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I have exactly the same problem! I really don't understand why. I have already change all the optocouplers. If some could help me (us?), thanks.

Here's the lead's schematics of Quad - Mark 2c+ - Mark4

The only way is to sloder out the opto, apply 2 Volts on the LED and check the résistance value (with ohmmeter) when LED it's ON.

I think it's possible but switch has to disrupt a high voltage DC current. It could be more careful to use it when amp is off. Mostly for the"switch life".

The optocouplers are the switching machine of the Quad preamp. As you can see on the picture it's led+resistor. So it's not really a switch! When it's OFF the resistance increases to 50 Mohm => the signal is blocked. When it's ON the resistance decrease to 630 ohm => the signal is NOT blocked...

Pas de problème! I find it's a clever system to power down the amp but not so as on the 5:25 or 5:50 system...

It could be the optocouplers . It's the weakness of Quad preamp ...

Look at the schematic on tubefreak.com. You can see that switch occurs on C which is the anode voltage of 12aX7 phase invertor (it drops 436 volts to 112 volts). It's absolutely not a pentode triode switch like on the hughes & kettner VS 250.

You're right, I mean the phase inverter's 12ax7.

FALSE It drop down the anode voltage of the 12AT7.