Jim,
Check CR1372 and CR1379 for leakage. If leaky, replace with 1N4937. When one or more are leaky, it will give a too bright trace like you have. Also check the 22 meg resistor. It is likely out of tolerance. Use Vishay VR37 to replace it. When this resistor is high, the trace will be bright at turn on then dim as it warms up over 20 minutes without touching the intensity control. C1374 should be a film type to replace the electrolytic. Check VR1416. If bad (open), use BZX85B9V1. Check the 151-0367-00 transistors for leakage from C to E for leakage. Replace leaky ones with KSP10BU (BCE pinout). If you decide to recap the supply, use long life high temperature types to give a long life. Increasing the capacitance is fine. Watch for tantalums that are bad. They can be replaced with something like Nichicon ULD, 47mfd 25V for example. The 1.5A bridges should be replaced with 4A types. A bad solder joint may be on some resistors that should be raised in wattage which there are ones that should be a higher wattage than original. I can give you a list if you want. How far you want to go is up to you.
Welcome to the group. I hope you get your scope going to your satisfaction soon.
Mark
