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A Sound Repair

A couple years ago now, I cleaned up my old IIcx motherboard. It had some significant corrosion from leaking capacitors. There were a couple traces I found that had been eaten through, and in the case of R3 to C2, a via was completely gone.

I added bodge wires to fix the breaks I had found, and the machine booted and ran just fine. There was just one lingering problem — the internal speaker was mute.

The sound circuits were working. With headphones in, I could successfully test both stereo channels. But no matter what I did I couldn’t get the internal speaker working.

I had hoped my bodge wires would fix it. I thought maybe it was dirty contacts inside the headphone jack. Nothing helped.

I finally sat down recently with a copy of the Bomarc schematics and found the issue.

See, unlike every other device I’ve encountered before, the headphone jack for the IIcx has a DPDT switch. Most devices have an SPST switch to detect when headphones are plugged in. The IIcx though uses one switch to detect when the headphones are connected, and one switch to disconnect the internal speaker.

Following the schematic, I had continuity from the speaker all the way across the motherboard (in an internal layer) to the headphone jack. On the other side of the headphone jack though, continuity was broken. What I could see of the trace was fine. My best guess is it is broken underneath the ADB filter where I can’t see it.

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I ran a new bodge wire — from the headphone jack to the other side of R3 — and now continuity is good all the way from the sound chip through the headphone jack, and to the internal speaker.

And now it works again.

I had missed the startup chime.