These pocket radios can be modified for the 160m band by adding a product detector and some dexterous tuning. The additional circuit can be built using rats-nest technique and fitted in a convenient space inside the radio. The result is a smart looking pocket radio for SSB/CW single band 160 meter use.
These radio's often use 2x 1.5v batteries, ignore these. Select the radio that uses a PP3 type 9 volt battery and has 4 canned transformers (1=Osc + 3x IFT). Some radio's only have three transformers; these are useable but not so sensitive. The MW radio covers 550KHz to 1.6MHz and gives 1MHz of band coverage. We are going to retune it to top band.
THE PRODUCT DETECTOR
The product detector is a self oscillating mixer circuit using a dual gate FET and a 455KHz ceramic resonator. No input tuned circuit is required. The output transformer is a transistor interstage AF transformer "robbed" from another of these radios. If you can't get a transformer then use a 1K ohm resistor instead of the primary. Couple the output audio via a 1.0 uF cap. leave the decoupling capacitor in place as shown. If the oscillator stops then reduce the 1K resistor value until it starts again.
MOD TO THE RADIO:
This drawing shows the circuit of the last IF transformer (usually colour code BLACK) of the radio. Cut the one wire or PCB track to the volume control and insert the product detector PRODECT1.GIF as shown. You can usually lift the volume control tag off the PCB with the aid of a soldering iron, and this saves gouging out bits of PCB.
Tune in any old M.W. signal to near zero-beat, then adjust the three I.F. transformers for maximum audio, or the most negative voltage on the ANODE of the detector shown in MOD2.GIF. If possible, mount the PRODECT1.GIF as close to the receiver AF amplifier as possible to keep direct IF pickup to a minimum.
RF ALIGNMENT
The radio has 4 adjustable components which must be aligned;
1. Ferrite rod antenna - Slide the coil up and down the rod.
2. Oscillator Coil - - - Ferrite slug, colour coded RED.
3. Antenna trimmer - - - Capacitor on the main tuning capacitor.
4. Oscillator trimmer. - Capacitor on the main tuning capacitor.
To identify 3 and 4, touch them individually with your finger whilst tuned in to any old station. Touching the "Oscillator Trimmer" will cause the RF frequency to go whizzing off. Touching the Antenna Trimmer will only cause the signal to get a bit weaker.
Set the two trimmers (3 and 4) to mid position, screw the Oscillator coil (RED can) all the way OUT (up, anti-clockwise) and move the ferrite rod antenna coil to the end of the ferrite rod.
Find any station at the HIGH FREQUENCY end of the dial. Adjust the antenna trimmer capacitor (3) for maximum signal, (most NEGATIVE voltage on the cathode of the detector diode).
Find any station at the LOW FREQUENCY end of the dial. Adjust the ferrite rod antenna coil (1) for maximum signal, (most negative voltage on the cathode of the detector diode).
Repeat the above two steps until no further adjustments are needed.
Another MW transistor radio may also be used as a signal generator to aid alignment. The local oscillator of a MW radio radiates a weak signal that is 465KHz ABOVE the receive frequency. A radio covering 550 KHz to 1.6 MHz will therefore radiate a signal tuneable from 1.015 MHz to 2.065 MHz. With this you may check the coverage of your modified radio.
Have fun, de HARRY, Lunda, Sweden.