Odd problem, Low K Ohm Resistance Scales Out Of Tolerance.


I have an old Data Precision (USA) 1350 "3 1/2" Digit LED type Multimeter f rom the 70's era. It works great, except after performing a calibration th e low K ohm scales are displaying readings are generally out by more then 5 percent.

So, the first step of calibration is adjusting the time base or oscillator to 40 KHZ( +/- 200Hz) for the AD7107. The next step is the Gain calibration by inputing a DC reference voltage of 1.900 volts, and adjusting a pot to this voltage on the display. The last step is balance, where a lead wire is tied back from the red banana jack to pin 36 (Hi Ref) AD7107, and is adjus ted to 50 millivolts (+/- 50mv)

The multimeter readings are very good, except for the Low resistance scales .

Does anyone have any ideas on why?

Examples: 475 ohm 1% resistor it reads 430 ohms. 2K Ohm 1% resistor, it displays 1861 ohms

Thanks, Neil

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Corrossion at a terminal or joint somewhere? tried different probes? tried twizzling stuff while reading a resistor ohmage under test?

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Used it to measure voltage and partly fried some of the low-value resisters, seems most likely.

Reply to
Clifford Heath

Three possibilities: (1) weak battery - on low resistance, it might take extra current to develop a measurable voltage across the terminals, and battery voltage droops enough to decalibrate (2) series resistance in selector switch contacts (assuming there's a low-ohms switch contact that selects a resistor to ratio against, and the switch resistance is not insignificant) (3) recheck the 'balance' calibration, maladjusted offset to an op amp would make low-volts inputs measure incorrectly, and that step probably involved twisting a knob; sometimes they spring back after an adjustment.

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Neil, Do you have a scanned PDF of the manual for the 1350 DMM? If you do, could you post it somewhere on the net? I recommend the sharing service at

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You don't need an account, no signup, no credit card, etc. Totally free. Just upload the file there and let us know that it's there.

A friend has one of those meters and needs the manual for it.

To your problem, what kind of voltage reference did you use to calibrate the

1350? If it wasn't accurate, then that means that the basic DC voltage function of your 1350 is calibrated incorrectly, and all other functions and ranges will read incorrectly. All functions and ranges are dependent on the basic DC voltage calibration in the DMM being accurate.

Dave M

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Dave M

On one my meters, an odd problem which I can't remember details did arise. It was just reading wrong, otherwise it was fine. I actually opened it up, expecting something wrong with the range/function switch, but nothing. I have no idea how I got that far, or why I backed up and changed the battery, but it was a weak battery. That had never happened with a DMM with me before or after. Maybe I'm just lucky, I forget to turn off the meter, so the battery dies completely, rather than landing in some spot where it functions but not well.

No, maybe that wasn't the battery. It was the fuse. It was the original, but for some reason had gone up in value, but still a finite value. So it didn't make a noticeable difference with volts (I know I wasn't measuring anything for preciseness at the time), but did affect the ohms range at lower values, since it was something like 1K or thereabouts.

That was the odd thing, I hadn't expected the fuse to introduce a level of resistance.


Reply to
Michael Black


** Early DMMS likely use a different method of measuring resistors than more modern types.

DMMs made since the late 70s use a ratio method, where a fixed voltage is applied to the unknown resistor and reference resistor connected in series and the value displayed relates to the ratio of the voltages across each.

Readings are as accurate as the reference resistor - normally 1% or 0.1% metal film types are used.

Older instruments might feed a fixed current like 1mA, 100uA, 10uA etc into the unknown part and read the resulting voltage. Reading are then only as accurate as the internal current source, which requires occasional recalibration.

... Phil

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Phil Allison

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