The amp exhibits intermittent crackling noise, which I think is parasitic oscillation as it stops immediately I touch the base of Q409 with a scope probe or an insulated screwdriver. The amp will function perfectly for most of the time, but occasionally exhibits noise on the LH channel independent off the volume control but mute-able with the balance control which is on the front panel pcb after the preamp o/p. I've changed Q411, 414, 415 to avail. I've tapped components and used freezer spray. I don't believe the noise source to be intermittent component breakdown because of the way it stops as soon as some electrical contact without mechanical force is made at a Q415 base or Q409 base. What could be the cause of the noise? The schematic appears to confuse LH and RH. I have the speakers connected to the correct terminals, the balance control works in the correct way, the input phonos are correct, but when I inject a mains hum signal by touching Q409 base (when amp is working ok) it comes from the LH o/p, and Q402 gives me RH o/p.
EDIT: Changing out Q412 seems to have eradicated the noise: it would seem that my grounding Q415 base through a 10uF capacitor was causing a current spike which temporarily banished the noise in Q412. Previously I couldn't imagine an electric connection stopping (for a while) a noise caused by intermittent component breakdown: now I know it can. BTW neither freezer spray nor shaking/tapping were any use in this case.