repairing analog multimeter

[mdanh2002]

I have an old analog multimeter (Chinese brand) that is capable of measuring current, voltage and resistance. I have not used it for a long time and only tried it recently. Measuring current and voltage still works fine, however measuring resistance only works if the scale is set to x1, x10 and x1k. If the scale is set to x10k (the highest resistance scale on this meter capable of measuring up to 2Mohm resistors), all resistors read as open circuit. Other resistance/voltage/current scales seem to work fine and the measured value is correct.

Any ideas what the problem is? I open it up, the fuse (which should not cause the problem) is still fine. The PCB is using five-band resistors with strange color code, some of them have faded with time. All connections seem secured and all resistors value seem reasonable.

 

[ericgibbs]

hi,
What is the condition of the internal battery.??

 

[mdanh2002]

It has 2 battery slots, one for 9V battery and the other for 2xAA 1.5V batteries. Either slot can be used and I am using the 2xAA slot. Batteries are not required when measuring voltage/current. I already tried to replace the battery but it did not help.

 

[Nigel Goodwin]

It has 2 battery slots, one for 9V battery and the other for 2xAA 1.5V batteries. Either slot can be used and I am using the 2xAA slot. Batteries are not required when measuring voltage/current. I already tried to replace the battery but it did not help.

You can't use 'either' you have to use BOTH - the 9V battery will be used for the highest ohms range, which is probably why it doesn't work.

 

[mdanh2002]

You can't use 'either' you have to use BOTH - the 9V battery will be used for the highest ohms range, which is probably why it doesn't work.

Nigel, are you very sure that it needs 12V for the highest resistance scale (x10k) to work? I did remember that I tried all the resistance range when I first got it half a year ago and it seemed to work fine with only 2xAA (3V) battery. Anyway I will buy a fresh 9V battery and try again tomorrow.

 

[Nigel Goodwin]

Nigel, are you very sure that it needs 12V for the highest resistance scale (x10k) to work? I did remember that I tried all the resistance range when I first got it half a year ago and it seemed to work fine with only 2xAA (3V) battery. Anyway I will buy a fresh 9V battery and try again tomorrow.

It doesn't need 12V, it only needs 9V - the 9V battery is used only for the highest ohms range, which should work with just the 9V battery.

The old Avo Meters used a 15V battery for the same reason, along wth a single 1.5V for the lower ranges.

 

[Reloadron]

I believe you will find Nigel's advice to be right on target. With the exception of a few old VOMs I had that used a 22.5 volt battery all the later ones have used two batteries. Generally a 1.5 volt D cell and a 9 volt battery. When measuring high resistances the higher voltages are needed to get enough current through R unknown to drive the analog meter movement.

Ron

 

[mdanh2002]

Just an update. I tried with a 9V battery today and it did not work. In fact with a 9V battery alone, all resistance scales do not work - it only measures resistance and voltage, as good as when no battery is present. Was about to conclude that the 9V battery slot is somehow faulty when i decided to try both battery slots, e.g. 12V. And yes, it works well with 12V. All resistance ranges, including the x10k range capable of measuring up to 20MOhm resistors), work well.

So my meter needs 12V for full functionality.

 

[KeepItSimpleStupid]

I have an Eico kit meter, but it hasn't been used in years. It used 3 9V batteries in parallel for lifetime and a "D" cell.