Antique Fairchild 7000 Digital Voltmeter - Page 1
Forum section: electronics / repair

author: Cyberdragon
author_rank: Super Contributor
timestamp: December 04, 2015, 09:00:32 pm
content: 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.

author: PA0PBZ
author_rank: Super Contributor
timestamp: December 04, 2015, 09:14:45 pm
content: Well, you could always buy another one for $ 1000http://www.ebay.com/itm/Fairchild-7000A-Digital-Multimeter-/151042103289But honestly, it looks like a very high input impedance, what does it do with a 10M resistor on the input?

author: dom0
author_rank: Super Contributor
timestamp: December 05, 2015, 03:05:46 am
content: Yeah, this is a Hi-Z voltmeter. Totally fine, nothing wrong with it. The input capacitance simply integrates leakage currents.

author: Cubdriver
author_rank: Unknown
timestamp: December 05, 2015, 04:12:33 am
content: Quote from: PA0PBZ on December 04, 2015, 09:14:45 pmWow!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!!-Pat

author: Cyberdragon
author_rank: Super Contributor
timestamp: December 06, 2015, 06:03:58 pm
content: Quote from: dom0 on December 05, 2015, 03:05:46 amSo it's normal to read 5V or more when floating? How odd, most digital meters only read a few millivolts when floating.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 06, 2015, 06:11:36 pm
content: 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.

author: edpalmer42
author_rank: Super Contributor
timestamp: December 06, 2015, 07:31:31 pm
content: 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

author: Cyberdragon
author_rank: Super Contributor
timestamp: March 12, 2020, 02:44:21 am
content: Old faithful bit the dust today.On VDC with no connection the reading either floats wildly (normal) or fades away from the test value (decaying from 1) but does not respond to any voltage input on either input terminal. (Bad switch contact?)However, ACV reads correctly but the decimal point is wrong, the reading being exactly 10% even in auto.Voltage values are same as before exept the 30V being a bit high at 32V. All board contacts cleaned with no change. Caps on the power supply board are starting to show signs of physical decay but they haven't blown their guts yet (I doubt a recap would actually help though).Voltage at test point FG (reference?) reads what the tubes display in test mode and when switching out of test mode (when it the input is not floating, again, regardless of any voltage on the terminals).My guess is either bad switch or something wrong with the attenuator board. Does the AC voltage reading go through the DC attenuator for any reason? That would explain the low AC readings but if not then it's possibly seperate faults.

author: klimm
author_rank: Contributor
timestamp: May 12, 2020, 04:39:03 pm
content: I've got one of these today. An 7000A multimeter . With the input shorted on DC mV function /100mV range is counting endlessly and will not sense if I break the short.On AC Voltages, input shorted, won't count, stays on 0000Converter is double slope as I read in a booklet I could find.I wonder if someone has a schematic/manual of this Fairchild 7000A 'cause I badly need it