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286 laptop POST-ing fine but speaker dead

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First post, by z00m3r

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Hi guys,

as the title says - I picked up this Toshiba T3100e, amber plasma screen in perfect condition, mostly everything else not so...pristine

few days later I got the floppy working (de-crudded, head clining, some oiling of the rails), the keyboard (some lines were not working, still acts funky sometimes like the function key is pressed and I need to press it to get the "normal" keys to work, other than that works fine), HDD was dead, now it's working fine

anyway, the only thing I couldn't fix was the pc speaker, and since I don't have any 8bit soundcard (tried an 8bit VGA card on the port, got a decent image, albeit it's slower than the CGA, the card is an abysmally slow early OTIVga card with dual 9pin and 15pin D-SUB outputs, it's the only 8bit I ever got) the computer is as mute as it gets

the speaker itself seems to work (tested it on something else) and it's always on the warm side. Which is consistent with the readings I got on the PC Speaker pins, while using a multiscope - constant +5V as long as the computer is powered on. What could cause this? Was thinking a short, but then again the computer works like a dream, why would there be a short on the PC Speaker port?

Anyone else encountered anything similar on any other vintage board?

many thanks.

Reply 1 of 3, by snufkin

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I think the PC speaker works with a constant +5 and then toggles the other pin at whatever frequency beep is needed. So constant +5 is ok. Speaker getting warm might indicate a short to ground on the other pin?

Reply 2 of 3, by z00m3r

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thanks, I'll crack open the case and fiddle with it when I get home from work. I was always under the impression that current is fed into the speaker to produce sound, not lowered based on frequency. I can't find much information about how the pc speaker is supposed to work nowadays except for the wikipedia page and a few others, as googling pc speaker nowadays returns more about Intel HDA than pre-pentium era boards

Reply 3 of 3, by snufkin

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Found a thread here, with a useful post: Re: Why does the PC speaker have four pins?

Only pin 1 and 4 are commonly used: Data and +5V. I think the ground pin is only used if the cable to the speaker needs a shield. So when current starts flowing (pin 1 driven low) the cone moves one way, then when the current stops (pin 1 probably left floating, pulled high by the speaker +5 supply) the cone moves back to where it started. Toggle the data line fast enough and it makes a note. If your speaker is getting warm then current must be flowing through it, so I guess the Data line is stuck low. It might be difficult to pick out, but can you hear a faint 'tick' from the speaker when you turn the computer on and current starts to flow?

There's also a bit about the PC speaker, mostly about how to program it, here: https://wiki.osdev.org/PC_Speaker

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