    KK6IYM Ham Member QRZ Page

    Jim suggests a 5 to 5.5 MHz tunable IF. The design I am using has the variable IF from 2.4 to 2.9 MHZ. I have depended on Bob Weaver's on line bandspread calculator to aid in creating the transformers and oscillator coils and selecting the correct fixed trimmers and padders. All of my ganged capacitors are straightline capacitance variable capacitors. The second IF is 455 KHz to take advantage of parts from other radios as Jim has sugested. I have found there are as many mechanical problems to solve as there are electrical.

    Norm
     
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  6. K9STH

    K9STH Ham Member Staff Member QRZ Page

    The T195 PTO covers 1500 kHz to 3000 kHz. Using high side injection with a 500 kHz i.f., the basic receiver would cover 1000 kHz to 2500 kHz. That range definitely encompasses the 160-meter band. Then, crystal controlled converters would cover the various amateur radio bands.

    The T-195 PTOs are fairly common these days as are other PTOs with other frequencies usually between 1000 kHz and 1500 kHz bandwidth.

    Here are examples from eBay:

    https://www.ebay.com/itm/COLLINS-PT...881592?hash=item33e5ed88b8:g:3WAAAOSwkvFaaTmU

    https://www.ebay.com/itm/COLLINS-PT...882603?hash=item33e5ed8cab:g:GBsAAOSwqfZaaTnU

    https://www.ebay.com/itm/COLLINS-RA...919733?hash=item213d278075:g:o7UAAOSwL3JauqhV

    https://www.ebay.com/itm/COLLINS-R-...512791?hash=item213cf38417:g:uoUAAOSwslda0OBh

    Using such is only a step from using parts from surplus equipment like the ARC-5 variable capacitor. Yes, the receiver would not be completely home-brew. However, starting shortly after World War II, when surplus radio equipment and parts started being available, until at least into the 1960s, many assemblies and parts, from the surplus market, made their way in home-brew equipment.

    Using a surplus PTO would allow accuracy that would be very difficult to achieve with "normal" variable capacitor circuits.

    Glen, K9STH
     
  7. N2EY

    N2EY Ham Member QRZ Page

    In case anyone has not found this mother lode:

    http://americanradiohistory.com/

    A simply incredible collection of books and magazines about radio, from the beginning to the recent past. Including many editions of the Radio Handbook - including the 16th edition. All free for the download!

    Excellent plan - that's how I do it too.

    The classic "tunable first IF" concept, used by Collins, Heathkit, and many others.

    I had an NC-173 some years back - good rx for its time and price. Good parts too.

    Yes!

    I was surprised that the receiver you describe used the 6BE6 as a self-excited converter, rather than having a completely separate VFO. Seems an invitation to drift and such. However, they might be OK because the frequency is so low.

    73 de jim, N2EY
     
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  8. G3YRO

    G3YRO Ham Member QRZ Page

    If I was making a Valve Receiver again, I wouldn't bother with Dual Conversion . . .

    If it was just a Receiver (ie not a Transceiver) I would have a nice 4-gang variable capacitor, so you have 3 tuned circuits on the signal frequency, and one for the Local Oscillator. So it would track properly, rather than have to adjust a pre-selector control.

    And I wouldn't bother with band crystals . . . I would just switch coils to make the Local Oscillator cover the desired frequencies for each band. (one of the reasons for having a VFO and mixing for each band was to achieve a Linear Dial . . . but with the ease of Digital Frequency readout these days there's no point.)

    For band-switching I would just use miniature relays.

    The IF frequency? Well depends on what filters you want to use, as well as the bands you want to cover . . . being a low-band operator, I would just use 455kHz with readily available filters.

    Line-up would be something like 6BZ6 RF Amp, 7360 balance mixer, ECF80 VFO/Buffer, 6BA6 IF Amp, 6BA6 IF amp, 7360 product detector, ECF80 BFO/AGC Amp, ECL86 Audio. Of course you'd also want an S-Meter Driver, Notch Filter, maybe an Audio Filter for CW, etc etc.

    Roger G3YRO
     
  9. KK6IYM

    KK6IYM Ham Member QRZ Page

    Jim,

    I have been watching for drift and it is pretty stable. I still haven't geared the dial down and the tuning for SSB requires careful adjustment, but once on a station it stays there. So far the noise level is very low. The modular design allows for great shielding and separation between stages. I had a station on 20 meters the other night where I could compare it to my TS 830S and the homebrew brought it in better. Hopefully that sort of performance holds up; however, I am still a long way from complete. To add a band one needs a crystal, two transformer coils, and a choke coil. As Jim said earlier, a single crystal can serve two bands with one tuning backwards. Since my dial is a drum type (not connected yet) it is easy to create new printed scales. I won't have to try and hand number and mark a set of concentric half circles.

    Norm
     
  10. N2EY

    N2EY Ham Member QRZ Page



    Well that's one way to do it.

    The problem with that approach is that it compromises stability on the higher bands. The way to maximize stability - both mechanical and thermal - in an analog VFO is to have a single, low frequency tuning range with solid construction. That's why all the fuss with tunable IFs and such, which predate transceivers quite a bit.

    This month's Electric Radio has an interesting article by KG7TR about a dual-conversion receiver he built. It's also on his website

    http://www.kg7tr.com/75s-2b-receiver.html


    If someone wants to go that way, fine.

    But it seems a slippery slope to me. First it's digital readout to avoid calibrating an analog dial. Then it's a huff-puff stabilizer. Then a DDS kit instead of a VFO. Zeners instead of VR tubes, silicon rectifiers, an IC for the audio section, varactors, programmable dividers....pretty soon there's nothing left.

    If somebody wants to build an SS radio, more power to them! But let's call it what it is.

    For a band or two, they can do the job well.


    That would be good up to 40 meters or so.


    A lot depends on what one has on hand or can get inexpensively. The 6JH8 is often easier to find and less expensive. G4OEP used one in a receiver and did interesting work with it:

    http://g4oep.atspace.com/retro/retro.htm

    There's also the Pullen circuit.

    There's also the premixer method, used in the Drakes and the Southgate Type 7.

    73 de Jim, N2EY
     

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