Glenn B wrote:
At least there is hope that some of the diodes, resistor, or possibly cap has failed.
For what it's worth, a better test of a VU meter is to provide an audio signal from a generator at 1 KHz. If you do so, a signal of approximately 1.087 VAC RMS at 1 KHz should be about +3 dB (typical meter full scale). 0 db is approximately 0.77 VAC RMS.
In a pinch, you could record a 1 KHz tone on something, measure the AC voltage with your voltmeter and adjust it to the right level then hook it to the VU meter for testing.
The above assumes common industry practice that +4 dB is 1.22 VAC RMS at 1 KHz.