The Zenith a.m. 6D030 was working admirably until it started swacking very loudly. Thinking it was the IF cans I disassembled them, replaced the mica capacitor sheets, re-assembled them and re tuned the radio, amidst the high volume swaking at higher volumes. If the radio were less sensitive and the oscilloscope wasn't around I would not heard a station. At that point the switch broke and would not function at all. Were two problems created here, or just one; being the switch. The oscillator maybe?

Possibly a microphonic audio tube or hardened rubber socket vibration mounts on same (if so equipped). If the set has been recently recapped, another possibility is that the audio section lead dress has been disturbed such that regenerative audio feedback is now happening by stray audio signal coupling. A less likely situation is that the rectifier tube is very weak and so high-current demand from the audio output stage (under high-volume-setting conditions) causes audio-frequency signal fluctuations through the B+ bus. Generally this last problem shows up as poor sound quality first, not howling. Problem probably has nothing to do with the RF/IF stages of the set since these stages are not affected by the volume control setting. Did you test all the tubes prior to reworking the IF padder caps?

:Possibly a microphonic audio tube or hardened rubber socket vibration mounts on same (if so equipped). If the set has been recently recapped, another possibility is that the audio section lead dress has been disturbed such that regenerative audio feedback is now happening by stray audio signal coupling. A less likely situation is that the rectifier tube is very weak and so high-current demand from the audio output stage (under high-volume-setting conditions) causes audio-frequency signal fluctuations through the B+ bus. Generally this last problem shows up as poor sound quality first, not howling. Problem probably has nothing to do with the RF/IF stages of the set since these stages are not affected by the volume control setting. Did you test all the tubes prior to reworking the IF padder caps?
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The tubes were functioning before and after. The hollowing only occurs on mid to high volume, but it seems to be getting worse the longer the sets on. I don't have a tube tester, so I had to trust the salesman when he sold me the NOS stock.
How stable is the 12BA6?. While the radio was live I may have touched two posts at the same time to test voltages. ZOT!!!If the tube is vulnerable to shorts that may be it. The rectifier tube barely has 5 hours of working time, but its still 50 years old, like the rest of them. Know any companies that make new tubes for old radios? Ill accept russian made if they don't charge a mint.

::Possibly a microphonic audio tube or hardened rubber socket vibration mounts on same (if so equipped). If the set has been recently recapped, another possibility is that the audio section lead dress has been disturbed such that regenerative audio feedback is now happening by stray audio signal coupling. A less likely situation is that the rectifier tube is very weak and so high-current demand from the audio output stage (under high-volume-setting conditions) causes audio-frequency signal fluctuations through the B+ bus. Generally this last problem shows up as poor sound quality first, not howling. Problem probably has nothing to do with the RF/IF stages of the set since these stages are not affected by the volume control setting. Did you test all the tubes prior to reworking the IF padder caps?
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:The tubes were functioning before and after. The hollowing only occurs on mid to high volume, but it seems to be getting worse the longer the sets on. I don't have a tube tester, so I had to trust the salesman when he sold me the NOS stock.
:How stable is the 12BA6?. While the radio was live I may have touched two posts at the same time to test voltages. ZOT!!!If the tube is vulnerable to shorts that may be it. The rectifier tube barely has 5 hours of working time, but its still 50 years old, like the rest of them. Know any companies that make new tubes for old radios? Ill accept russian made if they don't charge a mint.
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"another possibility is that the audio section lead dress has been disturbed such that regenerative audio feedback is now happening by stray audio signal coupling".

How do I insure proper wire distribution in the audio section lead area if I'm really sure the wires were placed almost exactly where they were before? If this is the case manipulating the wires will live shouldn't be a problem.

Tim: Do you have the Rider's information for your Zenith? Many include great photographs of the underside of the chassis and you can see where the wires should be running. Also, regarding older tubes. Most hold up very well over time because they are sealed airtight. I don't think you need a brand new one to insure that it is working okay. Mark from Kalamazoo

:Tim: Do you have the Rider's information for your Zenith? Many include great photographs of the underside of the chassis and you can see where the wires should be running. Also, regarding older tubes. Most hold up very well over time because they are sealed airtight. I don't think you need a brand new one to insure that it is working okay. Mark from Kalamazoo
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Oops. Meant to say Photofacts, not Riders! Mark from kalamazoo

::Tim: Do you have the Rider's information for your Zenith? Many include great photographs of the underside of the chassis and you can see where the wires should be running. Also, regarding older tubes. Most hold up very well over time because they are sealed airtight. I don't think you need a brand new one to insure that it is working okay. Mark from Kalamazoo
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:Oops. Meant to say Photofacts, not Riders! Mark from kalamazoo
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Having the pictures would have helped with the basic wiring but preventing unnecessary work and creating more problems is tough to avoid without the theory. Its like hit and miss compared to isolating the problem and knowing its going to work with the repair. CV's good at keeping at a basic level. Thankfully.

::Tim: Do you have the Rider's information for your Zenith? Many include great photographs of the underside of the chassis and you can see where the wires should be running. Also, regarding older tubes. Most hold up very well over time because they are sealed airtight. I don't think you need a brand new one to insure that it is working okay. Mark from Kalamazoo
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:Oops. Meant to say Photofacts, not Riders! Mark from kalamazoo
:
Having the pictures would have helped with the basic wiring but preventing unnecessary work and creating more problems is tough to avoid without the theory. Its like hit and miss compared to isolating the problem and knowing its going to work with the repair. CV's good at keeping at a basic level. Thankfully.

If you don't have access to a tube tester, the other option is to buy another set of tubes and try substituting each one, one tube at a time. Tubes are very robust and you are unlikely to damage one by misprobing. About the worst that could happen is that you might burn open the heater filament, which would ruin the tube of course, but in a series-wired set like this such a fault would be immediately obvious. It's unlikely that you could induce an inter-element short at the voltage levels in a typical radio.

If the set has been definitely recapped or rewired, you can use a wooden chopstick or other hefty, nonconductive probe to move wires/caps around in the audio amp sections while the set is playing to see if that has any effect on the noise. But if it were me, I wouldn't bother to do that until I had exonerated the tubes one way (tester) or the other (substitution). Just because a tube is NOS and tests good doesn't guarantee that it is actually "good".

Did you check the B+ voltage? Low B+ voltage can cause all sorts of weird symptoms. If it is low, that could be a clue that you have problems with the filter caps.

:If you don't have access to a tube tester, the other option is to buy another set of tubes and try substituting each one, one tube at a time. Tubes are very robust and you are unlikely to damage one by misprobing. About the worst that could happen is that you might burn open the heater filament, which would ruin the tube of course, but in a series-wired set like this such a fault would be immediately obvious. It's unlikely that you could induce an inter-element short at the voltage levels in a typical radio.
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:If the set has been definitely recapped or rewired, you can use a wooden chopstick or other hefty, nonconductive probe to move wires/caps around in the audio amp sections while the set is playing to see if that has any effect on the noise. But if it were me, I wouldn't bother to do that until I had exonerated the tubes one way (tester) or the other (substitution). Just because a tube is NOS and tests good doesn't guarantee that it is actually "good".
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:Did you check the B+ voltage? Low B+ voltage can cause all sorts of weird symptoms. If it is low, that could be a clue that you have problems with the filter caps.
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I should have thought of that before tearing the IF cans apart. When the switch and NOS tubes come in Ill dig into it.

:Tim: Do you have the Rider's information for your Zenith? Many include great photographs of the underside of the chassis and you can see where the wires should be running. Also, regarding older tubes. Most hold up very well over time because they are sealed airtight. I don't think you need a brand new one to insure that it is working okay. Mark from Kalamazoo
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Thanks for the reply. I replaced all the caps, resistors and wires as well as rewound the oscillator and wave trap coil. II Couldn't have done that without the schematic. The only component I cant replace is the switch. Its values are so specific I am told that only a genuine replace will work. Its in the mail. About the tubes. I learning radio theory as I go. Tricky stuff. Not dressing the wires correctly can cause excessive noise. The schematic did not tell me that.