In addition to the Yaesu FT-901DM Service Manual (200+ pages) which you should have on your repair workbench,
FoxTango (Carol Maher) has extensive photographs, service bulletins, and notes for DIY Repairs on this radio.http://foxtango.org/FT901/FT-901-902%20Web%20Page.htm
Yaesu FT-901DM SURVIVAL GUIDEhttp://foxtango.org/Files/FT-901DMsg.pdf
C-1701 (100 pF, 3 kV) is a common problem on the PB-1715A PA Board as these radios have aged,
it is the coupling capacitor between the Driver tube Anode and the PA grids.
When it becomes leaky or goes bad, PA tube damage is possible.
The rectifier A and C boards are known to "blow" the Elna (Japan) capacitors --
On the Rectifier A board, you need to CHECK IF the diode modification
has been performed in the Screen Supply of the PA with S/N radios prior than 08xxxx, otherwise ARCing of PA tubes can occur.http://foxtango.org/FT901/graphics/FT-901_A_Rectifier_Unit.jpghttp://foxtango.org/FT901/graphics/FT-901_C_Rectifier_PB-1717.jpg
so replacement was a good idea.
Given the age (> 30 years) of these Yaesu models, the power supply/rectifier boards should be re-built with modern 500 VDC, low ESR, 105 degree capacitors.
I have not replaced the 2 big caps on the rec. B board, they LOOK to be OK for now.
Still the tubes don't heat or light.
The Rectifier B board supplies the 850 VDC for the PA anode (tubes).
Carol notes that R1405 and R1406 should be checked for proper value (470k), which is part of voltage divider. IF they differ in value (unbalanced), then one of the 2 capacitors will be handling close to its 500 VDC voltage value !!