You could have a shot at fixing it. I purchased a (broken) Philips 3216, and in the end it was a fairly simple fix in the power supply (a bad trimmer)
Mine is a PM-3212
The screen seems to be dimming on it. Tried adjusting the brightness contrast settings but it doesn't seem to help.
Any ideas or suggestions as to what I could look toward in terms of troubleshooting the cause?
The first thing you should do is measure the power supply rails to see they are accurate and without ripple. Electrolytic capacitors are usually the culprit there. (Stay away from the >2kV circuit unless you know what you are doing)
Presuming that the trace is bright when you turn it on and then dims, but the size position and focus of the trace are unchanged, then I would suspect there is a problem with the Z-axis waveform. That comes in two variants: the "LT" version (range 0-150V) and exactly the same waveform "shifted up" to be between the CRT's grid and cathode at 2-3kV. The "DC restorer" circuit does that, and the HV diodes and capacitors in it often become
subtly faulty with age. "Subtly" means that a simple diode tester won't spot the problem.
Contrary to popular belief, it is not necessary to have scope to debug such problems; I've done it with just a multimeter and a lot of thought and understanding, and care. The easy test would be to put a multimeter across the LT Z-axis waveform and measure the mean voltage (somewhere 50-120V, depending on timebase and other front panel controls). If that remains constant while the trace dims, then the problem is likely to be in the DC restorer.
When dealing with DC restorer problems, it is usual to simply replace all those diodes and capacitors rather than to diagnose which is more faulty. Make sure you get diodes with appropriate voltage, current and turnoff times. Capacitors need to have appropriate voltage, but the capacitance value does not have to be exactly the same.
Be aware that voltages will be stored on those capacitors for a long time, and make sure you understand the practical consequences of "soakage" or "dielectric absorbtion".