Haven't found out how to fix the focus problem on the Tektronix 464, so I left it and took the HP 3312A out of the storage.
The only schematics I've found are low res scans but the power supply is quite common +/-17VDC, a few inductors for filtering and some resistors and zeners for lower voltage lines.
Both +17V and -17V rails are shorted to ground, that's why main fuse has blown.
Started to remove the big 500µF capacitors, tested with the DER EE 5000, still good.
Traced the +17V and -17V lines, not that easy as it's double sided boards, removed some inductors and resistors, then finally found two shorted 2.2µF tantalums.
Replaced them, quick test with a 40W bulb in series as I don't have 6x32 / 160mA fuses in stock, but after a few minutes the bulb was glowing and a 10 Ohms resistor has burnt.
So I've replaced all tantalums and now it works.
Then back to storage waiting for a good restoration until I'll do a Mouser order for the fuse and a few capacitors.