QRP Guys Lidia 40m Receiver

In this video I build up the QRP Guys Lidia receiver. Unfortunately they only sell the boards now:

Here is the web site of the Wim the original designer:

At the heart is the MC3361 Narrowband FM IF which is used as a direct conversion mixer

Video Index
0:00 Intro
02:02 Build Order
03:20 Power supply
04:35 Bandpass Filter
07:20 Preamp
09:10 Audio Amplifier
10:35 Mixer and Tuner
11:15 Faceplate
12:50 Tuning circuit completion
17:13 Testing the LO Frequency
20:08 On the air

[TheEmbeddedHobbyist]

Nice little kit build, takes your mind off lockdown here in the UK, one week left of lockdown V2.0

[risu]

Enjoyed watching it. Refreshed some childhood memories with dc rx and spending hours of trial and error to suppress the bc interference. This gives some soothing in this covid-19 times.

[dxexplorer]

Nice kit and lovely video. You make me wish I had an oscilloscope... I'm pretty sure it would save me a lot of time while learning and testing a lot of stuff hehehe. Not in my budget right now though. By the way about the RF buffering "preamp"... I tried the exact design in my own receiver and there must be something wrong with the design. I'm not as smart to figure out what could be wrong... but it didn't work for me, so instead I used a simple similar circuit with just a 2N3904. Works like a charm. 73 and have an amazing weekend.

[nickwood]

Hi! Thanks for this - an interesting design. The double hump on the spectral display of your band pass filter is actually the sign of an over-coupled filter. It's always a trade off between bandwidth and the steepness of the roll off but for the 40m band you shouldn't need to worry too much about maximising bandwidth particularly. I think that 10pF coupling cap is probably a bit too high. If you try putting a lower value in there you'll probably see a flatter response in the passband. The trouble with band pass filters are they are fiddly little devils and every time you change one thing it changes something else! Anyway, hope this helps.Well done again on a great build and a very informative video.

73

Nick
M0NTV

[amirb]

@ 7:50 the buffer is oscillating itself which is most likely due to a bad pcb layout design (not proper bypass/decoupling) or could be due to using a high impedance input of oscilloscope and large capacitive load you put in there (scope + bnc cable). you can play with the 1K resistor to reduce the gm of the FET and eliminate the oscillation but putting the mixer in the circuit which will load the buffer with a lower impedance can take care of that too. The thing is I dont know why you would need that buffer anyways

[herbertsusmann]

That JFET pre-amp is connected as a source follower. Looks like someone got that wrong in the design phase or something. Should have been a common source amp with tank circuit in the drain with tapped winding off to the next stage. Also, that switch should have been a SPDT toggle switch to select either non-amplified original signal or amplified signal. Brings into question who designed this thing in the first place?

[rayprada]

I think the CW/SSB switch adds R in // in the low pass audio filter feedback to reduce the bandwidth.
I'm building this ckt just because I got two MC3361 chips handy, scrounged from a Medion computer wireless keyboard 27MHZ receiver,
but instead of the varactor oscillator I'm going to use a DDS VFO I also have laying around collecting dust. (Arduino + LCD ) That way it can tune 3 - 30 MHZ easy as pie. (The datasheet says the MC3361 chip is good to 60MHZ - the DDS will be fed into pin 1 with an attenuator)
I haven't decided what to do about the RF input tuning circuit, something with a variable capacitor?

73 de,
Ray WW0AW

[rayprada]

I should've learned this in school had I been paying attention but why is it that a DC receiver doesn't need a BFO for CW/SSB ?

Ray, WW0AW

[herbertsusmann]

Also, seems like a better choice of chips would have been the NE602 or similar. Smaller part, has some gain, built in osc. Lots of kits out there using NE602s for simple Direct Conversion receivers. I am not seeing why anyone would pick this FM radio chip??

[amirb]

there is no pre-amp. it's a buffer (gain <=1) i dont know why is it there really...

[nevmarr]

For what it's worth, here's my 10 cents on the CW/SSW switch. I wonder if it inserts some sort of audio offset in the CW position? If say you had the frequency display installed and wanted to hear CW on 7.010, ordinarily you'd hear nothing on the carrier freq and have to move the LO 700ish Hz. Put another way, the CW switch allows you to hear CW on the displayed carrier freq. To test my dumb theory tune a CW signal and then fiddle with R11 - by my theory the tone will change. If it turns out I'm talking bollocks, please delete my comment LOL!