Saturday, February 13, 2010

No solder smoke this weekend

At the start of the week I ordered a copy of fellow blogger Bill Meara I0/N2CQR's book SolderSmoke - A Global Adventure in Radio Electronics as a consequence of reading several positive reviews. It is entirely typical of my experience of purchasing anything that it arrived today, the day Bill announced in his blog that the publisher is offering 15% off the price of the book if you order it this weekend. I haven't had a chance to look at it yet so I can't give you any encouragement to buy it other than the chance of getting it for 15% less than I did, but that's probably a good enough reason!

I had hoped to make some solder smoke of my own this weekend, building up a permanent version of Roger G3XBM's amazing little two transistor transceiver the XBM80-2. However my 3.560 crystal still hasn't arrived. Instead I contented myself with trying a few mods with the circuit still on the breadboard.

Following good results using a pair of 2N3904 transistors in Darlington configuration for the audio stage, I tried an MPSA13, which is a pair of transistors in Darlington configuration in a single TO-92 case. This provides a worthwhile increase in gain. In fact, as I'm using a crystal for the American 40m QRP frequency (which is in the digital modes sector over here) I am actually being deafened by the sound of loud RTTY stations in the crystal earpiece. With the MPSA13 the receiver gain doesn't seem to fall off when using a 9V supply instead of 12V, which is good as such a tiny transceiver should really be run from a tiny power source like a PP3 battery.

I also experimented with some of the modifications made by Alan Yates VK2ZAY in his version of the XBM80-2. He uses a 2N7000 MOSFET to switch the crystal trimmer out of circuit on transmit to provide frequency offset so you can hear a station that replies on your frequency. This seemed such a worthwhile convenience compared to using a switch that it had to be worth a try and I am happy to report that it works perfectly (and with no perceptible chirp using the 40m crystal.)

Alan also adds some filtering to keep RF out of the audio amplifier stage, and a two-stage low-pass filter to tame some of the harmonics on transmit. I added the filtering, which made the receiver sound a bit cleaner, but I can't try the 80m low-pass filter until I have the rig on 80m.

The oscillator circuit of the XBM80 will oscillate just about anything, so I used it to find out the operating frequencies of the 2m FM transmit and receive crystals I bought on eBay. All of the receive crystals operate around 14.9MHz, even three of the ones that weren't marked as receive crystals. That explains why I spent a couple of hours failing to find their frequency of oscillation at all, as I was hunting around sub-multiples of 145.500. Grrr!

Out of the batch of 15 crystals I have two for 145.500 and three for 145.550, all but one of which are probably 36MHz third overtone crystals as they oscillate around 12.1MHz at a frequency a bit below the two metre frequency divided by 12. The other 145.550 crystal oscillates around 8.08MHz. The plan is to make an FM version of Roger G3XBM's Fredbox. So I need to look for a way to frequency modulate the oscillator in Roger's circuit. Suggestions, as always, are welcome.
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