Have all dashboard lights on but car runs perfect! Speedometer doesn't work

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I have 2002 hyundai xg350L with 118K miles on it. Runs perfect. Recently I have all the dashboard lights on. The brake, TCS (ABS) lights on and when they are on, the speedometer doesn't work. There is no problem with the car. It runs good. But thats the only problem and a mechanic whom I showed the car couldn't find the problem even after he connected it to the computer. It doesn't happen all the time but happens at the wish of the car. It runs normally sometimes. i am having the problem since 2 days! Someone told me there could be a problem with alternator. If so, is there any way that the alternator repaired or I should only replpace it at any cost?