How unrelated thoughts lead to an "easy" repair of the Icom IC-756
If you’d asked me a few months ago about the IC-756 sitting on my bench, I’d have told you it was a lost cause. When I got it, the radio had already been through the hands of several amateurs. Each had taken on a handful of minor repairs: fixing loose connections, patching up weak solder joints, or replacing aging components—and clearly spraying all the connectors with so much DeoxIT contact cleaner I’m quite surprised it hadn’t drowned itself. By the time it ended up with me, it was functional on some level but still had a few nagging issues that no one had quite figured out.
At first, I tried troubleshooting it here and there, occasionally poking at it without much focus. I figured I’d either cannibalize it for parts or let it sit until I had more time. Then, something unexpected happened.
Well, talking to someone helped some ideas to all fall into place.
We couldn’t be more different. I’m all about physics, maths, and electronics, while she’s into biology and English. We understand each other’s interests usually (if I didn’t understand biology then I think both my biologist parents would have disowned me) but I am much more passionate about physics and maths. Sometimes, conversations with her just leave me silent and dumbfounded at the idiotic things she says; other times, I’m confused by her apparent intelligence. In one way or another, something she says always provides me with something to think about (hybrid rat hedgehog people for example...). Now, I hear you say, “Well, why are you talking about this girl? What’s her relevance to a radio from 20 years ago?” (also all this preamble is useless for fixing the radio really, but may set the background for other posts)
Well, one night, during one of our relatively common phone calls into the early hours of the morning, we were talking about literally everything and nothing at the same time. I forget exactly what we were talking about to begin with, but something she said got me thinking about the reliance of systems working flawlessly, either by nature's design or that of humans.
I couldn’t stop thinking about it. Even when I fell asleep, the idea stuck with me. When I woke up, my brain had somehow connected it all to the IC-756. It happens sometimes. Technical solutions seem to appear after I go to sleep, or when I hear a song which reminds me of something I saw in a YouTube video years ago when I was just getting into electronics, or randomly in the middle of the day when something you see sparks an idea. Now this is the interesting part where it stops being a blog about my life, and starts being about the problem with the radio.
In this specific example, I woke up with one thought only. The internal ATU.
This is where the IC-756 gets tricky. Even when the tuner is bypassed, all RF still runs through its circuitry. I’d always assumed the tuner couldn’t be the problem—it wasn’t even supposed to be in use! But what if that was wrong? What if this seemingly unrelated part was the key to the high SWR issue?
Sure enough, when I opened up the tuner unit, it was clear after I poked a few bits with screwdrivers. There are relays switching inductance (toroids and fixed axials) and two stepper motor-driven air spaced variable capacitors. A stepper motor works by rotating in small increments, controlled by pulses of current. When I inspected the motor, I found that the spindle leading to the gearbox had broken apart over time, and the tube-shaped magnet had broken free from the shaft connected to the gearbox, causing the motor to get stuck in whatever position it was before the total failure occurred—unluckily for me, that position was the fully closed, i.e., maximum capacitance position. That tiny failure was disrupting the RF path. It wasn’t obvious at first, but even in bypass mode, the motor’s malfunction was creating just enough of a mismatch to throw everything off.
Fixing it was intricate work. Not as bad as the time I had to resolder the CPU on a QMX transceiver (another future blog post), but it was pretty interesting. I had to first take care not to bend the motor housing while removing the top covering plate from the bottom shell. Then I photographed all the gears, including the position of the external spindle to ensure that the motor knew where it was. Then came the tricky part—removing those gears that reduced the motor’s ratio down to 64:1. One more plate out, and I was finally at the magnet. Lo and behold, my initial theory was right, and because I haven’t got enough money to just buy a new motor, I simply used some thick CA glue (like superglue but less runny) to attach the magnet and spindle again. Some sanding and cleaning later, I put the whole motor back together again. With no way to test it externally, I committed to complete reassembly of the tuner unit.
After reassembling the tuner and putting the radio back together, I powered it on and checked the SWR. No more bad SWR in bypass, and my end-fed random wire (I know, I know. Random wire is usually a certain length, but I think "random" refers to the fact that the wire is intentionally NOT resonant on any ham band or on a harmonic) presented an SWR of 1.3:1 into the radio on 40-10m. Bingo. The IC-756 was finally behaving the way it should.
It’s funny how things come together sometimes. Before, I’d been treating the tuner like some random part of the system—just like how I’d always thought of myself as too different from someone like her to have much in common. But seeing how opposites can complement each other—not just in life but in radios—made me rethink that whole “we’re too different” attitude. Turns out, maybe it’s the things that don’t seem to fit together at first that actually make the whole system work.
The IC-756 is now back on the air, and I couldn’t be happier. It’s like when two seemingly mismatched parts finally click into place and start doing their job—you know, that feeling when you get the perfect SWR and everything’s just right. Now, if only I could figure out how to get that same balance in my life all the time...
73,
Daniel