Troepfler wrote:

I clean SMPL and others with a rotating brass brush (soft and slow).
Between 3 mods I exchanged parts to get better/full "start on" power.
The problem seem to be the 8(?) to 12(?) notches in the switch.
They seem to make not always the best contact to negative:
you hit the button left side: OK, you hit right side: not OK.
It seems the mod is worn out, too much play between electric parts.
I think I need some new tube mods,
but what to buy?

Yes, brass will scratch copper, so if you have a copper fire button you might have caused too much play between.
(would be possible if you have a brass button as well).
Soft and slow will just make it take a longer time, but you will end up there eventually.
(that's where an ultrasonic cleaner shines, as it does not diminish the metal, just oxide and dirt)

But it could also be that the spring is worn out.

Fasttech used to have replacement buttons for SMPL, sadly not anymore.

But you should be able to source it elsewhere