Avometer Meter Movement Repair

The AVO Mk 2 meter movement has a sensitivity of 37.5 µA, and any damage to the jewels, pivots, or hairspring can cause a reduction of sensitivity.

An overloaded meter movement often displays telltale signs of damage. Usually if you move the meter by approximately 45° in any plane and see the needle swinging more than 1% of full-scale deflection, then the meter may require balancing. In my meter, the needle was stuck and not moving in the slightest. A closer inspection revealed cobwebs in the movement, and therefore I used an air spray to gently blow them away.

An inspection of the hairsprings revealed that the return spring had detached from one end. After carefully reattaching the hairspring, with the use of a stereomicroscope, the needle was able to return to home position.

Another fault was that the needle was stopping mid way, which was due to metal particles in the magnetic channel where the voice coil moves. After removing these particles, I noticed that the needle was slowing down and then speeding up again. This indicated that the pivots might be sticking; therefore, I decided to soak the whole movement in a cleaning solution. After drying out, the needle was returning swiftly and unhindered.

Passing 37.5 µA through the voice coil caused a full-scale deflection, and the needle swiftly and elegantly returned to home position.

As you can see, there is a cobweb inside the voice coil channel, and plenty of dust particles.

Sometimes the hairspring can break away from its binding post and if you have the necessary tools, you can carefully solder the spring back to the post. This type of damage usually indicates heavy overload.

Even though the meter movement is robust, it is an extremely delicate piece of electromechanical engineering, and moderate overloads can cause it to go out of balance. Having bought a couple of second-hand meters on eBay, I encountered various faults associated with heavy overloads. Extremely heavy overloads can result in spring coils warped to the extent that the meter never reaches zero position again, no matter how much calibrating you perform. Large forces can also damage the fine pivots causing the voice coil to touch the magnets. If you are unlucky, the meter may have burnt phosphor bronze hairsprings, and sometimes a partially burnt voice coil works, but no longer exhibits the correct resistance or the sensitivity. Sometimes balancing the needle by adding more counterweight may help however, you are very much limited when extensive repairs are required. Usually the AVO Company provided replacement meter movements; however, today you may have to look for second-hand meters for the parts.

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