We have tried the easy ones, now the harder part [your choice, up to you if you want to do it, considering the age of this scope];
Please be careful, observe safety.
Use a working scope, set to AC, probe Divide by 10, Volt/Div as appropriate;
The scope under repair, set to 10mv/div, apply test signal into the input.
Probe with respect to GND, [if you are save the traces, please post it up]
Collector Q5, then Collector Q6,
Collector Q9, then Collector Q10
Emitter Q13, then Emitter Q14
Volt meter measure Voltage at Cathode of D10 and then Anode D14 with respect to GND.
[You sure you have checked the 5V, 12V and 20V?]
The above just merely divide the areas of search.
I suspect one of the common biases is OUT.