Click the thumbnails for the full size photos. Bill W4HBK writes:
"The QRP Labs MEPT is one of the sweetest little rigs I've ever used. Here's how I finished it up and made a few mods to tailor it to my particular needs.
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First, the Altoids tin cabinet. |
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How I added a tuning capacitor for easier frequency adjustment and a jack for an external keyer. The tuning capacitor is a small VHF type which I measured at 14pFand is in series with the original trimmer supplied with the kit. Even less pF's would be better since the tuning range is several times what is needed. Still, it's smooth and easy to adjust within a few Hz. |
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The styrofoam block I whittled out to fit over the xtal and which is pressed firmly in place with the lid closed. The styrofoam block around the xtal smooths out temperature fluctuations fairly well. Even in my unheated shack the drift is usually no more than one Hz over an hour, adequate for image stacking. |
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How I added a heat sink to the 2N7000 by folding a piece of thin copper strip around and soldering to the Altoids tin |
"The external keyer is QRS by ON7YN which I use because it has a very accurate and stable timing control which is required for image stacking using Rot'N'Stack. The AT13 keyer timing is subject to drift with temperature. The program's output is via the serial or parallel computer port either one of which will key the rig the same as the AT13. You can see the red wire which runs from pin 3 of the AT13 in the upper left of pic 2 to the keyer jack input at the lower right. To restore the keyer I just pull the wire out of pin 3 and re-insert the AT13.
"I'm also using a 9V ps which has increased the output power to 280 mW, compared to 60 mW when using 4 AA cells. The AT13 is designed for 5 v and QRP Labs cautions that anything over 6 V may kill it. Since I pulled the chip that's not a problem. At 9V the 2N7000 is barely warm to the touch, due in part to the little heat sink thingy. When going to a higher voltage be sure to follow the adjustment instructions carefully..I've read a few emails about burned out 2N7000's."