I have just restored a Model 80 with new electrolytics and replacement of all dogbone resistors (including Ron's Model 80 resistor mods). It looks like this one had its electrolytics replaced, as the original cans were empty and some axial caps were installed underneath (50's vintage caps). I also found that a gimmick was already installed on this unit with the same type of wire as the original wire. Could some of these Model 80's had gimmicks installed in later production runs? The radio pulls in many stations and sounds fantastic, except...
Every couple of minutes or so the volume drops off a bit and I eventually here a "pop", followed by a return to full volume for a few more minutes. I have seen others refer to this occurring on other models in this forum but have been unable to find a description of what might be causing this. Is it a failing rectifier tube or some other component in the DC supply section? The only one I haven't replaced is the 325 Ohm wirewound resistor to ground. Also, I haven't touched any of the bakelite condensors.
Any hints on this would be very helpful. Initially, I just chalked this up to "expected 75 year-old radio" performance. But, it seems like a problem that should be fixable.
I am also wondering if this may be caused by the volume control rheostat. I haven't touched it yet, but am wondering if there is something I can do to "clean" it up.
Assuming that you have emptied out and recapped the bakelite blocks, I suggest that you replace the largest (power) resistors- the ones electrically connected to the filter caps.
Most likely it's one of the large power resistors but if you want the radio to operate well, the highest values must be eventually replaced- generally all resistors above 20K. Work you way down.
You could also isolate the resistor with a can of freeze-spray. A light spray on a bad resistor will thermally shock the resistor into doing something better or worse. A good resistor will not be affected by the freezing spray.
I have replaced all resistors, but haven't touched the bakelite block caps. I was hoping to avoid those.
The popping you have most likely is a leaky cap in one or more, or most of the bakelite bock caps in the set. They
all must be re-built for reliable and safe operation.
Easy Bakelite Block Condeser Re-Building:
http://www.philcorepairbench.com/capbuild.htm
OK, I'll replace all of the bakelite caps. Do I also need to replace the two small valued mica caps? (I'd rather not go back up into the IF can again.)
No, usually not, as they are specific values for tuning and other RF/IF tuned applications. Micas rarely go bad. Changing these may make subsequent re-alignment impossible or troublesome if not changed out with the
exact capacitance values in the schematic or parts listing.
I'd leave them be.
Off-value resistors, wax-paper and electrolytic caps are 99% of the problems with a typical radio, aside from other problems like open coils, fried parts, bad tubes, previously attempted "repairs", etc.
Chuck
I refurbished all of the bakelite caps. The popping is now gone. Thanks for the hint, Chuck. I still get some volume wandering that, I believe, is due to the old rheostat. However, I think I may be just fine with this behavior.
One other comment. I performed the change in the #9 resistor value as described in Ron's Service Tip #43. I used a 1W resistor as was described in tip #3, but later noticed that that was for the Model 80's original 9K resistor. I believe that the 13K resistor needs to be able to handle at least 1.5W
So, I guess I am suggesting an update to Ron's tip #43 to specify using a 3W resistor if changing the #9 resistor.
I have been digging a bit further. All of the spec sheets I have found on the Type 36 tetrode show a maximum screen bias of 90V. However, the change of the 9kOhm resistor to a 13kOhm results in a screen voltage of about 120V. Even subtracting a 10V cathode voltage this is well above the 90V cathode-to-screen-grid values in the spec. I noticed that the Type 77 allows for a higher screen bias.
I haven't tried a 9kOhm resistor yet. I am curious to see how that might affect things.