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Poundshop HF Receiver

In 2005, I modified a FM 'scanning' radio to receive cw and SSB in the 40m band. I call it the Poundshop radio, since these radios are bought from such places!


Here is a diagram of the conversion. A summary of the conversion follows.


Summary of modifications to PMS Scanning VHF FM radio
to produce a 7MHz direct-conversion SSB/cw receiver.
-------------------------------------------------------

(1) VCO

1. Lift R1 (5k6) from pin 16.
2. Remove RESET, RUN and LIGHT switches.
3. Remove C5 (473).
4. Connect pin 16 to pin 14.
5. Remove L1 (osc. tuning).
6. Fit 7MHz tank in place of L1 (6.8uH / 50p / 20p trimmer)
7. Cut track between hot side of R2 (22k) and pot CW.
8. Remove "C6" link.
9. Connect R1 "loose" end to pot W.
10.Connect pot CW to Vp (+3V).
11.Replace C13 (103) with 15p (Reduces tuning span to around 115KHz).

(2) RF

1. Remove C1 (82p).
2. Connect pin 11 to antenna via 33p.
3. Connect other antenna end to pin 12.

(3) AF

1. Remove C8 (332).
2. Replace C10 (181) with 562.
3. Replace C9 (332) with 104.
4. Connect pin 15 to transistor base with 104.
5. Fit 22k from base to collector of transistor.
6. Connect pins 8 and 9 with 100n.
7. Bypass pin 3 the ground with 220u

NOTES
-----

1. Trimmer fits in LIGHT switch area, and a hole drilled in the light switch button makes it accessible without opening the case.
2. A better antenna, perhaps via a matcher, will produce more signal but increases BCI.
3. The basic radio can have a 'standard' LM386 amplifier added for more AF gain, but remember that the TDA7088 can use no more than 5V supply. 4.5V from three cellsis a good compromise.
4. In principle, any band from 1500KHz to 110MHz can be received by changing the tuning coil. I use ordinary axial chokes for coils. Screened coils may reduce BCI.
5. Loop antenna exits from light-bulb hole. The radio may be worn around the neck for convenience, or hung up, or laid on a table.
6. The supplied 'in-the-ear' headphones work well.
7. Strong SSB signals cause significant distortion, null them down by turning the antenna.


Acknowledgements
----------------
I would like to thank the many encouraging members on the Yahoo G-QRP Group, but especially Sverre Holm LA3ZA and Hans Summers G0UPL for their contributions.

The idea and conversion details were originally published in SPRAT, the journal of the G-QRP Club. Anyone interested in low-power radio should visit www.gqrp.com , the Club's website.

Here are some pictures of a converted radio.


Showing the PCB, the redundant switches and the coiled-up loop antenna.

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