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Multimeter Problem.
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4098 Posts
Question

Please help. I have a second hand P.O. meter no 12 C/1. It is a moving coil meter. It reads o.k. on Volts but under reads on Ohms. The zeroing pot does not seem to work. A label inside states that it has thick film components. Why won't it read Ohms accurately? Can it be repaired.

Thanks,

Z.

12 Replies
AJJewsbury
3068 Posts

Can it be repaired.

Of course it can - anything can. Whether it's economic to do so however…

If volts part is OK, but the resistance range isn't, my guess would be something that's only in-circuit for the resistance mode must be the cause. I don't know the details of that particular meter but would guess at something like: a) the battery, b) dirty contacts on the selector switch, or c) one of the other components (e.g. resistor) used in the resistance circuit having gone out of spec.

   - Andy.

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4098 Posts

AJJewsbury: 
 

Can it be repaired.

Of course it can - anything can. Whether it's economic to do so however…

If volts part is OK, but the resistance range isn't, my guess would be something that's only in-circuit for the resistance mode must be the cause. I don't know the details of that particular meter but would guess at something like: a) the battery, b) dirty contacts on the selector switch, or c) one of the other components (e.g. resistor) used in the resistance circuit having gone out of spec.

   - Andy.

Perhaps somebody has used the meter set to Ohms on a live system and damaged a resistor. I can't think that resistors naturally degrade at 1.5 Volt use. I cleaned the selector switch and tried new 1.5 Volts cells but to no avail. The higher Ohms range uses a 15 Volt battery which I have not been able to source yet.

Z.

Chris Pearson
3195 Posts

Given that the zeroing pot does nothing, it seems that it is the problem.

mapj1
4321 Posts

Can you photo the inside - a bit of reverse engineering may be in order.

‘Thick film’ meant different things at different times - it may be that resistors are wire ended ceramic rods coated in carbon or nichrome and then dipped in paint, or there may be PCB chip mount resistors, or the real pain, there may be a PCB like structure with the resistors formed as tracks printed in a lossy material.

But seconded, most things can be repaired, the effort varies.

Mike

mapj1
4321 Posts

Generally PO meters were minor customization on the smaller AVO models of the day - 

is based on it one of these  ?

Nearly all had a minor variation on this circuit, with more or less ranges, perhaps additional options for audio level tests and some had extra sockets for a shunt for higher current readings.

Note that the dc input impedance is 20k ohms/ volt, so ~ 50uA for full scale. For AC volts sensitivity  is reduced, typically to half or 1mA full scale, reducing sensitivity to capacitive pick-up. If only modern meters did this.

Understanding the switch diagram requires you to realise that the rotor bridges the centre and outer contact in the selected positions, and there are two, one on each side of the board or concentric and single sided. So for example in the off position the meter movement is shorted and the ‘+’ terminal is open circuit. 

In some models one rotor (LH this diagram) is replaced by an ‘ohms or AC volts’ selector,  toggle switch as in the other states it does not do a fat lot…

Perhaps print out and then colour in the the link in the ohms position(s) and then re-draw the thing unwound with only the paths in circuit.

 

Ohms range faults  I'd be looking at battery condition, battery and switch contacts and then the adjuster pot.

 

 

d1948d524a51633a22b9a785b23ff271-origina

 

The ohms range part can be re-drawn as something like this

7c993095aa9582f68cafde5cb4b5dafe-origina

 

 

ebee
1380 Posts

Man and boy I`ve always loved those sort of meters. Ok they loaded the circuit a bit (no bad thing in some instances), easily damaged, parallax error to watch for .. And watching the needle waggle helped resolve what was going on in the input, such as mains ripple and cap charge/discharge .  But oh so beautiful . A “proper” meter   😀

 

About £200 at todays prices without the case

John Peckham
783 Posts

In my day the Meters Multirange 12A, 12B and 12C were made by Taylor not AVO. The 12A appeared in the background of Thunderbirds on a fancy control panel so know as a Thunderbirds meter. Nice brown leather case. 

Try a clean up of the battery contacts and the range switch contacts. I cant remember if there was one or 2 batteries? In any case change them for new.

When you were promoted from Technician to Technical Officer you could order up a meter multi-range  14C which was either a GEC instrument or an AVO 8 if you were lucky when you opened the box.

 I have a small collection of AVO 8s which I might be persuaded to dispose of for a reasonable sum if you want a proper analogue meter Zoom.

mapj1
4321 Posts

Well Taylor Electrical Instruments Ltd of Slough was taken over by AVO Ltd. in 1958 , long before thunderbirds were go,  so your post  office meter must go back  a bit - perhaps you are older than you look 🙄 if it predates  that merger.  more here 

Joking aside, I think for badge engineering purposes and to keep the high and lower cost laboratory and portable/toolbox ranges distinct, the two names were retained for quite a while afterwards.

In any case there are/were very few variations on the basic circuits used inside, though the exact ranges and so resistor values and switching arrangements vary a bit, and the mechanical form factor changed quite regularly.

Mike

 

PS basically the same circuit redrawn more obscurely from older manual

CCT.pdf

And the manual for the wrong model, but a nice explanation of the theory.

127A.pdf

 

Attached files
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4098 Posts

John Peckham: 
 

In my day the Meters Multirange 12A, 12B and 12C were made by Taylor not AVO. The 12A appeared in the background of Thunderbirds on a fancy control panel so know as a Thunderbirds meter. Nice brown leather case. 

Try a clean up of the battery contacts and the range switch contacts. I cant remember if there was one or 2 batteries? In any case change them for new.

When you were promoted from Technician to Technical Officer you could order up a meter multi-range  14C which was either a GEC instrument or an AVO 8 if you were lucky when you opened the box.

 I have a small collection of AVO 8s which I might be persuaded to dispose of for a reasonable sum if you want a proper analogue meter Zoom.

Hello John, I have an AVO 8, but it has been converted to a table lamp and is now a decorative feature. But I suppose that the lamp cold be useful if I need to use the meter in dark places.

I will further investigate the P.O. meter when I get time.

And yes, the P.O. meter does have a lovely leather case.

Z.

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4098 Posts

mapj1: 
 

Well Taylor Electrical Instruments Ltd of Slough was taken over by AVO Ltd. in 1958 , long before thunderbirds were go,  so your post  office meter must go back  a bit - perhaps you are older than you look 🙄 if it predates  that merger.  more here 

 

 

Nope, I look older than I am. I've ‘ad a ’ard life guv!

Gissus the price ovva cuppa tea Guv.

Z.

ebee
1380 Posts

Boys were boys and men were men and Dan Dare was in Eagle comic. Those were the days my froends

Chris Pearson
3195 Posts

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Hello John, I have an AVO 8, but it has been converted to a table lamp and is now a decorative feature.

Sacré bleu! 😯

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