Ampeg SVT Classic Mid control

[Wounded Paw]

So I've owned a vintage SVT for many years and one day the midrange control stopped working. No midrange, no difference no matter where the dial was. I tried having it fixed more than once and the problem just came back. I still have the amp but haven't used it for a while because of that problem.
Anyways, to the point. We rented an Ampeg SVT Classic for a backline of a little music fest yesterday and this new model has exactly the same problem. No matter what you do to the midrange control, even changing the frequency range, it does nothing to the sound. It's probably the main reason for using that amp, the mid range honk, but it's not there. Is this common or is it a coincidence. The rental amp has the same problem as my old vintage SVT? I also have an old V4-B and it's midrange control does exactly what I expect it to.
So, any thoughts, experiences?

[Jerrold Tiers]

The coil can open...... some are prone to doing that. Kills the mid control.

[Aharon]

Bringing this thread back to life momentarily...

This has just happened to my SVT 2 Pro. Something broke in the mid range selector area that was causing squealing and all sorts of nasty issues. I took it to a tech who fixed that and now just a few weeks later after maybe 2-3 hours of total use the mid control knob has stopped working.

What is the cause of this? Is it likely to keep happening like it did for Wounded Paw? Is the amp a "dud"?

Thanks.

[JimmyM]

Amp's not a dud, but obviously whatever he did broke again. I'd call him and ask him what he thinks.

[Aharon]

Well it looks like it will have to go back to the tech. Not ideal because he does not have a fast turn around and the last repair cost me £130

I was just a little curious about what Jerold said when he says that they are prone to doing it. Also what is the coil involved? Is the mid range just going to sit at 12 o clock or will it be at whatever it was before the problem happened?