There are at least two kinds of sampling diodes used in classic Tek sampling gear. The first are GaAs junction diodes. The second are Schottky diodes. It is easy for these diodes to get destroyed by ESD. They are designed to have low capacitance, which means that a small amount of charge can produce a potentially damaging voltage across the diode.
If the output of a sampler has a large DC shift even with no input to the sampler (an external triggering), and does not respond to the DC offset control, it is possible that one or more sampling diodes could be damaged. The original diodes are no longer available. When sampling diodes need replacement, there are two options. They can be scavenged from other gear or replacements can be improvised using modern parts. HP makes integrated pairs of high-speed Schottky diodes. These are a decent replacement for the original diodes.