Author Topic: Antique Fairchild 7000 Digital Voltmeter  (Read 2311 times)

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Online Cyberdragon

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Antique Fairchild 7000 Digital Voltmeter
« on: December 05, 2015, 08:00:32 am »
For a 40 to 50 year old digital voltmeter it at least tries to work. (It says "multimeter" but only AC and DC volts were ever installed in the unit. It's labels suggest it's custom so I'm just calling it a voltmeter. I think it was part of production testing.)

On power up it just displays random numbers except if I set it to "test" then it reads 10000 (probably what it is supposed to, the decimal changes with the range). If I short "hi" and "lo" it drops to zero then when the short is removed it slowly climbs back up from a few millivolts to several volts. If I connect a battery to the terminals the random numbers jump to a value very close to what I read on my portable dmm from the same battery meaning it is measuring. Upon removing the power source being measured the value stays there for a bit then starts climbing. The rate of climb gets really slow after a while so I don't know how far it will go. If I connect the portable dmm to the input of the 7000 they both read the same thing for a split second then both drop to zero with the same climbing numbers when disconnected. This proves that the voltage is present on the input (not background voltage/electric fields) but is easily discharges (touching both terminals also makes it drop).

When reading a voltage if I switch the range the decimal does not move correctly. 3.2v on 10v reads 3.2, on 100v it reads 32 and on 1000v it reads 320 (or sometimes just 32 for some reason).

The 12v rail measures only about 10v but there are 470ohm resistors across it so I don't know if that reading is normal. All other power buses are fine.

Any ideas what is going on? Schematics would also be helpful.
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Online PA0PBZ

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Re: Antique Fairchild 7000 Digital Voltmeter
« Reply #1 on: December 05, 2015, 08:14:45 am »
Well, you could always buy another one for $ 1000  >:D  http://www.ebay.com/itm/Fairchild-7000A-Digital-Multimeter-/151042103289

But honestly, it looks like a very high input impedance, what does it do with a 10M resistor on the input?
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Offline dom0

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Re: Antique Fairchild 7000 Digital Voltmeter
« Reply #2 on: December 05, 2015, 02:05:46 pm »
Yeah, this is a Hi-Z voltmeter. Totally fine, nothing wrong with it. The input capacitance simply integrates leakage currents.
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Offline Cubdriver

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Re: Antique Fairchild 7000 Digital Voltmeter
« Reply #3 on: December 05, 2015, 03:12:33 pm »
Well, you could always buy another one for $ 1000  >:D  http://www.ebay.com/itm/Fairchild-7000A-Digital-Multimeter-/151042103289

But honestly, it looks like a very high input impedance, what does it do with a 10M resistor on the input?

Wow!   :o  He wants a grand for something he (paraphrased) "doesn't have the proper equipment to test, but it looks like it's in good condition", with 2 pictures?  Good luck with that, dude.  Especially considering I see an apparently working one went for less than $225 in September.  Gotta love eBay!!

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Online Cyberdragon

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Re: Antique Fairchild 7000 Digital Voltmeter
« Reply #4 on: December 07, 2015, 05:03:58 am »
Yeah, this is a Hi-Z voltmeter. Totally fine, nothing wrong with it. The input capacitance simply integrates leakage currents.

So it's normal to read 5V or more when floating? How odd, most digital meters only read a few millivolts when floating.
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Offline Kleinstein

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Re: Antique Fairchild 7000 Digital Voltmeter
« Reply #5 on: December 07, 2015, 05:11:36 am »
Its normal for a DMM, even a high qualily one to hav a bias current form input circuit of something like 10-100 pA. How much voltage in open circuit is read open circuit, depends on the input impedance. I higher impedance like with high quality meter will result in higher vorltage. Just simple meters with a relatively low impedance 10 M input should not read more than a few mV open circuit. With 10s of GOhm input inpedance 1 V is OK. 
 

Online edpalmer42

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Re: Antique Fairchild 7000 Digital Voltmeter
« Reply #6 on: December 07, 2015, 06:31:31 am »
I don't remember which one, but in one of Dave's videos, he was looking at a DMM with Gohm input impedance.  It slowly counted up until it overflowed and he commented that that was okay  (even typical) for very high impedance meters.

Ed
 


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