Yesterday I felt like repairing radios, so that’s exactly what I did.
I have two radios that needed minor repair or adjustment. The one I decided to work on first was my Icom IC-751A HF rig. I was having issues with the higher bands; there was no received signal above about 15MHz. This can happen on those radios when the PLL can’t lock on to higher frequencies due to mistuned circuits that reduce the PLL lock voltage. The Service Manual (section 6-2) has a procedure to adjust it, and I found a good video on YouTube detailing the procedure. So I decided to give it a shot.
I don’t have an oscilloscope (yet; I’m likely going to buy one soon), but all I needed to do for this procedure was to measure voltage at a couple of points; my trusty voltmeter can do that. When I started measuring, I found the problem; all the voltages were way off. The circuits for the lower bands must be more forgiving; they worked with the lower voltage. Basically, where it should be 6.5V, I was seeing around 4.8V to 5.0V. There are four bands to check, so I did them all. To adjust the voltage, you tweak a variable inductor, so I carefully did that while measuring the voltage, got it right (well, within 0.1V or so) and then moved on to the next band.
After you tweak the inductor, you then have to check the bands at a different point and tweak 4 trimmer caps to get that voltage correct. Those trimmer caps can sometimes go bad, and I was hoping I didn’t have to replace them. Again, the voltages were pretty far off, so adjusting was done; the caps still seemed to work fine. I adjusted all four bands successfully. Lastly, you verify a voltage on a diode; it was fine, probably due to the adjustments I just made.
Once I was done, I fired it up and the higher bands worked! This was the first major adjustment I’d ever done to a radio, so I was extremely happy. And it gave me the energy and confidence to take on a second repair on another radio.
I have an IC-471H 440MHz all-mode. It seems to work fine, but the internal speaker doesn’t usually work. I mean, it does, but it doesn’t; sometimes it cuts in and out, or just stops working altogether. The headphone and external speaker jacks work fine all the time, so I knew the audio chain was good up to the speaker.
No one can find a service manual for the IC-471; it’s possible it was never written. But I found a schematic online for the IC-471A/E, which is close enough. It showed that the speaker and the external speaker jack were on a separate little board mounted to the back of the chassis. I found the board, and it was super simple: the jack, the 2-pin connector for the speaker, and a filter capacitor. So I unmounted the board and started testing things.
Continuity tests were fine. The cap wasn’t shorted. I didn’t see any cold solder joints, but I resoldered the 2-pin speaker connector anyway. I even tested the speaker; it was fine, both electrically and physically. Then I started looking at exactly how the speaker signal went through this little board. And it went through the external speaker jack; when you plug in the external speaker, the jack physically disconnects the internal speaker. Could the jack be bad?
Looking at it, I didn’t see anything physically wrong, but it was old and dirty and corroded. So I gave it a good cleaning with a Q-Tip and alcohol, which came away quite dirty. Could this be the issue, a dirty connector? So I tested the speaker.
I connected the speaker, powered up the rig, and tuned to a repeater. I threw my call at the repeater and heard it clearly from the internal speaker! I adjusted the volume and the squelch; they both worked fine. Wiggling the wires didn’t cause static or any other problems. It was fixed! So I put everything back together and it kept working!
The moral of this story? On a 30 year old radio, make sure everything that has a physical connection is clean. A lot of grunge and corrosion can build up over time and it can cause all sorts of weird issues. Basically, that jack was acting as a very crappy resistor and preventing the audio from getting to the speaker.
Now I have two fully-operational rigs. Yay for productive days!