I recently picked up a Tektronics CFG250 function generator on ebay. It powers on but the output wasn't working. I found the schematic online and have verified that all the voltage rails in the power supply work (-30, -15, 5, 15, 30V, though both 30V rails were closer to 25V). I also verified that the triangle-wave, square-wave, and sine-wave generators work all the way up to where they enter the output amplifier.
That's where I get lost. I'm trying to understand how the output amplifier works, but the first pair of transistors gets me stumped. It kind of looks like some sort of current mirror, but has the emitters connected rather than the bases. I'm guessing that this configuration lets the DC offset be applied to the output of 2nd base, with the collector carrying the AC signal to the rest of the amplifier. Is this supposed to be the case? While the 3rd pair of transistors looks to be a push/pull configuration, I'm confused by the middle pair, which seem to be driven only by the 15V rails.
Currently the output is stuck at ~-25V and Q13 gets very hot. I'm probably going to replace Q13, but I can't trace the AC signal that far into the circuit and was wondering if I should be looking to replace the other transistors. I've already verified that all the electrolytic caps in this circuit are fine.