EEVblog Electronics Community Forum

Electronics => Repair => Topic started by: vidarr on October 31, 2018, 11:50:41 pm

Title: Analog VU Meter
Post by: vidarr on October 31, 2018, 11:50:41 pm
This VU meter came from an audiometer my wife found the other day. This would go great with the amp circuit I built a couple weeks ago. I took it out and am trying to test it, but I am getting nothing. I am using a 3.5mm audio jack to the VU meter from my computer. I tried different combinations from the jack to the two terminals on the meter. There doesn't appear to be anything wrong with the meter visually looking at it.

Do I need to open it up to fix something? What should I look for now?

Thank you!
Title: Re: Analog VU Meter
Post by: BradC on October 31, 2018, 11:54:24 pm
Yeah, that won't work. The meter will need a rectifier and amplifier to generate a correctly proportioned DC signal to drive the coil.
Title: Re: Analog VU Meter
Post by: floobydust on November 01, 2018, 12:09:21 am
An analog meter needs DC not AC, but your picture shows a mystery 4-pin black part which I think is a bridge rectifier. So the meter would be fine with AC coming in, but would do nothing and not move until >1.5V is present. You could test with a multimeter, or just turn up the volume.
Title: Re: Analog VU Meter
Post by: vidarr on November 01, 2018, 12:50:41 am
An analog meter needs DC not AC, but your picture shows a mystery 4-pin black part which I think is a bridge rectifier. So the meter would be fine with AC coming in, but would do nothing and not move until >1.5V is present. You could test with a multimeter, or just turn up the volume.

Yes. The multimeter worked. It sent the needle to max.

This whole circuity for the audiometer is AC and DC. There is a transformer off the mains. There is no bridge rectifier right there, just a 12v LM340 voltage regulator. It seems each piece of the circuitry has its own bridge rectifier (or not).

Thank you again!

edit: I forgot... When I connect this to my amp circuit I will put it on one of the lines out to the speakers (its a stereo amp). Will the VU meter need an additional 1.5v supply? If it does, does it go to the terminal marked "+", or the blank one? I was reading on some music forums and it seemed everything they did was reversed. Like they put the ground to the "+".
Title: Re: Analog VU Meter
Post by: floobydust on November 01, 2018, 01:15:57 am
Try the multimeter with both polarities, the meter should move up either polarity if there already is a rectifier inside the meter's case (the little black cube).

Don't connect it directly to speakers, the signal is too strong. You need a resistor to lower it, depending on what volume you want to be full-scale. I would try 3k-10k ohm resistor in series.
Title: Re: Analog VU Meter
Post by: vidarr on November 01, 2018, 12:54:47 pm
Try the multimeter with both polarities, the meter should move up either polarity if there already is a rectifier inside the meter's case (the little black cube).

Don't connect it directly to speakers, the signal is too strong. You need a resistor to lower it, depending on what volume you want to be full-scale. I would try 3k-10k ohm resistor in series.


OK. Thank you for that advice.

I was also wrong about the bridge rectifier(s). I was wondering if AC current could go into the LM340 voltage regulator and maybe not right? I took the main board out today anyway and found I think a bridge rectifier buried in the corner in circuit before the LM340. 

Thank you.