Finally I put my Tek 7603 oscilloscope into service. A friend (the one from optical experiments) needed to verify a circuit he's building. Those were routine tests, not troubleshooting.
While probing for a couple of signals (ca 3 kHz, 15Vpeak) I noticed a strange random behavior: either the signal was as expected, or was passing through a high-pass filter (only spikes instead of edges).
The first problem was easily tracked down to a cold solder joint since it showed properly when touching the generating IC.
The second ghost was trickier: all joints were ok. It was a digital signal... where was the high-pass effect coming from? While checking for continuity with an ohm-meter backwards from target to source pins (with ICs removed!) I came to a homebrew connector with 2.54mm (1") pin headers ... on the corresponding pin tip I could not get a proper contact, while at the pin base it worked alright. Very suspicious. I touched the pin and asked right away: "what did you do to it? Glue?"
He admitted that the building instructions showed a way to hold the strip while soldering: put it into a roll of packing tape. !!!!!!!!!!!!!!!!