New Year’s eve building marathon: Rockmite QRPp 30m transceiver
2015-01-07 Leave a comment
I was always curios about WARC bands and in particular the 30m band my trusty Kenwood TS-520 lacks. I have it thanks to my FT-817ND but still, in part due to my love of experimentation with radio kits, last spring I ordered a Rockmite 30m kit from Rex W1REX (qrpme.com) and one more 40m version of it. This used to be a very inexpensive kit costing $27 only. These days it’s $45 and comes with a spare pair of crystals. Any mods (AF gain, TX efficiency, etc) are extra. The kit has successfully arrived to Toronto after 5 weeks. During BoM inspection of the 30m kit, I noticed one band capacitor supplied was wrong value and that I got two SA612 mixers and no amplifier IC. Luckily I had enough capacitors of various values ordered from eBay before so finding a replacement was not a problem. The amplifier chip I borrowed from the 40m kit and immediately ordered a pack of 20 on eBay from China for a mere $4 in case I ever need a spare handy… Rex W1REX (from QRPME.com) admitted the kit sorting and handling issues he endured while training a new helper and shortly sent me missing audio amp chip at no charge along with my next order. After finishing sorting components according to the BoM I started building the kit.
I had built kits before so building this one was not too difficult and took about one and a half hours from start to finish. The PCB is small and the parts count is fairly low. Although I find it easier installing SMD components and actually enjoy winding transformers (but that’s probably just me…), all components are through the hole and there are no toroids to wind so building is very straightforward. The builder’s manual comes somewhat handy, addressing most frequent questions builders get. All in all, this kit could be a challenge for a first time builder, but if this is your second or third kit, I think you should be fine. I have no anti-static mat so I used aluminum kitchen foil connected to the station grounding bus with a crocodile clip instead. To discharge your body (which holds charge!) before handling static sensitive ICs, I just touched the foil / grounding wire which worked just fine. Still, an anti-static mat would be a good investment. To simplify installation on a fairly crowded PCB I started with installing resistors and capacitors first, then, keeping attention to polarity, went the diodes. Next were IC sockets, crystal sockets, output RF transistor socket, transistors themselves, and finally connection wires to off-board buttons, connectors and AF-gain potentiometer. I used tiny SIP sockets provided with the kit to install the power transistor and the crystals to simplify their replacement if necessary.
After thorough visual inspection for cold solder joints and bridges I inserted the ICs and connected keyer, headphones, 50 ohm resistor dummy load to the antenna socket, and finally 12V power from a small gel battery. After power on, there was a happy ’73’ morse sound in the headphones meaning that the rig was working. However, there was a bad 59+ harmonics noise in the headphones even though I had a 50ohm resistor instead of an antenna. I removed the crystal on the RX side (left bottom) and the noise disappeared. Seems like I had some RF leakage from LO into the RX path. After inspecting the board again, I noticed a cold joint at one of the capacitors and re-soldered it as well as a few more suspiciously looking ones. This solved the issue completely. Next step was to measure the TX power. The power reading on my QRP power meter was 0.2W which is lower than 0.5W expected. I guess will need to play with the output transistor and install the power efficiency mod recommended by W1REX here: http://www.qrpme.com/docs/RM%5D%5B-40%20v3%20PwrEff%20Mod.pdf The frequency was slightly off – 10.104 instead of 10.106 so I adjusted the variable capacitor with a small screwdriver while monitoring my signal on a separate radio until the signal was finally heard on 10.106. Then I repeated this for 10.116 crystal. Next step was connecting to an antenna and testing it on the air. I sent several CQs with no response, however, got one or two spots on RBN. Modifications: The sidetone sound out of the box is uncomfortably loud and raspy. The proposed fix as per the builder manual is to change the value of C8 and add one or more R-C networks (series-R, shunt-C to ground) in the path [from PIC pin 5 to C8] to soften the tone. I added a 10 ohms resistor and a 10 uF capacitor as suggested by the manual. For that, one pin of the C8 (which goes to the IC) was desoldered and resoldered to a 10 Ohm resistor another end of which went into the board instead. A small electrolytic 10uF capacitor was soldered to this joint (positive end) and the negative end went to the board ground. This cured the sidetone sound issue completely. The rig lacked volume control (AF gain) but there is a simple modification described in the manual – replace R5 with a 1MOhm potentiometer resistor. I was able to find one seller on eBay from Thailand… so the project went on a shelf for another 5 weeks until the postman brings the shipment… and then again until this New Year’s eve when I finally found some quiet time to enjoy finishing this project. To switch the crystals, Rex offers SIP sockets. The connection is not too good so I built off-board platforms to keep the crystals and a switch. A variable capacitor was added in series with a crystal to tune it to the right frequency (10106 and 10116). Without a capacitor, the oscillator frequency is off from marked by 2-3 kHz.
All in all, I didn’t expect much from this little pocket size transceiver… especially given its 0.2W output power… It is a fun thing still but may not justify the cost and time spent. If you are thinking of a first radio – this probably not the best choice. I would rather suggest getting a 35 years old FT-101 or TS-520 or somewhat newer at a hamfest for $200-300 in a working condition. However, if you are into CW ragchewing, reside in a RFI-quiet location, or hike on mountain trails, and enjoy tinkering with a soldering iron, then a QRP transceiver like this may be a right thing. While studying the QRP market, I also came across a few very reasonably priced competitors: 1. CRK-10A CW Transceiver Kit A tiny 40m or 30m CW single frequency transceiver kit designed by BD4RG. Same features but whooping 3 watts (!) output and comes with a nice tiny case. Price: $55 + 8 shipping.
2. Youkits EK1A 2014 3 Band QRP CW transceiver kit Covers 40m, 30m and 20m band. Comes with 4-5 watts output, VFO, keyer, LCD screen, wide RX receiver. Price: $149
There is also a bigger brother – Youkits HB-1A MK2 2014 QRP CW transceiver which comes fully built (!) for $219 and features adjustable IF filter, RIT, and an even more advanced four-bander HB-1B for $299. Palm-sized, comes with internal battery.
3. Last but not least – AT (Appalachian Trail) Sprint trail-friendly radio – available in small batches by KD1JV via yahoo support group and its cousin ‘Mountain Topper’ to be available from LNR Precision group in early January 2015 for $225: I wonder what the front-end of this tiny rigs perform like and is it any close or better than of mine FT817ND which gets fairly easily overloaded…. Could not find any official test results yet, except for Youkits radio reviewed by ARRL Test Lab who gave it a very favourable review in a recent QST magazine.