Porthole Audio Problem

[bgadow]

This is a Zenith model 28T925-R/ch.28F20 (sams 64-15) that I got running a couple years ago. It used to have good audio but now I can only hear a whisper. More than once I've heard this blamed on bad caps in the audio section (silver mica?) and am wondering, is there a specific one or two that usually go? I will probably need to order them and was looking for a shortcut if I could find it. (I have to order some others, would like to do so before next week when I am going to get busy)

(note that all paper/electroytics are replaced already)

[Tubejunke]

Look for a resistor that has probably increased in value. Some people say as long as its no further than 20% out of value, leave it alone. If I have a schematic I tend to go with the tolerance listed. Naturally the higher the value the bigger the increase in resistance. I think thats why folks think that the higher values are more apt to large change of value. Anyway it is always easier to find a bad resistor than a bad cap in my mind. Less variables I guess. You do have to disconect one end on many of them to keep from getting the reading of a lower value resistor I believe in parallel. Series doesent matter. Someone correct me if I'm bass ackwards on this. This may help you some as it is highly probable if you didnt go through them when you replaced the caps.....

[Chad Hauris]

I would try running the 4.5 mhz IF output from the sound IF amp to another set's sound discriminator/output section...I have done this with FM radios, it will tell you what part of the circuit the trouble is in.

Silver mica caps are usually located inside IF transformers..have fixed radios by removing the mica sheets and trimming the contact leads, then adding capacitors outside the can. I'm not sure if the Zenith TV has caps inside the Sound IF cans or not.

I restored a 28T962 with the same chassis about 12 years ago, and had a similar problem. The Zenith TV sets of the late 40s have the same IF transformer problems as their radios of the same time period. These are the transformers which have the silver-plated mica disc capacitor assembly in the base of the transformer, and they are well known for intermittent connections and leakage caused by migration of the silver on the disc. I repaired the defective transformer on mine by measuring the capacitance of one good part of the capacitor with my capacitor checker, and then I removed the disc and wired in new disc capacitors of the correct value.

Check all of the voltages on the tubes in the sound IF, detedtor, and audio circuits.