Hi Stuart,
This is excellent. I would say basically you have found the problem but not the cause of the problem. What I think is going on is that LDR1 is not being powered on all the way. This might be caused by the LDR going bad or the capacitor that is across the LDR control being flackey. Because there is no change in volume for channel B it could be either of these items or maybe something entirely different. As before we try things to narrow down the possible causes.
Now for the capacitor and LDR I am talking about. Look at the second schematic. Towards the bottom are a bunch of transistors. To the right you will find the 1/2 DR jack input. Now go to the right of this input and up a little bit and find the resitor R372. Below it is a diode, LDR1, and a capacitor that is across it with a value of 100uf. If that capacitor is going bad it will partially short the diode and not allow it to turn on fully. If the LDR is going bad this could also cause the noise. There are a few ways to proceed.
First procedure is to touch up the solder joints around these parts. I would stick with the capacitor, LDR1 and R372. If this works you are done.
Second procedure. If touching up the solder joints did not stop the noise then remove the capacitor and see if it stops the noise. If it does then buy a replacement and put it in. It will not hurt your amplifier to run without this capacitor. What the capacitor is doing is slowing down the transition from on to off (or off to on) of that diode. This stops the amplifier from making noises when you are switching the 1/2 DR function. If the capacitor does not stop the problem then you can replace the LDR and see if this does it although I would post back before trying to replace the LDR. This part is going to be harder to find than the capacitor.
Third procedure. Use a voltmeter to measure the voltage at the junction of LDR1, the capacitor and R372. Do this without using your shorting cord in the 1/2 DR control. The voltage should be about 1.4 volts. If it is lower than this and wondering around, particularly when the nosie is present it would indicate the capacitor is bad. When the noise goes away see if the voltage has risen. If it has this is further indication of the capacitor being bad. If the voltage stays the same with or without the noise it is more likely the LDR. Now ground the cord that is attached to 1/2 DR input. The voltage should go up to about 3.2 volts.
If you are good with electronics and have the voltmeter then the third procdure is the way to go as it allows you to look at the problem without doing anything to the parts. If you are not so good with electronics but can solder then the first and second procedures are the way to go.
BTW DO NOT do any soldering on the amp while it is on or even plugged in. I don't mean to sound like your mother or anything but I would feel very bad if you were injured or if you messed up your amp. If you are in over your head with this stuff then get some help. I do think we are close to solving this but this stuff can bring you some surprises. So post back your results, even if it is to let us know that the amp is ok, and we can continue. Again we all learn from these things. Hope this helps.
Gary
BTW that the DEEP only affects channel A says there may be a problem here also. Your choice to fix or not. You also mention hearing the switching for DEEP and MODERN but not for 1/2 DR. This is normal. The first two are switching relays. Relays are mechanical and can be heard. The 1/2 DR is an electronic switch. Nothing mechanical to hear.
Channel B is the dodgy one originally thought it was A my mistake !!
