I'd verify my power supply was good. Appears to run off +15VDC, so is it indeed more or less 15 volts at the regulator, and is it clean? Then are the reference levels OK, meaning are all the signal pins on the various op amps sitting at about 7-8volts?
And the REticon chip is the heart of this thing, so look at pin 6 for DC and see if it is about 7-8 volts there too. The input bias control sets the voltage ther. It should be appoximately half the supply voltage.
Note the NE571 IC has its own power coming through that 100 ohm resistor center left on drawing. Is there indeed 15v on pin 13 of the 571? Hoping the 100 ohm is not open nor the cap there shorted.
I'd verify there was a clock signal at pin 1 of the Reticon IC.
I'd trace signal through the circuit. There must be signal at pin 7 of the input chip or you would not have clean signal. SO is there signal coming out of the NE571 at pin 5? How about coming out the Reticon IC at pins 3,4? Then how about at pin 1 of that op amp in the lower right corner of the schematic? That feeds the other half of the NE571, so is there signal coming out of it at pins 10,11?
SO, we checked for power, made sure there was a clock pulse to step the signal through the circuit, and then followed the signal through to look for where it gets interrupted.