[PortugalTV]

Its strange to see retrace lines with the brightness "only" at 75%. i would recheck voltages at the crt base. especially g1 and cathode
if those lines dont go away even with a good picture there can be an open component on the retrace supression circuit. usually its a winding on the lopt connected to some components wich connect also the the vertical output transformer and to CRT G1 (brightness pin). there may be a diode stuffed in there and they often fail. the purpose of the retrace suppresor is to kill the beam on the retace period ( when moving from line to line)

yes i think adjusting the agc without a signal is pointless, but without a signal i think you should not have any AGC voltage or atleast very low.

remember i havent looked at your schematic yet but usually the agc is generated by a circuit with a tube adjacent to the video amp, the idea is the more stronger the signal, the less agc voltage is produced. agc voltage is negative with respect to chassis and it will bias down the tuner rf amp, and the IF stages, sometimes it also acts on the video amp itself on some sets. usually you can artificially apply agc with AA batteries, connecting positive to chassis and negative to the point you wish to apply it.

the boost cap, from my point of view is the most urgent part to replace on a set, it may take the lopt down if it shorts. choose a good ac pulse rated cap like the philips mkp series. a regular cap may not last very long because they are rated in DC volts and that means nothing in that section, they are subject to high amplitude pulses... if you dont have the correct value on your local store you can parallel a couple of caps.
at this point i would just replace that cap and apply a signal and go from there, maybe you dont have any problem with the set

PS sorry for the bad English