I would like a digital "VTVM"

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I would like a digital "VTVM"

Post by Ragwire on Sun May 26, 2013 10:25 pm

Anyone know where can I order a reasonably affordable attachment to allow my Fluke 117 to read some grid bias voltages, or other low voltage/high impedance circuits where a VTVM would have formerly been used? Or do I have to make something myself with an op amp or something? I believe the 117 has an 11MΩ input impedance.

Thanks!

Rob
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Re: I would like a digital "VTVM"

Post by Steve Johnson on Mon May 27, 2013 8:22 am

I would suggest picking up a VTVM. It's not hard to rebuild one as long as the meter is good and they are pretty common at swap meets.

The 117 is really not the meter for the job and I don't think you will be happy trying to use it's analog bar display in place of a real analog meter. I would be very concerned about running into high voltages. The 117s input will only handle 600 volts and the fuses are expensive. Flukes have pretty good protection circuits but why test them? A high voltage probe could be used but the 117 is really something I would consider being an electricians meter or a meter for digital circuits.

Here's some more info on high voltage probes used on VTVMs: http://www.ohio.edu/people/postr/bapix/vtvms.htm

An 11MΩ input impedance is pretty good anyway. Don't try to be to accurate with vintage tube radio voltages. You'll drive yourself crazy.

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Re: I would like a digital "VTVM"

Post by Ragwire on Mon May 27, 2013 10:18 am

OK. Thanks, Steve.

Rob
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Re: I would like a digital "VTVM"

Post by Ragwire on Fri May 31, 2013 10:45 pm

Well, Steve, I bought a VTVM from ebay. I have had good luck in the past with stuff from there. Hopefully this will be OK for the job.
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Re: I would like a digital "VTVM"

Post by Steve Johnson on Thu Jun 06, 2013 8:17 pm

Let us know how it turns out.
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Re: I would like a digital "VTVM"

Post by Ragwire on Sun Jun 09, 2013 12:01 am

Simpson 312...It works like a champ! :mrgreen:
I tested it on about every range and function against a known good digital meter. The condition is excellent...even the probes. It arrived today, well packaged, from an Ebay order, and I couldn't be happier.



Inspection sticker from 1977:


The only thing it is weird about is reading RF (IF, actually) at the convertor plate, through a .01 MFD cap. It says there is 50VAC-RMS there, whereas my DMM with an RF probe reads about 4.8V. Other places, like on the oscillator coil-on a couple different pins-read similar RF voltage as the DMM....

So what might be a ballpark IF voltage level off the converter plate in a normal receiver with no RF amplifier? Surely just a few volts RMS, if even that...right?

cheers
Rob
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Re: I would like a digital "VTVM"

Post by Ragwire on Fri Jun 14, 2013 7:52 am

Well it turned out that the AC input impedance to the meter was much lower than DC impedance, and the circuit was oscillating.

Anyway, I got the old 312 calibrated last night with all the steps in the step by step procedures in the owner's manual. I love this instrument!
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Re: I would like a digital "VTVM"

Post by 75X11 on Fri Jun 14, 2013 12:37 pm

The best test equipment are those you are comfortable with and confident in.  Simpson is a good choice.
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Re: I would like a digital "VTVM"

Post by Ragwire on Fri Jun 14, 2013 9:02 pm

Thanks!
Now that I know to isolate the leads' capacitance and stray voltage I can use the digital too; but I prefer the VTVM for most stuff. It has a "coolness" factor for me--and really works quite well on these old tube radios. Very accurate.

Naturally, the calibration required removal of the chassis from the instrument and the insides look as good as the outside. Clean and no visible wear on the moving parts. I think this was someone's bench queen and then got put away. It may need re-capped someday (and probably should be done anyway) and I can do that if it ever requires that...or if I just get bored.

cheers
Rob
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