Sunday, 10 January 2010

The Flea 40m CW Minimalist TCVR

Had the bits in a plastic box since SPRAT issue 137 and finally managed to build it. Manhatten style. Just aligned it and tested and it puts out a healthy 1.3W from a SLA battery.

It was fun getting the superglue to secure the pads when the shack temp dipped below freezing. The PCB etch solution was full of ice cystals!

Anyway I include a close up image too. I found the signals stabilised when the xtal case was grounded.

The layout wasn't critical EXCEPT the T1 MUST be wound with all three windings (I used SWG28) in phase. The primary end connected between the trimmer and 56pf cap, the secondaries in phase to the supply railand the antenna. If not it doesn't work!

Nice fun little rig. RX signals from a Vertical 6BTV perfectly readable. The winding direction of T1 is critical though I wired the antenna connecton in
reverse initially once corrected adjusting C3 brought in the signals.

Managed to use it on 7.025 (FT243), 7.030 (HC49) and 7.040 (HC6) (no qrs qso yet!) and I have used one half of a turned 8pin DIL socket for the crystal between two manhatten pads so changing frequency is not a problem.

Using a small 10uV signal source and step attenuator I could receive signals down to about 0.4uV.

Changing C1/C2 and using a different C3 would mean bands 80m-20m should be possible anyone else had success on other bands that they might share with this design?

A 2N2222A also works for Q1 but with a much lower output c150mW.
Increasing V1 seemed to shift the RX offset (syntony=tuning?) a lttle bit more. I used a 50k pot for the'sintonia' pot. This is for the RX offet. Should be 22K I didn't have one and found 50K to give good results.

The 2N3866 was one of those given out at the Vicarage for the last GQRP Rochdale rally.

I have xtals at 7.010, 7.020, 7.030 and 7.040 so will box this up and try it out. I hear no sidetone though which is a bit hard so might add a k1el, kd1jv or norcal keyer.


Roger G3XBM said...

Dom, that's a very neat looking Flea. Look forward to a QSO with you in the very near future. No worries, it will be S L O W....

Roger G3XBM

Anonymous said...

Good job!
My Q is: where you got you 40m X-tals?
Here in Poland I cant find it.
73! de SP2SWR

M1KTA said...

The HC49 7.030 MHz from GQRP sales the FT243 was a gift the HC6 was from a rally stall.