I have a QDX (board rev 3a) for 80-20m that I'd used successfully on a number of outings. On a recent SOTA attempt, however, transmit on the 20m band stopped working after about 15 minutes. I switched to 40m and that transmitted fine. But even after rebooting a couple times, I could not get transmit on 20m to work again.
When I got home and put the QDX on the bench, it worked fine again to transmit on 20m. Troubleshooting intermittent failures is fun!
On the day of the 20m transmit failure, the temperature was cool out, maybe about 45° F (7° C). That was the coldest conditions I had tried operating in. So I'm wondering if there's a small crack in some component that caused an issue when cooled, something that's part of the diode band-switching circuitry.
Which components should I focus on as the most likely culprits for an issue like this?
Solder joints can behave like this. Imagine surfaces pulling apart as they cool and shrink.
With focus on the 20m LPF, I suggest checking the inductor leads first. A poor joint that is the result of inadequate enamel removal can wreak havoc.
Next comes the capacitor solder joints, with emphasis on the ground leads. Good solder joints need heat and the inner ground? planes suck that away from the joint.
If after all that you suspect something is wrong with PIN diode switching, a voltmeter and Hans' excellent circuit operation description comes to the rescue.
I think the most likely is the first possibility, inductors,? though.
On Sat, Jul 13, 2024, 1:38?PM Leigh KG7WED via <ham=[email protected]> wrote:
I have a QDX (board rev 3a) for 80-20m that I'd used successfully on a number of outings. On a recent SOTA attempt, however, transmit on the 20m band stopped working after about 15 minutes. I switched to 40m and that transmitted fine. But even after rebooting a couple times, I could not get transmit on 20m to work again.
When I got home and put the QDX on the bench, it worked fine again to transmit on 20m. Troubleshooting intermittent failures is fun!
On the day of the 20m transmit failure, the temperature was cool out, maybe about 45° F (7° C). That was the coldest conditions I had tried operating in. So I'm wondering if there's a small crack in some component that caused an issue when cooled, something that's part of the diode band-switching circuitry.
Which components should I focus on as the most likely culprits for an issue like this?
Just to be clear, when you're talking about checking the inductor leads first, you mean the hand-wound toroids (L2-L4, L6, L8, L10)? As opposed to the L1, L5, L7 and L9 radial inductor components?
Yes, exactly. The toroids are the ones to worry about.
The 47 uH molded inductors suffer in a different way. They are a bit brittle and are subject to physical damage. That too can shut down an LPF by denying PIN diodes their required forward bias current.
On Sat, Jul 13, 2024, 2:03?PM Leigh KG7WED via <ham=[email protected]> wrote:
Thanks John! I'll start with those.
Just to be clear, when you're talking about checking the inductor leads first, you mean the hand-wound toroids (L2-L4, L6, L8, L10)? As opposed to the L1, L5, L7 and L9 radial inductor components?
With focus on the 20m LPF, I suggest checking the inductor leads first. A poor joint that is the result of inadequate enamel removal can wreak havoc.
Well, it was hard to confirm visually, and I was unable to recreate the fault on the bench, but this may have been the source of trouble. I removed the solder from the 20m hand-wound inductor leads, gently scraped at the enamel with an X-acto knife blade, and then resoldered each one. (I did one lead at a time this way, so the inductors would not fall off the board!)
A follow-up SOTA activation was successful on 20m, so crossing fingers that it's now fixed. Until I test it in colder weather I won't be sure however.
Leigh KG7WED