TMK Model 700B Multimeter Calibration - UK Vintage Radio Repair and Restoration Discussion Forum

adamant · 22nd Jan 2018, 6:58 pm

I recently acquired a TMK Model 700B analogue multimeter in good condition, first impressions are it seems like a decent meter. Resistance readings are spot on, however for AC and DC readings it is reading way off, especially on the AC. The readings are all consistent, but I cleaned the switches anyway. I have inspected the inside but can't see any obvious defects. I was wondering if anyone has the user manual, a schematic, or the calibration procedure for this meter, (there are 2 adjustable pots inside) since mine came with no paperwork. There are some copies of the model 700 schematic and user manual floating around, but I couldn't find anything for the 700B version.

Any help would be greatly appreciated!

Adam

orbanp1 · 25th Jan 2018, 5:03 pm

Hi Adam,

RM has a page on the TMK-700 with the schematics that you can download:
https://www.radiomuseum.org/r/tmk_an...meter_700.html

It is probably the same as the B version, the B version with some improvements.
The 700 has a DC sensitivity of 20kOhm/V DC, and 4kOhm/V AC.
If your instrument is marked the same then the meter movement has the same sensitivity (50uA), the resistors would have the same values as in the schematics there.
If your meter has a different sensitivity (it is usually marked at the bottom of the scale, or at the back of the meter) then the resistor values would be different, but the circuit probably still the same.

The schematics shows a pot marked "AC CAL", probably that is where you would calibrate the AC scale.
There are two other pots in the schematics, but they do not say what are they for (probably for Ohm zero, and Ohm infinity).

There is usually no user calibration in the DC mode for analogue multimeters, the manufacturers calibrate the meter movement for nominal current and resistance, and the dropping and shunt resistors in the DC mode are sized to that sensitivity and should give accurate readings without any additional user calibration.
You can verify the accuracy of the meter movement in the 25mV 50uA setting, if I see that correctly from the grainy schematics.

I would suggest you follow the schematics in the various modes, and verify the functions of the other pots as well.

Good luck, Peter

karesz* · 29th Jan 2018, 11:47 am

Hi All,
I see only difference by TMK700B on "AC 60.000Ohm/V" instead 20KOhm/V as TMK700..
rgds, Karl

22nd Jan 2018, 6:58 pm	#1
adamant Retired Dormant Member Join Date: Jun 2013 Location: Northamptonshire, UK. Posts: 3	TMK Model 700B Multimeter Calibration I recently acquired a TMK Model 700B analogue multimeter in good condition, first impressions are it seems like a decent meter. Resistance readings are spot on, however for AC and DC readings it is reading way off, especially on the AC. The readings are all consistent, but I cleaned the switches anyway. I have inspected the inside but can't see any obvious defects. I was wondering if anyone has the user manual, a schematic, or the calibration procedure for this meter, (there are 2 adjustable pots inside) since mine came with no paperwork. There are some copies of the model 700 schematic and user manual floating around, but I couldn't find anything for the 700B version. Any help would be greatly appreciated! Adam

25th Jan 2018, 5:03 pm	#2
orbanp1 Heptode Join Date: Nov 2009 Location: London, Ontario, Canada Posts: 675	Re: TMK Model 700B Multimeter Calibration Hi Adam, RM has a page on the TMK-700 with the schematics that you can download: https://www.radiomuseum.org/r/tmk_an...meter_700.html It is probably the same as the B version, the B version with some improvements. The 700 has a DC sensitivity of 20kOhm/V DC, and 4kOhm/V AC. If your instrument is marked the same then the meter movement has the same sensitivity (50uA), the resistors would have the same values as in the schematics there. If your meter has a different sensitivity (it is usually marked at the bottom of the scale, or at the back of the meter) then the resistor values would be different, but the circuit probably still the same. The schematics shows a pot marked "AC CAL", probably that is where you would calibrate the AC scale. There are two other pots in the schematics, but they do not say what are they for (probably for Ohm zero, and Ohm infinity). There is usually no user calibration in the DC mode for analogue multimeters, the manufacturers calibrate the meter movement for nominal current and resistance, and the dropping and shunt resistors in the DC mode are sized to that sensitivity and should give accurate readings without any additional user calibration. You can verify the accuracy of the meter movement in the 25mV 50uA setting, if I see that correctly from the grainy schematics. I would suggest you follow the schematics in the various modes, and verify the functions of the other pots as well. Good luck, Peter

29th Jan 2018, 11:47 am	#3
karesz* Heptode Join Date: Jun 2010 Location: Zurich, Switzerland Posts: 538	Re: TMK Model 700B Multimeter Calibration Hi All, I see only difference by TMK700B on "AC 60.000Ohm/V" instead 20KOhm/V as TMK700.. rgds, Karl Attached Thumbnails