Hello,

I have an old Data Precision (USA) 1350 "3 1/2" Digit LED type Multimeter from the 70's era. It works great, except after performing a calibration the 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 adjusted 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

On 15/10/2015 10:34, wrote:
Hello,

I have an old Data Precision (USA) 1350 "3 1/2" Digit LED type Multimeter from the 70's era. It works great, except after performing a calibration the 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 adjusted 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


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

On 15/10/15 20:34, wrote:
Hello,

I have an old Data Precision (USA) 1350 "3 1/2" Digit LED type Multimeter from the 70's era. It works great, except after performing a calibration the 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 adjusted 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

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

On Thursday, October 15, 2015 at 2:34:51 AM UTC-7, wrote:

I have an old Data Precision (USA) 1350 "3 1/2" Digit LED type Multimeter...
after performing a calibration the 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 bas
The next step is the Gain calibration by inputing a DC reference voltage...
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 adjusted to 50 millivolts (+/- 50mv)

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

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.

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
http://www.c-c-i.com/exchange/. 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


wrote:
Hello,

I have an old Data Precision (USA) 1350 "3 1/2" Digit LED type
Multimeter from the 70's era. It works great, except after
performing a calibration the 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 adjusted 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

On Thu, 15 Oct 2015, whit3rd wrote:

On Thursday, October 15, 2015 at 2:34:51 AM UTC-7, wrote:

I have an old Data Precision (USA) 1350 "3 1/2" Digit LED type Multimeter...
after performing a calibration the 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 bas
The next step is the Gain calibration by inputing a DC reference voltage...
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 adjusted to 50 millivolts (+/- 50mv)

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

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)

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.

Michael

(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.

neilw...


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


** 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