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Saturday, October 29, 2016

DONE! Jan's AMAZING Mate for the Mighty Midget Receiver


Hi Bill,
  
Finally the Mate for the Mighty Midget is finished, just in time for the G-QRP Valve Day 12-13th of November.
Got the LO fixed for 40m by lowering the parallel capacitor from 150 pF to 100 pF.
Also the 68 pF series capacitor was lowered to 33 pF for some more band spread on 40m.
It now receives from about 7.0 to 7.4 MHz and from 3.45 to 4.0 MHz
Had to exchange C1 in the end, the one used initially quit every now and then.
I only had a large 3 section variable in the junk box covering 10-550 pF, which works fine now.
For the lower end of 80m I had to add additional 47 pF next to the 47 pF trimmer caps, so there it is about 600-650 pF max!
At the high end of 40, it is also just not too much.
The meter was used as a position indicator for C1.
Tried several ideas, but with no separate tube for AGC, I couldn’t get it to work as a S- meter
Read something about audio derived AGC, maybe this is worth a try.
 
The BFO can be switched off for AM reception.
Simultaneously the input on the mixer side of the crystals is disconnected but still coupled by some capacitance of the switch wires.
AM reception is possible, but not very good.
Need to find a better solution which doesn’t degrade the crystal filter properties to much.
(By the way, the detector regen. control ads about 4 dB to the AM sensitivity)
The receiver will mainly be used for CW/SSB reception, so maybe it stays this way for a while J
 
I hooked it up to the W1TS transmitter, which was very loud.
Didn’t foresee a T/R relay (learned a lot from this project ;-)  ), so added this one between the front plates next to the RF and audio gain control.
The quit down everything a little, the RF gain pot is lifted of ground as suggested by James, N2EY at QRZ.com.
It helped a lot, but was still too loud if tuned exactly in the bandpass of the crystals.
The T/R relay now also switches an adjustable potentiometer at the input of the audio pre-amplifier.
 
The dial cord has no lag, and works very well for fine tuning.
Unfortunately the reduction drive went from 1:19 to 1:9…, the tuning capacitor only has a 180 deg. span.
Something to remember for the next receiver.
 
It’s a nice little receiver and quite stable after warm-up.
The only extra luxury a next receiver will have, is AGC.
But with no AGC it’s easier to tune the antenna tuner by ear J
There’s now a complete homebrew station here, antenna, feeder, tuner, receiver, transmitter, power supply, al home made J
 
About the Mystery Hole….
If you haven’t guessed it by now, or Pete hasn’t told you, it is revealed in one of the pictures below.
 
I also made a little video:
 









5 comments:

  1. Congrats to PA3GSV for a really cool homebrew receiver. I like his matching rigs side by side. I also like the creative way he built his tuning/dial mechanism--beautiful work!
    Al, N8WQ

    ReplyDelete
  2. Thanks Alan, it was a nice project, learned a lot.
    I always wanted to build a tube superhet as a boy, but never had the money or the patience to do so.

    73 Jan PA3GSV

    ReplyDelete
  3. What a great piece of classic homebrew. Who nowadays goes to the trouble of constructing a dial drive mechanism like that? Thank you for keeping the art alive Jan - and not an Arduino in sight either :-)

    73,

    Dave
    AA7EE

    ReplyDelete
  4. hi Dave, on my day job there's only "contemporary radio" So for stress relief this stuff is doing really well :-)

    73 Jan PA3GSV

    ReplyDelete

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