Commodore PET 2001 with screen problem

I assume you hae the scan lines (if you turn the brightness up?) but no vidoe modulation. If you ahve hre raster lines, then most odf the circuitry is working. In particular, the deflection circuits are oorking, and most of the electrode ovltages are right. Most of the time, the modualtion is applied ot the CRT cathode. The more postiibe the cathode is, the more negative the control grid is with respect to it, so the lower the beam current. Typically, the catode is drive by a common emitter stage with a supply of a few 10's of volts. With the trasnsitor cut off, the cathode is sufficiently +ve to cut the CRT off. When a vidoe signal si applied ot the abse of that transsitor, the cathode voltage is lowered,, turning on the electron beam in the CRT. My first test would be to measure hte CRT cathode votlage. This is normally pinb 2 of the CRT IIRC. Expect perhaps 40-70V there. If it's very low, or zero, susepct a problem in the power supply to the video amplifer. Nomally this will give an uncontrollably bright raster, but it doesn;'t hurt to check. The voltage for this stage is almsot always obtaiend from the flyback transformer. A tamp on one of the windings is halfwave rectified ans smoothed.So if the supply is missing, chekc that diode and cpacitor, when you've foudn them. Simialrly , fif the cathode votlage looks OK, check that video transistor and associated components. Now, how to find them... If you don't ahve the schematic, just trace that cathode lead back. You may find varios series inductors and resistors, but you will come to the collector of a transistor. That's the one to look at. -tony