Q410, Q412, etc are 0367. Check all that have this on them from C to E. Likely leaky.
Dan is right about U820 possibly being defective. I had one of these bad before. It is possible dirty contacts, if this one is in a socket.
If you are not getting any sweep on standard or delayed, I have seen Q812 open B to C. This transistor will make U820 look bad if the transistor is bad. A KSP10BU will replace this and the 0367 types. I have had that happen at least once. The Cherry switch on the right side of the plug-in at the front not physically mounted right. That will be open or grounded to P911. See if it is closed and open by a multimeter when you pull and push the time base selector. This is done with no power applied. The screws that mount to the readout board (A12) are oblong and allow movement of the switch. Make sure you have the pin that activates the microswitch there. You will see it with the right side panel removed. R479 and R939 should be 1W. These are stressed and could be high to open. I have heard of someone saying U856 can go bad. This is a 7474 flip-flop
Pulling the time base knob engages the delayed sweep. I do not know if you did or did not test this. See if you get sweep this way. The small intensity knob to the left of the time/div knob. That could be too low. Turn that CW to see of you get sweep. When working right, it will get very bright at full CW. The contrast to the right of this control may need adjusting. The trace sep. knob my be off.
If your S/N is below 60000, it is possible the tunnel diodes are bad on one or both triggering boards.
It is possible the internal dc bal. and/or trg. sens. pots were turned. They may need a slight adjustment. It may bring the triggering back.
Cleaning the switches might help if you have already done so.
I hope this helps.