I’m always working on improving my ham radio activities. And, all too often, one adventure that goes off the road leads to another.
In this case, I built a PTT cable with relays for my IC-9100. It worked fine until one of the phono plugs didn’t make an electrical contact, leading to a burnt-out LNA. Read about my repair work while learning about SMD desoldering and soldering.
6 and 2 Meter Airspy SDR and LNA
I use two Airspy SDRs, one for 6 meters and one for 2 meters. I also use separate Antennas-Amplifiers preamp, bandpass filter, and transmit-receive relay for each band. I’ve written about using it with the Airspy SDRs and my IC-7100 on 6 meters and IC-9100 on 2 meters. It’s a great setup. I also use the Leo Bodnar GPSDO at 10 MHz to drive both SDRs to ensure they are on frequency.
The switching from receive to transmit uses Downeast Microwave LTRS sequencers. The PTT signal comes from the IC-7100 and IC-9100. For the IC-7100, the only way to drive an external PTT is via the ACC connection, while the IC-9100 has a phono jack or the ACC connection.
Some time ago, I built an ACC connection cable with relays that would separately activate HF/6 meters and 2 meters. I’m using that on the IC-7100. However, the IC-9100, using just the phono plug, activates the 2-meter sequencer even when I’m operating HF. So, I decided I’d better build another ACC cable.
Fried Preamplifier, Anyone?
On testing the new cable, the transmitter activated but not the transmit-receive relay, resulting in a fried preamplifier. The cause was a poor connection between a phono plug and a jack, which I repaired. Next, I’m repairing the preamplifier.
I contacted Goran, YU1CF, at Antennas-Amplifiers for advice. He offered to repair it for a very reasonable 15 euros, but shipping it back and forth to Serbia wasn’t all that attractive. He also provided a photo, schematic, and directions to repair it myself.
Goran further advised me on what parts to purchase. The Mouser website has about 100 different options for 1 nF SMD capacitors. He pointed me to the right one. I bought several and some standard lead capacitors, just in case. I also picked up the PGA103+. They arrived the next day from their warehouse here in the Dallas area. That was nice.
Precision? SMD Soldering
The first task was desoldering the metal cabinet that houses the LNA/PTT/Bandpass Filter. He advised that I would need a 200-watt soldering iron. I confirmed this by trying my 100-watt large iron, and it just couldn’t budge the four solder joints. I ordered a 200-watt iron that had good reviews on Amazon. Then, I started heating things and wedging the lid off the unit.
Next up, I watched several helpful YouTube videos on SMD desoldering. I used a special flux and alloy to get everything off the board using a much smaller soldering iron set to 350 degrees. That was cleaned up with a desoldering braid and isopropyl alcohol. Then, I tacked the tiny capacitor and PGA103+ onto the board.
It took quite a bit of fiddling to get everything lined up. But on testing, everything worked! Hurrah. So now I have lots of tools, solder, flux, braid, and one giant soldering iron. I guess I’m ready for the next adventure that runs off the tracks.
My other insight is that you should try this at home. It’s not that challenging, even for my shaky hands and poor eyesight. Plus, there are lots of helpful videos available to guide your efforts.
Update: June 2024
Well, I’ve taken my own advice and tried this at home two more times! I’m getting way too good at this with way too much practice. My 2-meter sequencer and PTT cables did me in a bad way, wiping out the preamp twice more now. I believe that I’ve found all the issues and expunged them from my station, but who knows?
