Hi Robert!
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I've never liked the keypad beeper on the FT-980. Mine is VERY quiet and quite high-pitched. During normal operation I can't really hear it over the normal HF hiss. Since there is no "side-tone" in the audio path, if you are wearing headphones it might as well not exist. Additionally, there is no volume control for it that I am aware of.
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So, that being said, let's dive in. For whatever reason Yaesu decided to implement a small piezo buzzer from Panasonic (an EFB-R series buzzer) right on the keypad. Now, there is a perfectly good audio path they could have injected it into, where we could have put volume control, heard it over an external speaker/headphones, etc. But, whatever ...
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The exact part was the Panasonic EFB·RE25D02, which is an EFB-R series, external drive (I believe), 2.5kHz resonant, small diameter piezo. These are typically fed by a 15Vp-p square wave. From the schematics, it appears that Yaesu might have used a 5V square-wave.
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From the symptoms it sounds like the piezo element in your buzzers is loose or broken. Odd this occurred in the same time period, but ... who knows.
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Although this buzzer does not appear to have been produced in some time, there are numerous ones that are very similar in specs/performance. You might want to check/compare the TDK PS14 series parts. These are < $1 at Digikey in quantities of 1.??
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The one thing I am not 100% sure is if the buzzer is internal or external drive. It "looks" external ... but without ripping my unit open I can't be sure.
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-Mat Breton, N8TW
Mat Breton N8TW