Vintage DECtalk voice synth repair

[artelse]

On the desk I am attempting to repair a DECtalk voice synth from about vintage 1990. It sports a 80C186XL10 and a 87C51 and various logic, an AD7541 ADC, etc. Several caps were leaking and replaced all with equivalents. Also a voltage inverter LT1054 died and replaced it. Still the device outputs nothing sensible through the serial port and no voice greeting.
I do measure rather high peaks on the databus, up to 10Vpp.. due to ringing? I checked the 5V supply and it got some dirt on it, but can this cause the ringing on the databus pulses? Can these peaks damage anything? See the attached images: databus, Vcc and clock-out from the cpu which isn't that great either.
Any advice is greatly appreciated.

[Tomorokoshi]

Where is the probe ground connected compared to the pin you are checking? How long is the grounding wire? There could be some ringing if the ground is too long.

[artelse]

Good question. First I had a longer ground wire for easy probing over the board. Then I shortened it to the regular ground wire of approc.10cm and connected it right to the chip-under-test. I saw some reduction of ringing, but not what I hoped to see.
Any other idea?

Where is the probe ground connected compared to the pin you are checking? How long is the grounding wire? There could be some ringing if the ground is too long.