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"Antique" VU Meter

Conversion of a damaged 1940s era Multimeter into a VU Meter.

tim-savageTim Savage
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I bought this Universal Avo Meter Model 7 (believe it is 1940s or early 1950s vintage) off eBay several years ago. I didn't expect to receive a fully working meter, however, it was beyond repair, looks to have experienced a surge, with much of the wiring missing or burnt away.

It sat as more of a display piece for a couple of years before I thought to take another look inside, turns out the meter movement is still working fine, and I came up with the idea of turning it into a VU meter, the result is this project.

Utilising the existing battery compartment as a way get the 12V power supply into the unit and trying to minimise any change to the outside of the case. In the end, the only addition has been a 6mm TRS jack.

The circuit utilises a TL072 op-amp followed by a diode rectifier and a capacitor to provide peak hold. The resistance of the coil in the meter movement bleeds the voltage from the capacitor.

No working multimeters where harmed in this conversion.

Build is complete, action video!

IMG_20180324_134228.jpg

Back panel (for the Multimeter aficionados)

JPEG Image - 1.89 MB - 03/24/2018 at 03:45

Preview

  • Turning it off

    Tim Savage • 03/26/2018 at 11:36 • 0 comments

    One final tweak, behind each of the AC and DC switches are some mechanical switches. These are operated by small cams on the back of the rotary mechanism. This of course is a convenient way to turn off the unit without needing to add any more switches.

    With that along with a nice cable I'm calling this project done... 

    ...next project!

  • Power lead

    Tim Savage • 03/25/2018 at 12:14 • 0 comments

    Made up a power lead that connects to the old battery lugs to power the device. Just need to make a nice input cable...

  • Schematic and details

    Tim Savage • 03/25/2018 at 07:19 • 0 comments

    Attached is the schematic, consisting of number of common blocks, DC offset filter and bias, variable gain Op-Amp and a rectifier.

    This is a "slightly" tweaked version from circuitlib.com the main change being R3 to work better with my particular meter movement.

    The rest of the build was largely around laying out and building the circuit onto a PCB and making the brackets to mount it.  The brackets where made from metal salvaged from a rice cooker that already had M4 threads tapped. 

    Power is supplied using the existing battery contacts, an old school braided power cable would really finish the look.

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Discussions

Dr. Cockroach wrote 04/07/2018 at 19:56 • point

I have a fondness for vintage equipment and when I first saw this project my thought was " Nooo" restore it, don't change it. But As you mentioned, it was really shot inside so you did the right thing and at least kept the looks :-)  Well done :-)

  Are you sure? yes | no

Tim Savage wrote 04/09/2018 at 02:10 • point

Restore was what I was thinking when I bought it, however, it was really beyond what I could feasibly repair. I have kept most of what I removed, might go into another project or to somebody doing a restoration.

  Are you sure? yes | no

Dillon Nichols wrote 04/05/2018 at 12:36 • point

I have an old DeVry 1S14 meter that still works, but I don't need to use it as a meter and wanted to make it into more of a display piece. Something like this is a great idea. Do you have any other ideas? I was thinking something like a temperature or humidity display

  Are you sure? yes | no

Tim Savage wrote 04/05/2018 at 15:30 • point

I thought of a few things, CPU load, temp/humidity, light levels, internet speed test, etc etc. However, none of those ideas get the needle moving like a VU. I am also a musician and love the idea of movement from music.

  Are you sure? yes | no

Sadiq Mohamed wrote 04/04/2018 at 18:35 • point

Poor old meter! It has had a very hard life. This brought back memories. Back in the 70's I worked for the BBC at TV Centre in West London. We used the Model 8 Mk4 & Mk5. Beautifully made. There was a lab in the building which could calibrate them to manufacturers spec. I bought a Universal MultiMinor for my own use, and although it no longer works, I still have it!

Nice hack!

  Are you sure? yes | no

Tim Savage wrote 04/05/2018 at 05:30 • point

Thanks. Is still a very nice looking meter. Love the style from that era. I considered just replacing the internals with modern parts to keep it as a meter however the movement only sometimes returns back to zero.

  Are you sure? yes | no

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