AVO 8 Mk V Movement Problems

[rjmusto]

Hi Guys,

I've been given an AVO 8 MkV - yippee. But it does have a problem with the meter movement. It sticks. I have found that it swings freely if I hold the instrument upside down, but laying the right way up the needle tends to stick.

Looking at the documentation, there is quite a bit to dismantle to get at the movement and, I'm guessing there may be little I can actually do with it.

What do you think - is it likely I can fix it?

Thanks,
Ralph

[The Philpott]

This is the sort of fault where i would experiment tightening up the movement jewels by 1/8th or 1/4 turn to see if slack is causing the pivots to fail to locate in the jewels properly.
There's a locknut to loosen to enable the screw(s) to turn. I should imagine the rear screw is accessible without removing the movement from the meter, so just try the rear one initially. If it doesn't work (or the pointer sticks even more than before) return the screw to it's initial setting and nip up the locknut slightly.

Does the pointer always stick at the same point? If so there could also be ferrous flakes on the inside of the magnet concentrator, thus blocking the free movement of the pointer. These flakes can be very hard to see from some angles as they tend to be knife edged, thus invisible unless you look from as many directions as possible. I take it you have jeweller's loupes, obviously without them you won't see anything at all! Non ferrous tools must be used for this work since magnetism can be lost otherwise.

I am not particularly familiar with the Mk V, i stopped at the MK III. I believe that the above should apply equally to the Mk V however..

Dave

[pmmunro]

Ralph,

It is quite possible that you may be able to fix the sticking problem, depending on what is causing it.

Your best chance is, as you suggest, is to remove the movement from the front panel. This is failrly easy to do, but great care must be taken with the flexible wiring whose tracks are easily broken. The flexible tracks can however be by-passed with conventional wiring, but it's better not to incur the need if possible. Some desoldering is involved, and this too needs care as the printed wiring substarte will not tolerate too much heat. A cooler soldering iron bit than normal is advisable.

The correct proceedure is detailed in the service manual which is readily available.

If you can't find the manual,lets us know.

I wouldn't advise tryimg to dismantle the movement unles you are confident and experienced, but it is quite possible that there is some debris between the moving coil and the magnet which can be removed. There may be some slight movement axially which is bringniing the moving coil into contact with the foreign object. A piece of Blue Tak on a cocktail stick, or fine non-magnetic tweezers might be useful.

PMM

[rjmusto]

Thanks for the replies fellas.

I have a copy of the manual, so will take some time and dismantle the thing carefully. The mirror needs a clean anyway, so hopefully I'll get a good result all round when I'm finished.

Ralph

[rjmusto]

A quick follow-up.....

I dismantled far enough to get the plastic housing off the back of the meter movement. And gradually tightened up the bottom spindle screw, testing as I went.

And the movement now moves freely.

Ralph