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Technical Topics => The RF Workbench => Topic started by: ak7v on March 10, 2021, 2317 UTC

Title: Help with 22m beacon kit from Black Cat Systems
Post by: ak7v on March 10, 2021, 2317 UTC

Hello,

I ordered a 22m beacon kit years ago and finally got around to putting it together.  It works great as long as my supply voltage is 4v or less - nice clean sine wave on frequency, keying from uC works.  Anything above 4v, and it will tend to oscillate at a higher harmonic (and not make it through the output filter, thankfully).  When I first power it up, it'll oscillate on the crystal's fundamental, but if I adjust the pot, or "key" the microcontroller (either with the actual microcontroller or manually with a wire from pin 6 to VCC or GND), the 74HC02 will jump up to a harmonic and stay there until I cycle power to the board.

Anything I can do or anywhere I should look to diagnose/fix this problem?  I'm powering it off a bench supply for now, no regulator installed on the board (just jumper).  No extra caps, although I played with a few values in C16 without any change in this behavior.  I've checked my component placement and all looks well.  I measured 4.98v in the initial voltage check prior to installing the 74HC02.

If I run at 4v or lower, it functions exactly as it should.  This probably won't be a real problem since I plan on using a solar light w/ battery to power it, but I'd like to learn and understand what's happening and how to fix it.

Thanks!

Title: Re: Help with 22m beacon kit from Black Cat Systems
Post by: redhat on March 11, 2021, 0000 UTC

My money is on a bad IC.  It probably got zapped when you built it...I've had similar happen with CMOS devices.

+-RH

Title: Re: Help with 22m beacon kit from Black Cat Systems
Post by: ak7v on March 11, 2021, 1743 UTC

Yep, you were right - swapping out the 74HC02 fixed the issue.  I gotta be more careful!

Thanks