I have taken some update measurements on the 5V line with and without the keyboard/display pcb. This all started because I was and still are getting gibberish on the front display leds and also no consistency in responses to key presses. I measured the noise/ripple on the C6 and. C7 at the time and was seeing 50-60mv.
Now after recapping and fixing the Zener on the power supply I see the following. Am using a fluke87v and a Leader LMV-186A along with my scope. Leader pic attached

- 4.8mv AC between chassis and MP2 header (+5v) on power supply with and without the keyboard/display pcb connected
- R67 on display pcb (+5v line) to chassis 23mv AC. Keyboard pcb obviously plugged in.
- R67 on display pcb (+5v line) to C7 -ve side 30mv AC. Keyboard pcb obviously plugged in. Scope shot also attached.
- R67 on display pcb (+5v line) to C6 -ve side 24mv AC. Keyboard pcb obviously plugged in.
- ST20 pin 16 to pin 11 or chassis, 0.1mv AC. This is where the 5V comes into the front keyboard/display pcb. This is with the display NOT plugged in.
Clearly the 5V in at the ST20 ribbon connector to the keyboard is low ripple, not the same when the keyboard pcb is connected?