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 Post subject: Questions about VU meter diodes
PostPosted: Apr Sun 06, 2008 4:44 am 
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Location: Burbank, CA and Thailand
I have a number of bad VU meters (Triplett, Weston, Modutec) that seem to have faulty diode bridges. When I've taken some of these meters apart, I've found that the diodes are often sealed in dope or epoxy, with no identifiable markings.

Does anyone know what type of diodes are generally used in good quality VU meters, and where spares can be found? Since VU meters respond to very small AC voltages, maybe that these are special diodes with low forward bias voltages? Even germanium diodes have forward voltages of about 200 mv, right? Yet VU meters will respond to -20, which is less than 10 mv. Are the meter numerals or spring tensions scaled to compensate for nonlinearities in the diodes?

This topic has long been a puzzlement to me. Thanks in advance for any enlightenment and/or ideas for replacement diodes.


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 Post subject:
PostPosted: Apr Sun 06, 2008 6:15 am 
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With the help of Wikipedia, I found:

The behaviour of VU meters is defined in ANSI C16.5-1942

that spec should provide further information. But since it was issued in 1942, it's a sure thing that the original design used a copper oxide bridge meter rectifier - thats about all that was available then.

--Chuck


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 Post subject:
PostPosted: Apr Sun 06, 2008 7:24 am 
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The ballistics of a VU meter is very specialized. That is why you cannot make one using an AC voltmeter movement. It would indicate but not work for it's intended use.

I believe copper oxide diodes are used due a specified voltage drop. Even today's VU meters use CUO2 diodes, if they are indeed "real" VU meters.

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Last edited by renaissance.man on Apr Sun 06, 2008 7:31 am, edited 1 time in total.

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 Post subject:
PostPosted: Apr Sun 06, 2008 7:29 am 
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Diode thresholds are not a ridged wall. Low voltage diodes start to conduct well below their expected voltages.

1N34 (germanium) or 1N5711 (hot carrier) diodes, should work quite well in most VU meters.

RCW

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PostPosted: Apr Sun 06, 2008 7:47 am 
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Just wondering, how do you know they are bad? This seems somewhat strange for the diodes to have gone bad.

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 Post subject:
PostPosted: Apr Sun 06, 2008 8:03 am 
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Thanks for the replies and suggestions. I'll try some of the diode types that were mentioned. I'm curious to look into the copper oxides and 1N5711's.

Several of these meters read almost exactly 6 db low and respond to DC in one polarity but not the other, which seems to point pretty clearly to an open diode.

Thanks again for the helpful answers.


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 Post subject:
PostPosted: Apr Sun 06, 2008 8:50 am 
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[Mr Mackey Voice] Testing VU meters with DC voltage is bad...

Real VU Meters came with full wave diode bridges as well as series half wave bridges(with a shunt diode). If you burned out the diodes(which is highly unlikely), the meter was toasted long before that.

To connect a VU meter, the VU Meter with a series 3600 ohm resistor is connected across a 600 ohm load which is fed by some audio device at line level. It is quite possible that some of those meters have the 3600 ohm resistor built in. This would give nearly a perfect deflection drop of 6dB compared to the meters without the built in 3600 ohm resistor since you lose 48 percent of your deflection voltage across the built in 3600 ohm resistor.

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PostPosted: Apr Sun 06, 2008 9:40 am 
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I think I should have given a little more background info on this application and my own experience. I've worked in audio for 30 years and am familiar with VU meter hookup and usage. The meters came to me faulty, in equipment that was sent to me for repair and are specific models that I have seen many times. You're right that the wrong series resistor can cause level errors, but that's not the issue here.

The gear that uses them (an audio compressor) has a mode switch, which in gain reduction mode applies a small DC current to the meter to indicate the amount of gain reduction.

I observed that the bad meters read low by 6 db in audio mode and, unlike good units, tend to work in the DC mode with the leads connected in one direction, but not the other. (I guess my earlier message suggested that I was testing these meters with a battery or bench supply which could indeed could cause damage, unless the right current limiting resistor was used. :)) I'm pretty confident that this is caused by a faulty diode inside the meter. I'll try repairing them with the types mentioned, and will report back.

Thanks again to renaissance.man and the others who replied here.


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 Post subject: Re: Questions about VU meter diodes
PostPosted: Dec Sun 01, 2019 5:14 pm 
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Joined: Dec Sun 01, 2019 5:10 pm
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Hello, I am trying to figure out how to replace the bridge diode in a Modutec vu meter from a Urei La4 compressor. Just wondering if you ever had any success with your similar endeavor. Thanks. Michael


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