Wednesday, 14 March 2012

RockMite 40 low current mod

I gained my 500 Activator points certificate for SOTA on Monday, 12th March 2012. I began planning my SOTA expedition the week before. I kept an eye on the weather forecast and the solar conditions and decided that Monday, March 12th would be the day to do my expedition. My boss signed off my holiday, so I was all set - or was I?

Being a Monday, I thought that maybe SOTA chaser stations might be hard to come by. I wanted to continue my theme of activating for SOTA using PP3 powered rigs in order to increase my score for LA1KHA's PP3 SOTA challenge. Would I find enough chasers on 30m or 17m to qualify the three summits needed to increase my SOTA points to at least 500? I thought that it might have been a tall order. Thoughts turned to the 40m band. 7.032MHz has a reputation of being the most used frequency for SOTA.

I had built a RockMite 40 kit a while ago using 7.015MHz crystals, when I connected it to an antenna I discovered that the rig was overloaded with high speed CW. As a CW beginner, I had made a mistake to choose this frequency, so I put in some 7.030MHz crystals instead. The rig then sat in my cupboard for a couple of months.

Regulator fitted top right, next to red capacitor.

 The RockMite seemed a prime candidate for conversion to 9V, so last week I removed the PCB yet again and pulled out the 5.1V zener regulators (R1/D3 and R8/D4). The zener regulators do their job, but they waste lots of current. I replaced the regulators with a single TS2950ct-5.0 ultra low drop out 5V regulator which I hurriedly picked up at Maplin for 99p. The 5V regulator fitted nicely into the holes for R1, although as there are only two holes, an extra hole was drilled through the PCB on to the ground plane on the underside. The solder resist was then scraped off to allow soldering of the regulator ground lead to the ground plane. A 0.33uF capacitor was then soldered into the holes intended for D3. This cap was on the output side of the regulator. A jumper lead was then soldered underneath the board from the output of the regulator to one of the holes left after the removal of R8.

Regulator and capacitor in place of R1/D3

The current consumption in receive reduced by about half to just under 10mA. This is great for battery life! The picture to the right shows a previous conversion of a 30m RockMite (you might spot different coloured capacitors!).

I wasn't sure what the exact frequency of my RockMite 40 would be, all I knew is that it would be below 7.030MHz, as most RockMites run LF of the crystal frequency. I attempted to zero beat the rig with my FT817 to determine the exact frequency, the best match I found had a dial reading of 7.0286MHz. I would have to wait until later to find out how near I had got! I thought that the SOTA chasers wouldn't mind tuning down 3kHz, which would be good, as I would be rockbound!

My dipole on St Sunday Crag G/LD-010
I arrived on my first summit, St Sunday Crag, G/LD-010 for SOTA, at about 1100utc, Monday 12th March. I set up my usual inverted vee dipole, but this time I connected the extra elements for 40m. I sent an alert to the SOTAwatch website via SMS to state I was QRV. After a couple of CQ calls, I was answered by Åge, LA1ENA, who gave me a 559 report. A small run of stations waited in turn to call me, it was fantastic! I had worried about finding contacts to qualify the summit, but in just over 20 minutes, I'd worked 12 stations!

The views were simply stunning -

I moved on to the next summit, Fairfield, G-LD-007 and I was QRV by 1320utc. Another self spot via SMS brought the chasers on to my frequency very quickly, first in the log at 1326utc was Mike ,GM0OAA, giving me a 569 report from Glasgow. Again a nice run of stations called me, the last being George, GI4SRQ, giving me a new DXCC on CW.
Dipole on Fairfield,G/LD-007, above the clouds!

My last summit of the day was Seat Sandal, G/LD-022. I was at 494 points, so I only needed four more QSO's to get to my target of at least 500 points. I decided that it would be nice to finally earn my 500 point certificate using a home brew rig, instead of the RockMite. I fired up my MRM-39 on 30m. Yet again the self spot SMS service was used and I worked six stations including OE7PHI and OK1DVM for two new DXCCs. The RockMite 40 was then connected to the antenna, to allow the 40m chasers to work me. I worked another six stations, with the final contact being Michael, G0BPU, in Ipswich, who gave me a 559 report.

The little RockMite had performed beyond all expectations! I had a great day out and I didn't even feel tired at all considering the 8 Mile hike over the mountains.

My PP3 challenge battery is alive and well, and now I have three bands to choose from - 40m, 30m and 17m. Hmmm, there seems to be one band missing there! ;-)



  1. Cool. Thanks for sharing. I was going to order a RockMite 40 soon. I was curious how long you can operate on one 9V battery? This does not seem to be something I can find via google.

    Much appreciated,

    1. Hi Dave, the answer is that I don't really know! Kjell LA1KHA has had well over 500 QSO's on a single 9V battery and the battery is still in service as far as I'm aware. Kjell uses an unmodified RockMite 30 with a silicon supply protection diode in series with the DC socket. My own battery has managed 104 QSO's with lots of CQ'ing thrown in for good measure. My battery is still healthy. My RockMites use under 10mA on receive and less than 80mA on transmit. A typical modern high-end alkaline PP3 has a capacity of around 500 to 550mAH. An unmodifed RockMite will use about 20mA on receive and around 100mA on transmit (on 9V). I think it should be possible to operate for a long time on one battery - My battery has powered all my SOTA activations this year and is still going strong! :) 73