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Wednesday, October 26, 2011

Azorean Oscillator Re-build

With my JBOT amp ready for installation, it was time to go back and check out the rest of the circuitry on my old Azorean DSB/Direct Conversion 17 meter "Magic Carpet" transceiver. I was surprised to find that the oscillator, well, wouldn't oscillate. The MPF102 that I had in the main oscillator circuit was blown. I suspect voltage spikes from the T/R relay. I installed the required diode across the relay coil. (I seem to blow up a lot of MPF102s. Is it just me, or are these devices really fragile? They are junction FETs, not MOSFETS, so I thought of them as being more robust. But I seem to go through a lot of them.)

After messing around with the oscillator and buffer circuits, the nice clean Manhattan isolation pads that had been there at the start were all messed up, with big piles of solder with the ends of multiple axial leads stuck in there. I decided to start afresh. Out came the little PCB box that housed that housed the oscillator, buffer and amp stages. Off came the walls of the box. (When I built this thing the first time, I didn't realize that I would need an amp to get to the 7 dbm needed to drive the diode ring. I ran out of room on the main board and ended up building the needed amplifier on the back wall of the box!)

So I got to build this little circuit again, ten years and three countries after the original build. It was fun. I like building oscillators. There is that satisfying sense of closure and completion when, at the end of the effort, you turn on your receiver and hear the tone from your creation.

There was also a fun little bit of troubleshooting. After rebuilding the oscillator circuit I noticed that applying power to the "on the wall" amplifier caused the oscillator to shut down. At the buffer, I was seeing RF in and RF out, but the whole thing would shut down when I powered up the next stage. Obviously there wasn't a lot of BUFFERING going on! Sure enough, the MPF102 was bad. I replaced it from my dwindling supply, and all was right with the universe.

Now the amp goes in. But first I will build the low pass filter. I promise.

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1 comment:

  1. What a great story, Bill and outstanding photographs.

    I too launched quite a few MPF102s (actually they were HEP802s, same device) but I was 'driving the pants off' of them at 190mW input in one of my first HB QRP transmitters back in 1969-70. One would last for about a week or so. And they were quite expensive 'back in the day'; something like $12 ea. in today's money!

    Man, what fun.

    BTW, for your output filter, may I suggest the 'cookbook' model with values for all bands, posted by our dear Rev. George Dobbs of GQRP:

    http://www.gqrp.com/technical2.htm

    73 from the sunny Left Coast.......
    Steve Smith WB6TNL
    "Snort Rosin"

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