Metra Blansko Multimeters

Metra Blansko has been manufacturing electrical measuring equipment since 1911, in a town called... Blansko. There's even a link between them and Karel Čapek, the inventor of "Robot".

I bought two Metra Blansko multimeters at a flea market in Prague back in 2019.

PU140

As can be seen from the manual the PU140 was intended for automotive use.

The date on the back of the meter is 25 January 1974.

Nice Tesla resistors.

The switch has two inner and two outer contacts (here at roughly 1, 2, 5 and 7 o'clock).

Looking at things closely shows that the schematic is not as clear as maybe it should be.

The top switch row is formed by the two outer contacts and the bottom switch row is formed by the two inner contacts. Both are circular, the switch banks wrap left to right. In the schematic we're at the third switch position, "6000 r/min". Two to the left is the "6A" position, the top leftswitch would be two to the left which wraps to the contact at the far right, the top right switch connects the "A" socket to R13 and from there the meter, the bottom left contact is on the third position from the left (marked "6A" and like the bottom right contact not connecting anything. Clear? Vynikající!

Calibration

OK, first some reverse engineering.

• On the 6A setting, current flows through shunt R2 developing a voltage. 6A and 0.0166R is 0.1V. So the meter assembly (R13, R12, P3, R14 and the movement) is 0.1V FSD.
• On the 15V setting, current flows through R5, R6A, R6B, R7 and the meter assembly. R5+R6+R7=298K and drops 14.9V (because the meter needs 0.1V for FSD). Current is 50uA (so the meter assembly is 50uA FSD, and the resistance is 2K).
• On the 20V expanded scale setting the measured voltage (20V - the zener voltage which is probably about 9V) is generated over R3 and measured by the combination of the meter assembly, R4 and P1.
• On the dwell setting, there are a large number of components (resistors, an inductor, a couple of diodes, a capacitor or two...) in the signal path, one of these components is P2

So how I would calibrate this thing is:

• Set the meter mechanical zero
• Apply 15.0V on the 15V setting and set P3 for FSD
• Apply 20.0V on the 20V setting and set P1 for FSD
• I'm not going to bother calibrating dwell but you do that with P2 :-)

PU120

The PU120 is a later model than the PU140, it is also much more complex, with AC and DC measurement and the ability to test transistors.

PU120 User Manual (what do you mean you don't understand Slavic?)

8 hits since 2021-05-20.

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(This page last modified 2025-03-30)