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Tuesday, March 19, 2013

DeMaw's Barebones Superhet


A recent e-mail got me thinking about Doug DeMaw's Barebones Superhet.  June 1982 QST.  Mostly 40673 Dual Gate MOSFETS (this one's for you Dino!).  Barebones indeed.  Check out the schematic.

I literally blew the dust off my version and fired it up on Sunday.  It sounds really great.   I had lowered the values of the caps in the ladder filter to widen it out for phone.   Also, I see that I used LM386 instead of the op amp AF Amp prescribed by W1FB.   I notice that my version has much better audio than another version of this RX (with the op amp) that I'm using on 17 meters.  Could the difference be the LM386 vs. op amp?  There is a lot more audio with the 386, and the AF response seems wider.  

I feel the urge to put this receiver to use.  I am rehabilitating the W1VD CW transmitter that used with it during the late nineties, but I'm a phone guy now, and I feel compelled to use this RX as part of an SSB rig.  Of course, I could build a standalone SSB TX, but how about a diode switching scheme to make use of the Barebones' filter, the VXO and the BFO in a transceiver?

Here is my article on the original build of this receiver: http://www.gadgeteer.us/HBHOME.HTM

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4 comments:

  1. Hi Bill,

    My opinion is that you will need an additional section in the filter to make the slope steep enough to adequately reject the unwanted sideband. But what the heck, it's easy enough to try and see what happens, HA! You're already half-way to a complete XCVR!

    73.......Steve Smith WB6TNL
    "Snort Rosin"

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  2. As always, Steve provides good guidance. I'll leave the RX as is. I may build a separate SSB transmitter for it, perhaps using the filter Steve sent me a while back. I'll also have the VXO 6 watt CW transmitter to use with this RX.

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  3. I am by no means an expert, but I find that proper shielding, period, makes all the difference, whether or not you're using a chip LM386 audio stage or discrete component one. I know it's like plumbing: how that s--t backs up into the circuit is at times beyond one (unless one's maths skills are good--ought to be). With my Zack Lau tribander, am having a helluva time getting the thing to "ring" properly on all bands; I know that there are feedback problems (RF). An engineer pal here is telling me that "it's importante to have constant bypass to GND caps at many stages" and also that each circuit environment (layout, style)is critical. There is a lot of crap in the air here (t.v., modems, 2m and 70 cm r.f., CFE monitoring beacons) and it makes it hard.

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  4. Having Said That, I agree wholeheartedly. Don't 'Muntz-erize' it. "Mad Man Muntz" sold T.V.s that the circuitry was stripped to the bone so he could sell them cheap. The stuff pretty well backed up in those sets. But hey, he did bring us "The Jet" after all. :-P

    73.......Steve Smith WB6TNL
    "Snort Rosin"

    ReplyDelete

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