LC III: internal speaker works intermittently when pressing on the external audio jack. Audio plays through external jack when connected.

[theirongiant]

I have an LC III that boots and runs normally, but the internal speaker doesn't work. Well, at least I thought it didn't at first.

External audio works; I heard the startup chime, quacks, and wild eeps through headphones. By accident I was pushing the headphone jack while turning it back on, and the internal speaker played the boot chime. So I tried manipulating the jack with mixed results, and made a video recording of my experiments. The audio on this video is captured by a Shure MV88+ microphone plugged into my iPhone and resting right next to the speaker. I unplugged the fan for these tests.

Perhaps you can advise me on next steps to repair this? I've already used 99% isopropyl alcohol and a toothbrush on the logic board, and sprayed some WD40 contact cleaner inside the headphone jack. No luck. Is it time to replace the jack? Is this a difficult part to find?

Thanks!

 

[YMK]

It could be the jack, but it also might be as simple as reflowing the jack's pins with some flux and solder.

Also, it sounds like your LCIII is squealing.

This can probably be resolved by changing C4 to 47uF.

 

[XKSTEVE]

Getting the socket off can be a pain and risks trace damage if you are not careful (I have just fixed a LC that someone tried recapping and destroyed the pads for the cap and resistor infront of the socket which they then tried to remove ripping traces off everywhere in the process) so be careful, you'll need a lot of heat and a big tip as the ground plane will suck the heat out of your iron real fast and make sure you get all the soldier out.

If you are feeling brave the safe way (and this is what I do) is to destroy the old socket by cutting it off (carefully) so as to just leave the pins soldered to the board that way you can remove the pins individually which make the job much easier.

I get my replacement sockets from Farnell here in the UK (which reminds me I need to order some more)

https://uk.farnell.com/switchcraft-conxall/35rapc4bv4/connector-rca-phono-jack-3way/dp/2293877

You might just get away with a reflow (specifically pin 3 and 2 and 5), but I usually find the fault is component failure so for instance if the cutout switches in the socket are corroded or stuck that would result in this failure.

If you have a look at the schematic for the LC you should be able to figure it out as the LCIII circuit is very similar from what I remember.

HTH and Good luck.

PS - Just found out I have the Bomarc LCIII schematic - to aid clarity, I have over sketched the path in red and green