Loss of speed control on my Elna Zigzag?

This machine suddenly started going at full speed only. I looked inside the foot control and there were signs of charring on the coil where the contacts touched it and no continuity measured on meter across it. Got replacement universal foot control, wired this in as per instructions, but same problem. Foot control supplier tested the control and found no fault. Any suggestions? Are these foot controls variable resistance or varaible transformers? I have had a suggestion that a capacitor inside the machine has burnt out, but v. hard to get in there to check. Is there any test I can do from outside?