Spring problem

[Phono48]

I have an HMV 99 portable machine that had a broken spring, so I ordered a new one. However, it is a lot thicker than the original, in fact so much thicker that I had to grind the outer end down before it would hook onto the rivet on the drum. Having installed the spring, I'm finding that the power runs out before the end of a 12" record. The motor is small, admittedly and the spring is only 9' 6" long, but would the thickness of the new spring have a detrimental affect on the running time? Any advice welcome!
Barry

[Inigo]

Could be that being thicker, it has a surplus of force, but takes more room into the barrel.... The original spring is longer than the new one?
In the other hand, the barrel must be filled to a certain percentage only; of the spring fills more space, it cannot develope all its power.
What is key is certain relationship between the length, the thickness and the size of the barrel... So same length, if the spring is thicker it could impair its performance.
You squeeze the spring into the barrel. Then you wind it up; the number of turns it takes to full winding are a fixed number: it will give in return the same number of turns when unwinding. The space available within the barrel governs the number of turns available, thus the duration of the movement.
For a certain duration (number of turns) and the barrel size, there are optimum length and thickness of spring, and if you change one of these parameters, you remove the performance from its ideal optimum point. Then there is the minimum power required to move the platter under the needle at the outer edge of a record (the maximum power required is there). All these parameters enter into the design... The new spring should be exactly the same as the old one (in case the old one was the original proper spring for this motor; it could have been changed in the past...)

[Phono48]

The original spring was the same length as the new one, but being thinner, had quite a few more turns. It was the original spring (it had the HMV trade mark stamped into the outer end) and it conformed to the recommended length for the size of the barrel. In view of Inigos' remarks above, I feel the problem lies in the thickness of the new spring

Barry

[nostalgia]

I have never encountered this problem Barry, but so far I have also been lucky enough to use springs from scrapped portables that I already knew would fit.
Inigo's explanation sounds wise to my ears, and also your own observations. These small motors, with their tiny drums most probably had a highly calculated spring with a certain thickness and length,
and small variations can easily disturb the runs. I have a 99 here myself, but have so far not serviced it or even checked the motor and spring, and not compared the spring to the springs found inside 87, 88, 97 etc.
I had to check the HMG book, now, a 23B motor, I have to open it myself soon to check it.
I hope you manage to get a correct spring, if this one fails.