That you see no noise if you remove the marked cables is caused by the fact that these 4 cables provide the Clock-Signals to the p2CCD-Chips (those fragile ceramic unobtainium beasts of which one died in my PM3323 - BE CAREFUL when removing the board with the vertical amplifiers! The barrier between the P2CCD-Unit and the amplifier-board has springs on it, which send the metal piece flying once the screws are loose. This most likely destroyed the P2CCD-Chips of channel B in my unit.).
If you have another oscilloscope, check the output from the vertical amplifier-board. In my scope, one channel was quite noisy while the other one was ok. And then just keep changing capacitors until the noise is gone (didn't help in my unit, because one of the amplifier-hybrids was creating the noise while the supply-voltages were ok).
Should one of the orange hybrids be dead or damaged, I think it is possible to recreate one on a small pcb with off the shelf-components. Some of them even have their internal schematic drawn in the service-manual.
You also have to replace the capacitors on the Clock-Driver Board, which sits above the CRT.
In my unit I discovered the interesting phenomenon that there were some oscillations happening on that board. If you see the same and manage to find the source, please tell us here
Once the analog boards, the P2CCD-Unit and the Clock-Drivers are sorted and no longer noisy but there's still a lot of noise on the screen. take a look at the ADC-Board: There are also a few caps that needed replacement in my PM3320A. With new ones, noise dropped quite noticably.
Also noteworthy: My PM3320A uses a different ADC, which is housed in a metal can, than the PM3323, in which the Chip is under a golden hood on a ceramic carrier. The ceramic one has more than 6dB more noise than the one in a metal case.
From what I've seen so far in these scopes, basically everything can be repaired, some parts are even still being made and listed as "Active" on the manufacturer's websites, except for a few key components:
- P2CCD-Chips. If they're dead, it's Game Over until a donor-unit comes along! Replacing them with modern components basically means building a 1-Channel 500MS/s Sampler with a 50kS/s Output and a DSP to recreate the quirks of a P2CCD of which there is no datasheets available
- Programmable Logic Sequencers: NOS-Devices available and recreation with modern CPLDs possible, but getting the program out of them requires REALLY expensive special programmers.
- Transistor-Array-ICs - There are a few transistor-arrays on the boards, which are custom philips parts and not available any longer. They could be recreated with tiny SMD-Components, but like the Logic-Sequencers the work to be done would be extreme.
- The CRT (Obviously, although with enough work, a display-adapter could be built with a STM32-Chip to read the display-memory and write the content to an LCD - The Service-Manual goes into quite great detail about how the display-system works.)