Thursday, April 12, 2012


Well, I'm about 4 months late but here is the finished Red Hot Norcal 20. Purchased in 1999 and completed in 2012. The rig spent several months on the shelf because I got very frustrated chasing down a problem with the mute circuit which was absolutely vexing. When I had cooled off enough to think rationally, I posted a help request to the QRP-TECH group on Yahoo. Steve "Snort Rosin" Smith pointed me in the right direction (thanks for the e-mail!). I also downloaded the Spring 1999 issue of Norcal's QRPp journal (which they have thoughtfully posted on their website) that has the theory of operation of the rig. Ultimately I had a bad 2N3906 transistor on the TX +12 circuit along with the wrong resistor at location R99. R98 and R99 make a voltage divider that biases the base of transistor Q21 so that the +5 VDC from pin 5 of the TiCK keyer chip (U9) will turn off Q21. When Q21 turns off, it removes power from certain parts of the receiver circuit thereby muting it so the transmitter doesn't overload the receiver when in transmit mode. The bad 2N3906 (Q20) revealed itself when I was probing it's collector and found that the voltage went negative during transmit rather than going to +12 VDC.

Several more evening sessions had the radio completed but now I had a new problem, the receiver was deaf as a post! It turns out that when I was winding and installing T2, I got the primary / secondary leads mixed up so the signals were not getting out of the bandpass filter. That was very easy to correct but I took the opportunity to rewind both T1 and T2 to make a neater job of it.

Overall, it was a really fun radio to build, aside from the rework I had to do (which was my own fault). Hopefully one day very soon 20 meters will be open when I'm home to operate and I can get that first contact with the Red Hot Norcal 20 in my logbook.

Norcal QRP Club

No comments:

Post a Comment