Tuesday, 3 April 2012

MRM-30 in the workshop

In Summer last year, I built my first Manhattan style transceiver - I had upgraded my RockMite-20 with a PicoKeyer from Hamgadgets, this left me with a spare RockMite chip. I thought it would be fun to try out the Manhattan technique and build a RockMite 30. I had lots of help via email from Chuck, W5USJ - a great guy, always willing to help out newcomers and he knows his RockMites!

The result was my MRM-30 -

I was slightly disappointed that the rig only puts out 500mW on 10.116MHz - I was hoping for nearer 750mW. The rig does work quite well though, although I've only had it on air a couple of times. Like all RockMites, the receiver is quite wide so many stations can be heard at any one time! I had a nice QSO with HB9UH when I first tried out the rig into an antenna.

The circuit of the MRM-30 is pretty standard, although I have tried to tweak the PA stage for more power. I used a 4:1z transformer instead of the RF choke for the PA load/DC supply. I eventually upgraded the PIC to a PicoKeyer. I have fitted LED's to indicate the 'shift' state and also power on, mainly to make the rig look pretty!

The heat sink on the 2N3866 is a bit overkill - I was thinking that the rig would produce more power than it does, but still it doesn't harm anything and it adds a bit of eye candy! I made the case large on purpose - to allow later upgrades and also to make the rig a little easier to use. My original intention was to build the rig for SOTA use and I thought it would be good to have large controls to enable use with cold hands. I ended up making the rig too pretty - I wouldn't want to risk damaging it out on the hill now!

Ondra, OK1CDJ, announced a neat little kit he had designed, on the Yahoo RockMite Group a couple of weeks ago. The kit is a little audio filter for CW. Ondra had designed the filter for his own use I believe, but quite a few people showed an interest, so a kit was put together and was made available at HamShop Cz. The kits sold out pretty quickly, but I was lucky enough to buy one before they had all gone. I believe that this kit will be available again soon.

The kit arrived from the Czech Republic very quickly (4 days after order!) in a padded envelope. The included instructions were adequate for a seasoned builder. The PCB is of nice quality and the components used are all surface mount type. I like surface mount components, they feed my obsession of making things neat and compact. There are no components smaller than 0805, so soldering was not a problem.

The components soldered on to the board very easily, I again used lead-free solder, as this was on hand. The solder joints seemed to form nicely and were lovely and shiny! The finished PCB looks very professional - it doesn't look like it's been built as a kit at all -

My plan is to fit the little CW filter board in to my MRM-30. I have been planning to add an Op-Amp filter to the MRM-30 for quite some time, I even bought some of the components. The filter kit has made adding a filter really easy!
Two resistors fitted to rear of the PCB

I'm not sure at this stage whether I'll remove the little variable resistor and fit a front panel mounted variable resistor, it might prove useful to be able to adjust the frequency response of the filter during a QSO.

My MRM-30 shows slightly different shift frequencies depending upon which 'channel' is in use, so if the replying station is zero beat, they may be slightly out of the 'peak' of the pass band of the filter, if adjusted for one particular 'channel'.

I hope to fit the filter PCB in to the MRM-30 in the next few days. I have a Freq-Mite on order from Small Wonder Labs, so I hope to be fitting that into the SW-20+ also quite soon.


  1. Hi Colin, yes firts batch was sold very quickly. Next batch of filter KIT will be available in 14 days... GL and 73 Ondra OK1CDJ

  2. Hi Colin, did you fit the filter and if so how did you get on. My building skills are quite ltd at the moment so I would be interested to hear how you attached the filter PCB to the MRM-30.
    Cheers Richard M0AUW