Saturday, 17 November 2012

MRM-30 goes QRO

One of my first Manhattan style construction projects was a RockMite 30 circuit. I had upgraded my RockMite 20 with a Hamgadgets memory keyer chip, this left me with a spare keyer chip. I thought a great way to use the spare chip was to build a RockMite 30. I had seen pictures of other home brew RockMites on the Internet and I thought it would be something I would be able to do.
I wanted to make an 'ultimate' RockMite, so I put in a VXO circuit and also the output PA transformer modification promoted on the Yahoo RockMite Group. I used a 2N3866 transistor for the PA. I was hoping for about 800mW of RF, but I was disappointed that the rig would only put out 520mW. I then upgraded the output filter, this promotes a super clean transmit signal from the rig, but it introduced another 20mW of loss, I was now only getting 500mW from 12 volts.
I was still pleased with my efforts though, it was the first home brew transceiver that I had attempted and it did work. My local radio club put on a camping and amateur radio weekend in July 2011 and I tried out the newly completed using an inverted vee dipole. I was pleased to work HB9UH with some very shaky QRS CW! It turns out that Hans, HB9UH, is a member of the Helvetia Telegraphy Club, a group dedicated to promoting the use and teaching of Morse Code.
The QSO with Hans remains my only contact with the rig. My CW has improved a bit since then, I have been using another Manhattan style RockMite 30 rig which I built to take part in LA1KHA's PP3 challenge and also an MTR by KD1JV for my SOTA activations this year. The MTR is my favourite rig, I rate it above everything else, so my other rigs have been gathering dust! I decided that I need to try to use my other home built radios.
I was always disappointed with the low power output from the home brew RockMite 30 - (MRM-30), I thought it would be more useful for SOTA if it could put out a bit more power. A couple of watts on 30m should be very useful.
I looked in to a design for a 5 Watt RockMite 80 on the Yahoo group, the design used a CB PA output transistor, a 2SC2166. I thought that I could use a similar scheme for my rig. I managed to locate an Eleflow 2SC2166 at a reasonable price on Ebay.
I replaced the 2N3866 output transistor on my rig with a 2N2222A and I used an adapted version of the 1.5W PA circuit from the Small Wonder Labs SW-30+. I knew that the circuit would be capable of a couple of watts at 10MHz. I built the circuit in to the RockMite this week and I was pleased to measure the output at around 2 watts, I'm well pleased with that! I could probably get a lot more out of the circuit, but I happy to stick at the 2 watt level. The signal still looks very clean, so I'm really happy.
I hope to try the rig from a SOTA summit soon, Winter Bonus is only a couple of weeks away!:-)
I haven't added any RX filtering yet, I'm going to see how bad it is without first!
New 2 watt output stage squeezed in (right hand, middle)

1 comment:

  1. Hi Colin, this sounds very interesting! My standard RM30 was is giving me approx 700mw output, I changed the transistor to a 2N3053 which seems to have boosted the power. But it would be nice to have a couple of watts, have you had some good contacts?
    73s Richard M0AUW