[MrBungle]

Very little information on HP kit after late 1980s. I recently had to reverse engineer one of their digital scopes because there is no info for it. I couldn't repair it in the end as there was a cracked board under the attenuator where it had been dropped, so scrapped the entire unit and am selling bits of it for parts. That was unfortunately the only way out.

On the HP scope, I found a number of issues including a dead opamp before I came to that conclusion. As it's an ADC board, worth checking references and any buffer amplifiers for those references to make sure none are being pulled low, assuming there are any. This will affect the input range of the ADC and prevent cal. I've noticed a common theme with HP kit is they have a fixed reference, say 5V and then use a precision opamp to pull that down to 2.5V, buffer it with a heavily compensated opamp and provide that as a ref for any ADC.

I did this with a continuity tester, scope and some patience tracing out the entire analogue front end. Took about 3 days' of evenings to get that far.

Quick google for that particular HP instrument suggests it's card frame based which makes it even more difficult to trace out unfortunately, at least without a riser card available.