With the failed parts identified, we need a replacement for the FES8DT diodes: 200 V, 8 A, 0.95 V drop, 35 ns recovery time, 125 A surge current.
Something more rugged – because already on the datasheet, it says “Ultrafast, rugged” – what do you want more – are the BYV79E-200 diodes.
These are 200 V, 14 A, below 0.9 V drop, 30 ns, 150 A. These should last for another 20+ years, and come in the same packaging, SOD59, aka TO220AC.
Fortunately, I had some spares around from other activities, but you know – the best source is not xbay, but just some junk switchmode supplies that should be sitting around in any good electronics workshop – these are always keepers, and have come in handy as a source for parts, for more urgent (commercially relevant) repair jobs, rather than for the HPAK 8904A…
For the 8904A – one important thing! Never short out the RAM backup power on the main board – you will need to go through an activation routine, otherwise, your instrument will be rendered non-working. Don’t now why HPAK didn’t use EEPROM at the time, maybe, these were not yet invented… Following the instructions in the user manual (Thank You, HPAK, for providing all these manuals free of charge on the web!!!), the backup battery was replaced with a new Lithium cell. In the end, it might be a good idea anyway, to open-up the case every 10 years, get the the dust out, and a new battery in.
With new Schottkys, new RAM backup, new serial (and new code), the instrument is now fully functional, option 001 and 002, and will provide good service, hopefully, for decades to come!
Anyone with a broken 8904A – feel free to contact me!