[MrFixr55]

BTW, if you got the Riders schematic from the Nostalgia Air website, some of their scans are fuzzy.  I'm sure that they did the best that they can.  Nostalgia Air is a free website that has done a world of good to this hobby.

Attached is a copy of the service Notes for the RCA R120 from the 1933 RCA Victor Service Notes "Red Book".  The schematic is clearer in this document.

RCA R120 fr RCA 1933 Red Book.pdf Size: 301.89 KB  Downloads: 8

Capacitors in radio circuits serve 4 purposes:

• Power Supply or AVC filtering
  
• Coupling between stages
  
• Part of "tank" (tuned) circuits
  
• Cathode or plate resistor "bypass".
  

Essentially, there are 3 types of construction used in capacitors used in these vintage radios.  Each has their place and their issues:

• Electrolytic-  These are usually 1.0uF or higher in capacity and can either be "wet" or "dry".  They are usually used in the power supply section, and sometimes, as cathode resistor bypass caps. Early electrolytics were "wet", newer ones are "dry", actually a paste.  These capacitors dry out as they age. the wet electrolytics will usually short when they dry out.  This is very dangerous to the rectifier or power transformer, causing burnout of these parts from the shorted capacitor. When "dry" ones dry out, they lose their capacity and cause power supply hum.
  
• Paper / wax/ foil -  These caps are usually between 0.001 and 0.5 uF, and are built as described.  The plates are foil, the dielectric is wax paper, and the capacitor is sealed by wax.  These caps invariably leak or short.  A leaking coupling cap between the plate of the 1st AF amp and the Output Tube grid will cause a very positive grid bias, and excess plate current in the output tube, usually enough to cause the tube to "red plate" (overheat), burning out the tube, output transformer, etc.  A shorted B+ bypass cap will usually take out the dropping resistor in the least and could cause further damage.  In my experience, these caps can short at any time.  The ones most likely to short are the caps between the output tube plate and cathode that act as a tone control or HF filter, and coupling caps, but any of these caps in any location can short with disastrous results.
  
• Mica-  These caps are the lowest in capacity, usually less than 0.001uF, and are used in tuned circuits for the IF transformers and for rf filtering in the 1st audio stage.  Most experts believe that these caps rarely fail and usually do not change these out.  However, in newer sets (late 1940s till the end of the tube era, these caps can short due to what radio hobbyists call "Silver-Mica Disease" (SMD).
  

Using cause and effect troubleshooting for this issue is a laudable educational tool, but at the end of the day, pleas replace all electrolytic and paper caps.  The new mylar caps and electrolytics from quality brands will last any years.