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How to repair burned out resistors in a AVO (Ampère/Volt/Ohms) meter when you don't know their value



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When a precision resistor in an analog (or whatever) voltmeter burns out, its value is often no longer visible due to scorch and burn. How to repair that?

This demo gives a solution, repair of a burnt out (value no longer visible) resistor in the Ohms measurement scale of the analog or even digital meter, as long as everything is not made "tiny".

So what to do when you don’t know the replacement value? In many cases, nowadays (2023), these meters come without a diagram/schematic.

The video shows how to find out that unknown resistor value, thus repairing the meter. Thus: don’t throw your AVO meter away when one or more resistor(s) are burnt out due to a sloppy measurement setup or whatever reason. Repair is better for the environment and in this case often very easy.

Some considerations:

1) When you do this repair (in the demo: in the Ohms range) always connect the probes first, then align the Ohms measurement to “zero” with the Ohms adjustment knob (potentiometer) on the scale, after that try to find out that unknown resistor with a known-value resistor at the probes.

2) For alignment, if possible, use a metal film test resistor with that known value. When not available: a carbon film resistor with a tolerance of 5 %. Measure that test resistor first with a digital Ohms meter to know its value and to know how precise the alignment must/can be.

3) Every analog meter has its “natural” tolerance. Could be 3 % or even 5 %. Perhaps even higher… Take this in account. In many cases that tolerance is enough for measurements of voltages and resistances and currents for hobby use.

And, when you want to get precise: a good quality digital or analog AVO meter can measure extremely precise when you know what you are doing in electronics, even with respect to its high (3%-5%) tolerance. Especially analog meters can give here more precise measurements in the Micro Ampère range. Say between 1 uA and 50 uA. The pointer shows exactly even the tiniest currents. Only very expensive digital meters also can do that, but effects of changed currents in the Micro Ampere range are not very well visible with a DVM (in general).

4) You can use this setup also when resistors are burned out in the Voltage or Ampère range of your AVO meter (AVO=Ampère/Volt/Ohms). In that case you need a precise test Voltage on the probes of the meter (that needs repair) or a precise test current.

Compare Volt and Ampère values in these cases to a measurement with a DVM (=digital voltmeter): for voltage the measurement has to be parallel to the voltage source, for current the measurement has to be in-between the lead where the current flows. So in that way you can also find out and replace resistors in an AVO meter, burnt out due to a too high voltage/current or a combination of both.

5) Ohms law is valid here: U=IxR, thus Voltage = Current x Resistance.

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