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Is it hard to calibrate db meters on AMP ?

I have a Proton amp with a blue-lit db meter for each channel. Everything with the amp is A-okay except the meters don't match. At no input the right meter rests at 0, but the left meter rests up a little, not quite at 0. Also the right meter is more "active", dancing faster where as the left meter dances slow in comparison and does not peak quite as high as the right meter. I used a MONO signal input to verify it is the meter's problem.

I'm pretty handy- mechanically but does anyone know how hard it is to calibrate the meters to match? I notice at the base of the needles there is some sort of spring and 2 limits or stops, one on each side of the needle. On the left needle these limits are only about 120-degrees apart where-as on the right meter the limits are a full 180 degrees apart (give or take several degrees)

any help is appreciated
thanks!
draggcj
2 responses Add your response
richingoth
326 posts
 
For mechanical zero adjustment there should be a small adjusting screw at the bottom of the needle where it swings.
Turn the screw until the needle aligns with the zero mark and turn the screw a little the other way so that the internal adjusting mechanism aren't touching anymore, (so the temperature expansion doesn't move the needle). Also apply anti-static spray to the lens, as electostatic charges keeps the pointer from moving.
If these are power meters you will need a signal generator, volt meter and power resistor.
Somebody please continue... I have to get back to work (calibration).
richingoth
326 posts
 
Set the signal generator to sine wave 1 kHz and set the output voltage of the sig.gen. to the input sensitvity voltage of the amp with the AC volt meter. The two 8 ohm 100 watt power resistor will go across the speaker terminals + & -, one each for left and right. You then put the voltmeter across the resistor to be measured or an ampmeter in series with the resistor. Turn on the amplifier and turn up it's gain until you read 28.284 volts across the resistor or 3.536 amps on the ampmeter. When these readings are reached your dB power meters should read 0 dB or 100 watts. Those resistors should get HOT, be careful. For this calibration to be meaningful the test equiptment should have a 4:1 greater accuracy over the unit under test. (per NIST).
Good luck.
Regards,
Rich
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