AVO 7 Help/Information Please.

[ThePillenwerfer]

I have acquired a 1944 AVO 7 in very good cosmetic condition and complete with original case, leads and prods. However...

The DC voltage ranges are dead but I can see a loose wire inside so that will hopefully be a simple repair.

More of a problem is that the meter movement is very sticky. It looks as though this may have been got at as half of the head of the bottom adjusting screw is been broken off.

I would be grateful for any information on how to sort this out. I can't find a circuit diagram for this mark (there's allegedly one on RMOrg but it's actually the one out of the Mark II manual) so don't know where the loose wire goes or how to safely disassemble the movement.

Thanks

Joe

[John_BS]

Does this help?

John

http://photos.offog.org/avometer-7/p...4-IMG_0051.JPG

[ThePillenwerfer]

Thanks, John but I think that's a Mk II as some of the resistance wire is wound on bobbins and the meter magnet is in three parts rather than a single horse-shoe.

Cheers

Joe

Additional: I've looked at the rest of those photos and it's definitely a II.

[Freya]

Cant help with the loose wire but the movement is the same as my avo two panel valve tester.
It had small flakes of magnetic material that obstructed the coil as it swung over its travel, using a cocktail stick with some blu tac moulded on its end it was possible to remove them. The magnet can be removed by taking the side screws out, make sure all connections are removed prior to doing this.
The brass screw is the endfloat adjustment, either this end or the other has a spring loaded bearing. If it has been altered the hairspring will not lay flat.

Hope this may be helpful

Stephen

[ThePillenwerfer]

Thanks, I'll have a look tomorrow, if I'm feeling brave.

- Joe

[hilitevr]

Definately do not remove the magnet, it will lose some of its magnetism and will require re-magnetizing.

[John_BS]

I think I've found the Mk1 schematic? Looks "old" in style, and no PF terminals?

John

[Phil G4SPZ]

There are in fact very few differences between the circuit diagrams of the later Mk 1 and Mk II Avo 7s, although the internal layout varies markedly, but there is an earlier circuit that includes the bimetallic temperature compensator which I attach and this may help you. The best way to identify the various bobbins, wound multipliers and shunts is to trace the wiring from the range switches.

By all means disconnect the movement and remove it from the faceplate, as this allows better access to the coil, but I would also urge caution in dismantling an Avometer movement if you have not done it before. In nine cases out of ten, sticky movements can be freed without the need for dismantling, by following the advice already given by other members. If someone has got at it before you, this warning is doubly important, as it might never work properly again. I have dismantled and repaired several Avo movements but they are never predictable. The adjustment of the bearing screws is particularly critical, calling for a tiny amount of end-float but also the correct positioning of the coil fore-and-aft in the air gap. Too far back and the acceleration cut-out will bind on the front of the concentrator, or the pointer will catch on the scale plate; too far forwards and the moving coil will stick against the back of the concentrator. Only adjust these screws by a fraction of a turn at a time. A significant amount of trial and error is required.

Then there is the fiendish acceleration cut-out mechanism, which drives the pointer itself, and this whole mechanism is very delicate indeed. On these early models it is an integral part of the front pivot assembly, which is fixed to the moving coil with a dab of shellac and can easily be dislodged - if this happens, prepare for a long and painstaking repair and rebalancing process. You will also need excellent light, a steady hand and the strongest magnifying lens you can find! Good luck,

Phil

[ThePillenwerfer]

Thanks for that, Phil. It's solved the mystery of what the strange-looking coil next to the movement is; it's the temperature compensator.

It sounds as though the movement itself is beyond my skills but I'll have a look at least when I've got a clear bench.

Cheers

Joe

[ThePillenwerfer]

Slight success.

I've repaired the resistance which has got the voltage ranges working and cleaning the movement, an no adjustments made besides, has improved it but it is still a long way from right.

Before it was generally sticky but now only sticks at the high end of the scale. Besides that it won't zero.

There is a peg which the needle wraps around and this feels loose.

Rather than risk making things worse I'm inclined to leave well alone pending access to better equipment and more practise on less delicate things.

Thanks again for the information and interest.

- Joe

[Phil G4SPZ]

The pointer is designed to rotate freely on the peg, but the peg is supposed to be rigidly attached to the upper (front) pivot plate, and this in turn is supposed to be firmly fixed to the top (front) of the moving coil.

The pointer may simply have been moved to the right-hand side of the driving lever. It should be to its left, held gently against it by the shaped spring wire. This is the heart of the acceleration cut-out fitted to Model 7 and Model 40 Avometers.

[Leon Crampin]

A movement which cannot be set to zero can generally be reset by moving the fixed end of the other hairspring - which is on a pivoted mount. Set the "user" zero to its centre, and then move the other hairspring adjuster to achieve a true zero on the scale in the operating position.

Leon.