AVO model 8

[sparkymike]

I need to know the value of the resistance marked in photo please.
It is a model 8 AVO Universal meter.
Mike.

[AndyGilham]

sparkymike,
I believe that winding is 4.5 ohms (Wound with 10 turns of 30 swg wire)
Andy

[The Philpott]

That's what my circuit diagram says. (I've seen couple of those 100mA DC windings that have been abused- they still work but have stunk out the interior!)

Dave

[sparkymike]

I now find that although the coil looks shot, it still has continuity !!
Around 5.5 ohms I am getting.
I had thought that this coil was the cause of a problem.
On AC, the 1000 volt section works fine, but lower voltages do not read.
Any clues here would be welcome.

[The Philpott]

Typically with an AVO of this generation almost all of the windings on the bottom slab are DC current shunts (The 10A one being the large cylindrical coil)

Is it a Mk1,II or III?

Dave

[The Philpott]

That suggests a leaf switching problem.. Try applying AC voltage to a lower AC range, then grabbing the 'AC & Ohms range' selector at 12-o-clock and 6-o-clock positions and rocking it.

Dave

[sparkymike]

Hi Dave,
it is a model 8 (1)
Should I be looking on the upper board.? I was having fun trying to equate the circuit drawing to the actual meter.!!
It would be quite an exercise to draw the route through each switch from input socket to meter on CAD. Wonder if anyone has done similar ?
Mike.

[The Philpott]

The shunts (whether windings or component resistors) are cumulative (arranged in series,) so the fact that the 1000vAC range works and the lower ones do not, indicates to me that the shunts (which are indeed on the upper board) are most likely ok.

The leaf switches at the top of the 'AC & ohms' range selector are certainly the first thing i would look at. (Not particularly easily accessible, it has to be said, but possible to tweak unless completely worn out) They might just have a light film of black silver sulphide on them..

Dave

[sparkymike]

Thanks Dave, will check leaf switches later. At least you can see them !!
I cleaned the contacts on the overload switch and found that the movable contacts were easily removed, but I made the mistake of removing one of the screws on the fixed contacts, to make cleaning easier and then took half an hour or more replacing the nut !! I had thought that the screws were tapped into the Bakelite. Not so.!! I then cleaned them in situ..The nuts sit in a nut shaped hole in the support. In future I would probably hold the nuts in place with a dab of grease.
The needle floats quite a bit when case is moved.ie loss of damping. Does this indicate a reduction of magnetism in movement.?
Mike.

[The Philpott]

Hmm. Hard to say without seeing it. WIth selectors at 'DC' and '50uA', it is possible to short the plus and minus terminals and completely damp the mvmnt for transit- doing this could give an indication whether anything is awry.

Failing this i am not too far away (although busy this coming w/end) so we could always have a gander at it at some point. I have a worn out Model 8 without it's case which is useful to refer to sometimes.

Dave

[sparkymike]

Hi Dave,
that is handy tip to know for transportation purposes. I don't remember ever seeing that in the handbooks. I will try that and see what occurs.

Mike.

[sparkymike]

Going back to the AC voltage selection (or lack of) ,When selecting 1000v,the left rear leaf switch viewing when looking from the meter bottom, engages and stays engages through the whole of the AC volt range. As I see it, there must be one component (resistor or coil) that stops the remainder of the chain working. Is that right or am I missing something here ?
Mike.

[sparkymike]

Panic and scratching of head now over. In desperation, I gave the leaf switches a liberal dose of contact cleaner in open and closed mode and now working on AC volt ranges.
Now on to the ohms range......
Mike.

[The Philpott]

Easy with the contact cleaner so you don't get it in other moving parts!
The cardboard tube inside loo roll is abrasive enough (and the right thickness) for cutting strips off and cleaning those contacts. I use it soaked in contact cleaner (dry then wet then dry)
Best of luck with the ohms ranges, most problems are solvable, often down to elderly potentiometers making zeroing difficult. A low mileage meter will be less problematic in this respect.

Dave

[sparkymike]

The red plastic knurled cover on the input terminal was missing so for a temporary fix, I have a 8 Mk.5 that gave up the ghost years ago and found that the covers are still same size, except there is a hole in the top for the later probes. I then wondered if the complete later terminals would fit on earlier models ? I have always been a bit wary re. the probe spade terminals which so easily can be touched with enlivening results !!
At one stage, checking out the various resistors, I found that there are some that are impossible to get at. How on earth you change them without a major strip-down I don't know.
Mike.

[The Philpott]

There are certainly some accessibility issues, but having said that the resistors and windings were pretty good by the time of the Model 8,(Mk.I,II,III) and are unlikely to need attention unless they have been overcooked by misuse.

Although there are often some 'inexpensive' looking 5% and 10% tolerance resistors to be seen, they are not representative of the overall accuracy as they are used as trimmers to bring the main shunts up to their proper resistance values. (eg a wirewound resistor of 3meg might be trimmed up using a 15k 5% tol. resistor- and the effect of that 5% is low enough to be disregarded.)

One of the 'wow' factors about the Avo 8 and 9 is the ability to measure AC up to quite high frequencies with good accuracy.

Dave

[sparkymike]

Hi Dave,
one of the things about Avo's, was that they could pick up spikes that a digital meter would miss. At work they did not possess an Avo and I took mine in to help out and it showed the problem. I believe the fault was on a generator. (Several years ago now.Too many years !!)
Mike.

[sparkymike]

Going back to the meter above, on checking the resistance range, the needle can be zeroed fine but when touching the probes, the needle tries to go right off the scale. It only does this on one resistance range. The others are not working.
Dirty leaf switch contacts again on resistance range I wonder ?
Mike.

[sparkymike]

Spent a bit more time getting a bit more sense out of the meter.
I followed your advise Dave and cleaned up the contacts as you suggested and now have two of the ranges working. One of the leaf switches was hardly making contact with the blade, so I had to put a slight set in the blade with long nosed pliers. The needle in all ranges was sticking slightly in mid range and I have cured that by removing the mirror glass first and then bending the dial plate very slightly and now all is good. I don't know what they made that dial from, but it is tough metal !!. One range to sort now, the X -100. When probes are shorted needle goes fsd. Not sure what is going on there yet.
I also have to make a top cover out of 16 gauge alloy and print out the label for battery connection.I have a decent one to scan and copy.
Mike.

[The Philpott]

I too had a needle that was hitting the scale, on an Avo 7. I managed to bend the scaleplate back slightly, and yes, it did put up a fight!

Do you mean the needle goes as far as it can on ohms x 100 scale, and cannot be reduced to 'zero ohms'?

D