Bicycle Mobile Mic Amp Debug: It’s the Connector

The radio on Mary’s bike has been misbehaving over the last few months: the PTT button on the handlebars occasionally had no effect. Debugging this sort of intermittent problem is quite difficult, as it would sometimes fail and repair itself before we could get stopped in a safe place where I could poke around in the wiring.

After months of this nonsense, I narrowed the failure down to the short cable from the HT’s mic jack to the interface board: by positioning the cable just so, the radio would work fine for days or weeks at a time. I taped the thing in position and all was well, at least for a few days or weeks at a time.

These two pictures show what the interface looked like back in 2001 when I put it together (modified from another version I did in 1997!) and what it looks like today. The most significant change is in the plugs connecting the whole affair to the HT: a CNC-machined plate holds them perfectly parallel at the proper spacing and an epoxy-putty turd fuses them into a rigid mass. More on that sub-project tomorrow…

Loose plugs, it turns out, vibrate the HT’s jacks right off the circuit board in short order and those jacks are a major pain to replace. Ask me how I know…

The wire break seemed to be precisely where the mic cable exits the epoxy turd. You’d expect a fatigue fracture to occur at that spot, so I wasn’t particularly surprised, although I was amazed that the thing hadn’t failed completely over the months I spend fiddling with it. I finally resolved to fix this once and for all, which meant either flaying the cable and patching the wire in situ or rebuilding the whole connector assembly. Either choice requires enough fiddly work to discourage even me.

Sooo, disconnect everything & haul it to the Basement Laboratory, Electronics Workbench Division…

Before cutting into the cable, I measured the mic voltage on the PCB and tried to make the thing fail on the bench. The HT (an ancient ICOM IC-Z1A) normally presents 3.5 V DC on the mic wire and the external PTT switch pulls it to ground through a 22 kΩ (or 33 kΩ or thereabouts) resistor. The mic audio is a small AC signal riding a volt or so of DC bias with the PTT active.

The wire measured maybe 0.25 volts and the PTT dragged it flat dead to ground. Yup, through that honkin’ big resistor. Well, maybe the last conductor in that mic wire had finally broken, right there on the bench?

Measured from the 2.5 mm plug tip conductor (tip = mic, ring = 3.5 V DC, sleeve = mic common) to the PCB pad on the PC, the mic wire stubbornly read 0.0 Ω, regardless of any wiggling & jiggling I applied to the cable. But no voltage got through from the radio to the board…

Sticking a bare 2.5 mm plug into the HT mic jack produced a steady 3.5 V on the tip lug. Reinstalling my epoxy-turd plug assembly produced either 0.25 or 3.5 V, depending on whether I twisted the thing this way or that way.

Ah-ha! Gotcha!

Pulled out my lifetime supply of Caig DeoxIT Red, applied a minute drop to the end of the mic plug, rammed it home & yanked it out several times, wiped off the residue, and the PTT now works perfectly. Did the same thing to the adjacent speaker plug, just on general principles, and I suspect that’ll be all good, too.

Diagnosis: oxidation or accumulated crud on the mic jack inside the radio.

Now, to try it out on the bike and see how long this fix lasts. Anything will work fine on the bench, but very few things survive for long on a bicycle.

Memo to Self: It’s always the connectors. Unless it’s the wires.

Here’s the schematic, just in case you’re wondering. I wouldn’t do it this way today, but that’s because I’ve learned a bit over the last decade or so…

[Update: A more recent attempt is there.]