overdrive boost isnt working

past couple weeks on my tubemesiter 18 the 2nd button (boost) on the pedal wont activate the boost anymore. when i dont have the pedal plugged in i can use the button on the amp to activate the boost, but with the pedal plugged in it won't happen. any advice?

Hi Kirk.

I'm assuming this is a standard plain old 2 switch unit plugged straight into the 1/4" jack socket on the back and not something more exotic.

If it is working at the amp then there is absolutely nothing wrong there.  The footswitch is simply put in place instead of the internal push switch by the socket switches at the rear panel socket and it controls exactly the same circuitry as the internal switch.  The problem is most likely in your external switch unit.  This could be down to the actual stomp switch being faulty or it could be down to the cable being damaged internally.  It's a bad thing to wind up cables too tightly with sharp bends in them or just shove them into a bag so they kink.  Likewise a sharp jerk on the cable can break either connections at the ends or the wires inside.

Do you have a plain old RTS 3 contact jack plug or cable handy which has jack plugs which can open up by unscrewing the outer cover?  If you do then take the cover off the plug and plug the bare stem into the back of the amp.  Then just short out the sleeve and ring terminals of the plug.  This should activate the Boost exactly as though the footswitch had been used.

If you find it is the footswitch at fault then I would open it up, which is usually pretty easy if you are careful, and use a meter or continuity tester to check out the wires from the jack to the box.  If these are ok then it should be down to either a broken connection inside the unit or the switch itself needs replaced.

I've got a spare HK FS-2 box leftover from my TM18 so if you're in the USA and find out you need to replace the box let me know