Hi all-
I have a nice Teac 4 track deck I got for real cheap. It works great except for one VU meter.
I tore the thing apart last night just to get to the meter, they sure don't make this model easy to service!
Anyway, before taking the meter out I verified that it was getting signal, which it was. Now that I have the meter out I measure about 160k across the meter "+" "-" connections. The other meters read about half that (no connections to the circuit).
What would make a meter go this high? Is this what causes it not to respond to a signal? I did notice when I took the face off the front it does have a small "blob" that looks like a capacitor on the incoming wires. Not really sure what it is though, maybe a protection device of some sort?

I'd like to try and repair it, but I'm not sure if that's possible. What type of voltage do these things handle so I can test it out of the unit. Probably pretty small mV range I assume.

Any advice would be appreciated.
Thanks

The meter may or may not be repairable. The resistance reading may be due to a combination of the voltage provided by your meter and the internal rectifier in the meter. Below is a typical VU meter courtesy of http://sound.westhost.com/project55.htmYou may have a problem with a connection, series resistor or the rectifiers. Things in there will be delicate, and if the meter is glued together, virtually unservicable.

Curtis Eickerman

Thank you Sir for that information!
Time to take it apart further. I just took the scale plate off the front, there are two more
screws in the back that release the movement from the housing.
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.

Curtis Eickerman

Thank you. I had signal on the channel before I took the meter out. I measured both
left and right with the scope and they were both the same. The meter is free moving, so
it's not a mechanical binding issue.

I have an HP204C that should work well for testing this on the bench.

Thank you for the suggestions.

Okay, I disassembled the meter last night, and it looks like the "blob" is a bridge as it has 4 connections. It has a 560R resistor on the "-" side of the meter connection, two leads go to the movement, and the last leg goes to the "+" side of the meter connection. I have continuity through the meter, but still no meter movement using my HP204c. I should have checked the bridge with my meter, but I haven't done that yet.
Is it acceptable just to build a small bridge out of discrete diodes for a test? Would small signal diodes be acceptable?

Thanks

Yes, but you need diodes with a low forward voltage drop. So either germanium diodes like 1N34 or 1N64 or hot carrier diodes like 1N5711.

With regular silicon diodes the meter will be very insensitive because so much signal will be lost by the 0.6 V forward drop of regular silicon diodes.

Curtis Eickerman