LED VU meter problem

I have a commercial mid 1980s solid state stereo preamp/mixer, which has a dual LED Bar Meter. (about 6 green on the bottom, two yellow above that, and a red on the top).

The preamp/mixer works well, but the LED meter is flakey. I have never worked with one of these, so before I tear into it, I want to know if these are all complete units, and are they fed off the actual preamp, or do they have a separate amp of their own?

What it's doing: One channel lights up every so often, and moves like it should. Then it goes off for awhile. The other channel does not light at all.

However, both channels do momentarily light up the red LED for a split second when I turn on the power to this preamp. So, that tells me that there is obviously power going to this LED meter, and that the meter LEDs do work.

I'll begin by looking for any loose connections, but what is the way to troubleshoot these if all connections are tight? If I measure voltages with my VOM, what sort of voltage would be normal?

(I have not been able to find a schematic for this device, so I'm on my own).

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** So the make / model is a HUGE secret is it ??

Wot a PITA moron.

** Check the leds first, cracked solder on the legs is very common and so are dead leds.

.... Phil

Reply to
Phil Allison

A lot of these meters use an LM3914 (linear) or LM3915 (logarithmic) dot/bar display driver IC, or an equivalent. They take in a signal voltage, accept a "reference" voltage to set the full-scale reading, and drive LEDs or other display elements.

If one of these is what your device uses, it probably wouldn't need a separate preamp circuit - it ought to be able to sample the audio voltage on the main bus (or output) directly. Their might be a half- or full-wave peak detector circuit between the input and the display-driver IC.

The data sheets for these ICs are instructive.

Reply to
Dave Platt

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