Thursday, April 18, 2013
Talking to France Via My Electric Razor
I nearly threw away my worn out Braun 5569 electric shaver a few months ago. On second thought, I decided to at least check it for usable components before I disposed of it. It occurred to me a short while afterwards that it might be fun to attempt to convert whatever was inside it into an amateur radio
I got around to opening it up last week. It was mostly built using surface-mount technology (SMT), but at least it used discrete devices vs. a custom-made IC. A waterproof coating made salvage difficult, but careful surgery with a razor blade allowed me to remove all five SMT transistors and half-a-dozen diodes; one of which was a 5V Zener.
The transistors were soldered to bits of copper-clad "carriers" along with flying leads. One transistor didn't survive the transplant operation. The one non-SMT transistor, an NPN power device, refused to oscillate above 1MHz. The remaining three NPN and one PNP transistors appeared to function well enough at 14MHz that I thought I'd have a go at building something for 20m.
My first idea was to build this Japanese 10/6m DSB rig for 20m using a 14.3MHz computer crystal from my junk box.
After the standard amount of tinkering the transmitter appeared to be operational. Two of the tiny Silicon diodes taken from my electric razor matched well enough to produce a decent carrier null in the single-balanced modulator. I used a pair of NPNs in a push-pull PA stage. A forward-biased diode taken from the razor was used to set the PA bias. The RF CW output was ~90mW. An electret mic from an old telephone drove the PNP SMT transistor in the AF amplifier. The VXO provided a tuning range of 8kHz.
Unfortunately, switching the transmitter to the direct-conversion receiver mode produced the expected result. The 100kW, Nashville-based, WWCR, on 13.845MHz swamped the amateur phone signals. Placing a "balancing" pot in line with the the SBM diodes helped considerably, but not enough to completely eliminate the interference. Past experience tells me that at my QTH a narrow, triple-resonator, BPF is needed between the antenna and these simple diode mixers. I decided to pull the DSB rig apart and build instead a 20m CW station.
I built a 0V1, common-base, Colpitts autodyne regenerative receiver using two of the NPN SMT transistors. The Q-multiplied RF resonator easily shrugs off interference from the high power commercial shortwave transmitter.
The transmitter begins with a variable quartz crystal-controlled oscillator (VXO) made from the third NPN transistor. The final PNP SMT device was used in a common-base, RF power amplifier (PA). The transmitter VXO tunes from 14.055 to 14.061MHz using an xtal cut for the QRP calling frequency. The oscillator free-runs on transmit; only the PA is keyed. The RF output power is 75mW. A 7th order lowpass filter (LPF) holds the harmonics below -45dBc.
The current draw is 1mA on receive and 12mA on transmit using a 9Vdc battery as my power supply.
The bread-boarded transmitter appears in the above photo. The LPF is on the left-hand side, followed by the PA and quartz-crystal oscillator. Two of the transistors taken from the electric razor can be seen in this photo.
Here's a close-up of the two-stage receiver. The regenerative detector is on the left-hand side of the board, followed by one stage of audio amplification. The variable capacitor at the left tunes the receiver from 14.000 to 14.075MHz.
I was pleased to have worked four stations with this setup on April 17, 2013.
W4SX GA 569/339 K2 @5W
KB0PCI MN 569/339 12w
KD4ESO AL 579/559 100w
K5EST MO 569/559 5w
Coming up to the house later in the day I found three Reverse Beacon Network receivers had made captures of my 75mW signal. Oddly enough, all three were located in Europe.
On April 18, I worked three stations; my transmitter still driving an end-fed wire antenna with 75mW of RF power.
N0UR MN 449/229
W4SX GA 579/449
F6DCD France 559/519 K2 @ 5W
F6DCD very kindly called again 35 minutes later to inform me that my signal was still audible near Strasbourg. Denis increased my signal report to 529. Needless to say, I was ecstatic to have "crossed the pond" with the transistors taken from my old electric razor.
On 19 April I received an email from F6DCD
I was really pleased to contact you with your 75 mW. I heard you calling on frequency during 1 hour and more. Your sig was really nice on my K2.