@ snapdiode: The DC imbalance caused by malfunctioning Invert switch is mentioned in the 1A4 Instruction Manual, p.5-8, Table 5-3 "Circuit Isolation Troubleshooting Guide":
"3. No trace or waveform from one of the channels.
DC imbalance in the Input Amplifier for the channel affected. Check for faulty INVERT switch (could be caused by mechanical linkage that prevents switch from reaching the full-contact positions)."
http://bama.edebris.com/manuals/tek/1a4/
(- At PDF page 92).
I've never heard of anyone else having this problem with a 1A4, so maybe I've just been unlucky. But if anyone else has, perhaps they will consider joining the conversation: The more we can learn about these malfunctions, the better.
MDA.
