[ricard]

I'm assuming the 13.8V DC bias is on the emitter of Q109, not at the right hand side of C134.

It would be reasonable to assume that Q109 is biased at the middle of the supply voltage, so with a supply voltage of 27V, 13.8V would be about right. Doing some calculations, this means that the emitter current is 13.8mA (13.8V/R144=1kohm), which also makes sense considering the base resistor R141 is 220k and the BC107B has a gain of 200-400, so the ratio between the currents flowing through R144 and R141 will be the same as the gain as there are no other sources of bias for the transistor. With the emitter (and hence the base) being around mid-VCC, this means the ratio between the resistor values will be roughly the same as the gain which it is. This transistor is not providing any gain in itself, it's an emitter follower just acting as a buffer.

It's hard to say at a glance what the gain of the EQ amplifier is, but certainly there is a darlington pair at the input, so there will certainly be a lot of raw gain to start with, but then of course there is a lot of feedback too.

Regarding the input, since there is really only one input on the amplifier, it's not uncommon to lower the signal level for line level signals using a simple series resistor or divider, rather than skipping the first amplifier stage for high level signals.

What serial number does the machine have? There is an inherent problem with the circuitry on machines with serial numbers below 2209550 which causes erratic connections in the record/playback switches, usually manifesting themselves as loud scratches and pops in the audio channel when the switches are moved between their three positions. I'm thinking that the problem with clipping could be due to an erratic connection causing too much gain in the EQ amplifier.