Bright Switch issue on Super Reverb

[SleepLess]

Hi there!
I thought I'd be done with my blackfaced 1973 Super Reverb last week when I changed the optocoupler. It was the culprit and now the tremolo works perfectly. But now comes another issue. I start understanding why some guys say "when you buy a vintage amp, buy the tech that goes with it..." Here's the problem:

On the vibrato channel:
1. With the Bright Switch ON everything is normal and the amp works as it should.
2. With the bright switch OFF there is a continuous loud hum. I mean really loud. The amp still works and I can hear the guitar, but the hum is too loud to play this configuration.

Would you guys know where this problem coud come from? I swapped preamp tubes and the problem is still there. The thing that really bugs me is that the amp works just fine with the switch on the ON position... 

[sluckey]

Are you sure it's wired correctly? Sounds like you may be breaking the wiper or the pot ground.

[eleventeen]

I'll second sluckey's post. Either wired wrong or a defective switch.

[SleepLess]

Thanks guys, I'll look into that. The thing that bugs me is that the amp works fine with the switch ON. If the switch was defective I guess the amp would produce the hum in either position. Or at least in the ON position and not the OFF as it does... Strange stuff... 

I have also found microphonics. When I hit the knobs on the front panel they resonate through the speakers. I'll have to swap each preamp tube to find the microphonic culprit. Do you think that a tube could be responsible for the bright switch hum thing as well?

Thanks!

[phsyconoodler]

Listen to Sluckey.He knows what he's talking about.That switch isn't wired right or someone may have used it for a boost switch instead.If you know your way around a super reverb it's easy to spot those simple things.If not it can be a bear for you.
  It sounds like someone wired that bright switch up as a boost which might explain the microphonics when you hit the panel.

[SleepLess]

I ALWAYS listen to Sluckey  I will check the amp as soon as I've got enough time to open it up. I'll keep you posted. Thanks again.