Hi all, first time poster, long time reader. I bought an HP 5071A with a Symmetricom opt 001 tube confirmed working with <1500 V E_Mult. Within 48 hours, the following errors appeared: * Mass spec supply regulation failure * Error code: 000020a8 084a8000 * CBT load error 2 page 0 * CBT load error 2 page 4 * Bad CBT EEPROM format * Fatal startup error -1 in job 3 * Sequencer interrupt error -6273 Power-cycling repeated all of the above except the first two. A multimeter read 0.1 V on the +12 V rail, with a short narrowed down to the half-brick DC-DC converter in the A12 module. (I tried the more straightforward removal of the rail's 22 uF tantalums first -- I've been, ahem, burnt by a 3324A with shorted tantalums before.) Replacing the Vicor VI-J21-01 a.k.a. 0950-2184 rated at 28 W with a CUI VHB50W-Q48-S12 got the rail to just 11.9 V as it has inverted trim to the Vicor, i.e. trim up with resistance to negative sense, not positive. Removing the +SENSE SMD resistor and adding 56 kOhm to -SENSE brought it to spec, matching the negative rail's magnitude at 12.20 V. Note missing R6 and additional THT resistor to TRIM in the attached photo. The Vicor uses a mounting scheme and bracket that interferes with the CUI's dimensions, though nothing a Dremel couldn't fix. With a bunch of thermal pads the new converter runs steady-state at 43 C compared to +5 and -12 V at 42 C. Happy to report continuous operation since. Hope this is interesting to others, wondering how common an affliction like this is? Cheers! -Tim KE2EZH