M70 speaker distortion

I have a small scratchy distortion in one speaker. It's more obvious when playing piano music. I know it's the speaker because when I swap the leads at the amplifier the scratchy sound is still there in the same speaker. Can anyone give me a clue where to start ? It sounds like it's in the mid range speaker. Should I start with the speaker or the crossover board?

probably the spider is not supporting the cone properly, put the speaker upside down, does that fix it ? if so take out the offending driver mount upside down and it will be fine, it could be other things but this is likely if it has moved to much then this trick will not work and you need to get the driver repaired,

Thanks for the tip!

 I took the speaker off the trumpet stand and turned it upside down, plugged the lead back in and bingo!  --- no distortion. I will now follow your suggestion with the mid range speaker and I'll post another reply to advise the final result.

No! Is it the smaller cone, or the larger dome?

The smaller cone is the phase link, and the larger dome is the midrange. If it's the phase link, the magnet is beginning to slip. You'll need to epoxy the magnet. Well documented problem.

Hi Rich

I might let sleeping dogs lie!!  Since I inverted the speaker box and then turned  it back the right way and reconnected everything, the distortion has disappeared. At some stage however, I plan to remove the front grill cloth from each speaker and carefully examine the speakers individually. After all, they are more than 40 years old.

johndon:

After all, they are more than 40 years old.

And still going strong! Tim Jarman published a new review on them in the January 2017 issue of HFN&RR. You can order back issues if you want.