[Bruce B]

I am on my 4th old radio repair and now I have this radio working but it has an extremely sharp, like a crack, thin, tunning width. There is no width at all, it's like tunning a crystal detector, it's either on or off signal. The sound of the audio is somewhat distorted while the radio is warmed up. If I turn it off and let it cool, the audio is excellent when turned back on. It starts to deteriorte after about 3 minutes. 
  This radio has been completely re-capt. There are no original wax capacitors in it. All the capacitors are blue, Aerovox, capacitors. I have tested most of them under voltage for leakage and they all seem quite good and spot-on. I have replaced the capacitors adjacent to the audio tube anyway, because of the distortion thing, but no improvement was made. It's not fun to replace absolutely good parts, but I did it anyway. I can put an audio signal into the 2nd detector and the radio sounds as nice as an old radio can sound.
  The resistors are a different story they all seem to be original, dog bones and most are reading from a little bit high to somewhat high and I plan to start changing them out. I don't feel though that the resistors are causing this sharp band width problem, so I am trying to figure that out separately. I forgot to mention that as the radio warms up it will drift from its sharp tunning point and as the distortion begins. When I bring it back into tune again, the signal is still quite strong but with more distortion. I have aligned the IF and RF. The IF went well and came up and peaked quit easily but the the RF tuning was sluggish and had little responses to padding adjustments. I don't know if I have 1 or 2 issues here.  
  So after all of this being explained, I am in hopes that someone can guide me to the next thing I should be looking into?
Bruce B

[MrFixr55]

Hi Bruce,

Looking at the schematic, check cathode bias resistors R3 (RF, IF) and R7 (1st Detector). Also check R9 (B+ to oscillator.

Depending on where you injected the audio signal, good volume and tone from an injected signal rules out the 1st Audio and Output circuits. Changes in distortion, signal drift and changes in tunability indicate issues in the RF, IF and 1st Detector circuits. Maybe shotgun the resistors. Even though the caps looked like newer Aerovox units, replacing paper caps with mylar is still prudent, as paper caps will still break down.

[Bruce B]

Ok, I will get started on those resisters . I have felt that the problem lives in the association between RF and IF or just in the RF alone not producing a decent signal.to the detector. I used to work on these radios in the 1960s when I was in college. I haven't touched anything in Tv or radio since. I am having both fun and frustration trying to remember what I used to know, I think ?. That's one reason I don't want to just wholesale chang all the caps at once, I want to find out what capacitors are not doing their job and change them. Most of the capacitors I can put under the same stress as the radio puts them through, both AC and DC. If they don't support any DC current draw at high voltages any more than a brand new capacitor would draw, I let them stay where they are till I would find some other reason or symptom to replace it .Hopefully this will help me remember what I learned 60 years ago.
What I am curious about is the Riders 4-64 I have? It's quality is terrible!. I have to use a magnifying glass to see numbers around the components. It seems the other PDFs of the other models are quite clear, but the page I need is not clear. I see you have looked at the schematic and I was wondering if yours is also blurry? I can't believe Riders would have put our such a low quality print. Maybe it was just scanned poorly, and I should look for a better copy.
Anyway, thanks for the help, it's really appreciate. I will get started on the resistors and let you know of any joy I get.
Bruce B

[Bruce B]

Well I I've only gotten one pellet out of my shotgun, but there is much joy! You hit the nail on the head.  I was planning and still am planning on changing out all the resters you suggested.  I took on the most hard to get at resistor first R3, cathodes for RF and IF. What a deal! RCA had no intention of ever replacing this resistor.  R3 is soldered to the socket of the detector tube and they put the IF transformer underneath that tube socket, allowing no more than ½ " of space.  The Transformer can is riveted to the back of the chassis and one would have to grind out the rivets in order to move the IF can and get to R3.  R3 is supposed to be 1.0 K.  It tested at 1.4 K. I found a location under the RF amp and place a 3.5 K  from the cathodes to ground, or otherwise across R3 and that brought R3 back to 1 K ohms.
Well turned it on and wow! loud, and no distortion at all.  Still a little narrow on the tunning but it actually has a little bandwidth now.  I will take on the other bias and B+ resisters tomorrow.
Thanks for the help, Bruce B

[MrFixr55]

Bruce, you are welcome, I'm glad that the suggestions helped.

Eary on, Henry Ford developed the "Design to Manufacture" concept. This sometimes (or often) flies in the face of a "Design to be Repairable" concept. Your comments on the location of resistors and the need to perform major surgery to get to it underscore my point. Even on more expensive radios, like the upscale Philcos, this is nonetheless an issue.

[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: 5

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.