AVO Meter

[Mark Atherton]

Recently picked up an AVO Model 8 MkIV at a car boot for £3 (!).I all seems OK except for the Ohms scale will not zero,shorting the leads gives a deflection to 5kOhm and it will adjust left a right a bit with the zeroing potentiometer. Ohms x 100 and Ohms/100 scales both zero and adjust OK.DC and AC voltage scales are OK.Put a new 1.5 volt battery in it, the 15 volt battery measures 15 volts but dont think it is used for the Ohms scale anyway.Have a diagram for a Model 8 mark II which I found on the net. Any advice gratefully received. Mark Atherton

[Dave Plowman]

The two batteries supply different ohms ranges. I'm not sure how they split, but the high voltage one supplies the high resistance range, and it's likely it's not in the first flush of youth. It's a sign of a dying battery when it won't zero.

These batteries are usually for sale in the test equipment section of eBay if you can't find one.

[BillR]

If I'd known you wanted one you could have had mine! Trouble is I threw it out 5 years ago in favour of nice Fluke DVM

[Ray Cutler]

Ahhhhhh yes, the ubiquitous dvm, complete with digital-auto-parallax error ;) If you tell students at our place that there's an x% linearity error they tell you to go away, saying 'if it says 19.998 then 19.998 it is' Ray.

[bweebar]

I have a Mk V. The x1 and x100 Ohms settings use the 1.5V battery and the x10k Ohms setting uses the 15V battery, there is no Ohms/100 scale on the Mk V.

You can get the battery for a small fortune from: (search for BLR121) and (stock no. 196-6715) I just got the one from RS which was cylindrical rather than cuboid, had to pack out the space with folded paper.

[Paul Hewish]

Mark

Chuck the batteries.

If on joining the leads together it's impossible to get a zero, or if the pointer doesn't remain still but falls steadily, the battery concerned should be replaced. If you can't get readings on the ohmn x 1 & ohmn x 100 ranges, the 1amp fuse needs checking. Old 15v batteries may indicate a potential of 15v but their internal resistance can increase causing a loss of accuracy in the ohmn x 10k range. You can check the battery's short circuit current on the 100 mA d.c. range & if it's below 15 mA discard - only shot circuit for less that 2 secs.

Paul

[Dave Plowman]

They make decent money on eBay - certainly enough to make it worthwhile selling them.

[BigWallop]

It didn't come with resistive leads did it ?

[Andy Wade]

All good advice. To add my two penn'orth:

On the lowest range (x1), assuming a good cell, contact resistance is the usual reason for failure to zero. There are several spring contacts in series in the circuit, all of which might benefit from cleaning:

- battery holder spring contacts (not usually a problem, IME)

- fuse holder to fuse body (squeeze the clips together sligthly before re-inserting fuse)

- internal contacts between battery/fuse compartment in the base, and the main gubbins of the meter (which hangs off the top plate); dismantle by removing all the screws around the top edge of the base and lift the meter out of the case -- 'flat spring and pin' contacts are now exposed for cleaning (admire the construction and take great care not to introduce any contamination while the case is apart)

- meter leads to terminal posts; the usual moulded type of AVO leads are notoriously poor; ensure that the pin contacts of the lead plugs are a tight fit in the female contacts of the terminals -- if necessary prise the slotted male pins apart slightly with judicious use of a screwdriver, etc.

HTH

[Dave Plowman]

My posh Fluke won't zero unless you use the supplied leads.

[Mark Atherton]

[Chris Harris]

Really? The poshest one that I can lay my hands on is the Fluke 185 true RMS and on that one you can zero any leads, provided that their total resistance is less than 5 Ohms.

Chris

[BigWallop]

I mean do they have a resistor built in to the lead to extend the normal range of the meter. I have a seen sets of leads with a 10K 2W Resistor built in to the red lead and just wondered if the leads that came with the AVO in question had a 5K resistor built in. The original post says that the meter doesn't drop below the 5K mark, so it may point to some thing like this.

[Dave Plowman]

Mine is a 179, and it zeros ok on its own leads which are horrible stiff things, but not on the set from my rather old Maplin gold which uses silicone ones - it reads 0.1 ohm. Either set zeros on the Maplin meter, but then I repaired and re-calibrated that myself after a slight accident with mains. ;-)

[CJ]

I've got one of those maplin gold jobs. Analogue scale on electronic internals. Great meter - had it for...errrr.. about 15 years I guess. I'll have to try the leads from it on a modern Fluke and see what happens.