Metronome Repair.

Paquet Metronome

I was asked to have a go at fixing an old Paquet metronome.

I am loath to hack into something that may be an antique but this device had obviously been dropped as the cover had a glued split and the lower front panel had also been re glued.

The complaint was that the beat was uneven at low settings.

Testing using Audacity (see below) showed that set to 60 the rate was 60.72 but the ratio of the short beat to the next was 1 to 1.35.

After adjustment and the "corrections" shown below the result was much better. I timed 180 beat periods with a stop watch at 180.4 seconds. This compared favourably with a cheap AU$17 electronic unit that gave 178.61 seconds in a similar test.

The beat now seemed perfectly even to me. The average ratio of the shorter beat in a pair to the next from 4 random  measurements was 1 to 1.025 and the worst 1 to 1.043. By comparison the electronic unit when measured had perfectly even beats and also a superior ease of setting and range of rythms, pity its beat rate was not more accurate.

Paquet Metronome open front

As received, with the weight near the top the swing was obviously off to one side.

The spring was let to run down and I found that with the weight near the top, the pendulum came to rest about 15 degrees off centre.

This is a picture of  the metronome in an unwound condition after I worked on it.

Note that the pendulum is unlatched with the adjustment weight at the top and it is now perfectly vertical. 

Any unbalance will be exaggerated the further up the rod the weight is.

As the unit had evidence that it had been dropped I supposed that the lower weight had somehow shifted.


Metronome mechanism

To balance the pendulum some solder was added to one side of the weight.

Increasing the weight increases the frequency so an amount of the weight was rasped off the other side.

The aim was to get a balanced pendulum of the correct weight for the rhythm to match the printed scale.

A level platform was constructed and a microphone attached to my computer.

Note any tilt will effect the evenness of the beats. It is possible to "adjust" a slightly uneven metronome by packing one side up.

Audacity is a free programme that records sound, lets you view the wave form and make accurate measurements of time intervals.

Audacity being used to measure time interval between beats.

Metronome mechanism
The clack noise is produced by the escapement wheel peg hitting one detent or the other.

The impulse is transmitted to the case via a metal insert which the pivot shaft end rests against.

Even with a balanced pendulum the beat was uneven.

My aim was to get the escape to occur symmetrically in the swing.

The pivot shaft can be adjusted via the three screws  for equal escape timing.

I got the escape timing  adjusted to within 3% at a rate above 60 beats per minute but at slower speeds the beat was uneven and when the spring was more fully wound the pendulum slowed to the point where it stopped.  

At this point I removed the mechanism from its case and went hunting for the source of friction.

The nose of the shaft had worn a dent in the anvil that it pounds against.

Metronome anvil

A section of a snap off knife blade was glued on to the anvil.

The shaft nose had worn flat so it was rounded off.

This is the heart of the escapement mechanism and the likely source of the friction,

There is a  chip out of one face which from the wear marks is perhaps not involved in the sliding path.

As the pendulum shaft rotates the escapement pin is pressed against the face of one detent or the other.The friction of this contact increases when the spring is more tightly wound and has a bigger effect when the pendulum is slowly moving.

The detent faces get a hard pounding as the escapement pins hit them.

To make a complete repair some resurfacing of these worn faces would be necessary.

I decided to reassemble the unit, oil it and see if my efforts so far had improved things.

Unfortunately while giving satisfactory results (within about 3%) above about 60 BPM  at a slower tempo performance deteriorates.

However it is still usable at 40 BPM provided it is not tightly wound.

Note I have no expertise in this area and the above is just a report of my attempts at repair.

The following is the advice I gave to a reader who had a metronome with irregular beat.

Imagine a perfectly working metronome.
The click would occur at the centre of the swing when going in either direction and the beat would be even for all positions of the adjustment weight.

Consider the conditions necessary for a this to occur.
A free swinging simple pendulum will swing side to side an equal distance and time around the position that it comes to rest. A metronome beat is adjustable by sliding a weight and for the beat to be even at all positions of the weight the rest position must not change when the adjustment weight is moved.  The only way for this to occur is for the rest position of the pendulum to be vertical for all positions of the weight.

Checking the rest position.
The pendulum is driven (pushed) by from side to side by spring force acting via the angled face of the escapement. To find its free rest position this force must be removed. One can let the spring completely run down or remove the spring tension by applying winding force to the key. The pendulum will be almost certainly be near vertical with the adjustable weight at the bottom but it may not be with the weight at the top.

Assuming that the pendulum rest position is vertical regardless of the adjustable weights position (which means that the amount of free swing will be equal about the vertical) all that is necessary is for the "click" to be centred. The "click" may occur in different positions depending on the direction of the swing but should be adjusted to be symmetrical about the centre. This adjustment is made by moving the pendulum support shaft side to side while keeping the shaft parallel to the escapement wheel (adjust front and back support brackets).

Small adjustments to the evenness of beat perhaps can be made by adjusting the pendulum support shaft so that it is not parallel to the escapement wheel. If the shaft is not parallel to the escapement wheel the amount of "push" that is given to the pendulum will be different in each direction of swing. This adjustment can correct for slight errors in the angle of the escapement faces and may do something to correct for an unbalanced pendulum.

Balancing the pendulum.
To balance the pendulum it is necessary to move some weight around without changing the tempo of the metronome. To do this the mass must be kept the same and the centre of mass must be kept the same distance from the pivot.

Note. On my metronome I could see no evidence that the lead bob was adjusted by filling or adding lead in the factory and the slow tempo pendulum beat was way uneven. This leads me to speculate that the unit was dropped and the shaft was bent but that was not obvious. Bending the shaft to correct the balance has the advantage of keeping the weight the same and not changing the distance from the pivot but I did not attempt it.

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