Channel switch question - Mark III

[jhreid]

Apologies if this has been dealt with before.

Quick question on channel switching on a Mark III purple stripe - when I pull on the lead knob should I expect the channel to switch instantaneously or should I expect the lead sound to fade in over a short period of time?

 

[Whoopysnorp]

It should switch more or less instantaneously. What you're describing is probably a symptom of worn-out LDRs (light-dependent resistors), which control the switching in these amps. This is typical after 30 years of operation. Another symptom is bleeding of the lead channel into the clean channel, which manifests itself as a quiet distorted sound mixed in with your clean tone.

 

[jhreid]

Thanks for the response. Haven't noticed bleed yet. Will get my tech to look into replacing LDRs soon. They are getting expensive!

 

[Whoopysnorp]

I think I heard that the US company that used to manufacture them has ceased doing so, which is probably one reason why. Another reason to get that job done sooner rather than later. I had bleed happening in my purple stripe and had that job done as well as a cap job (well...technically, I tried to do it myself first, then had Mike B clean it up when I failed).

 

[jhreid]

UPDATE:
Just got the amp back from my tech. He replaced all the power supply caps and when he put it back together no more lead channel! He chop-sticked about inside and found a tiny crack in the circuit board - just before the power supply to the switching and the GEQ. He bridged this and tested again and now the lead channel switching is instantaneous as expected. He thinks that the crack had been there but not completely broken and that there was an intermittent interrupt to the power to this section. Seems to be fixed now!