Zenith 12-S-256 number one.
One or more of these tubes needs to be wiggled to make this radio play. I found tubes two and five to cause problems.
I went over this chassis cleaning up solder connections, wrapping lead wires around solder lugs, cleaning and lubricating. This chassis received a full alignment.
The dial scale was cleaned and the dial needle stripped and painted white.
Any original resistor was replaced and the Candohm resistor which is known to go bad or intermittent was replaced with wire wound cermet resistors.
If the speaker is intermittent, (audio output transformer, voice coil, field coil, plug or wires), it would render the radio silent. The dial lamp and tube filaments lit and glowing. The B+ high voltage flows through the field coil and the audio output transformer. The audio modulates the voice coil from the secondary of the audio output transformer. I do not have the speaker here to test.
I should mention the 456 kHz IF transformers were way out of alignment. It is surprising how this radio received at all. Perhaps the IF misalignment inhibited the radio from receiving. It was simple luck that moving tubes and cycling power allowed the radio to receive. The band adjustments were considerably out of adjustment as well.
Tubes 2 and 5 caused the radio to quit playing when moved around. I cleaned and retentioned the sockets. All the tubes were tested with a Heathkit emissions tester and passed. Tube 12 the eye tube is a bit dim.
In conclusion, this radio was restored by some one. And I think they cared. Each lead wire was dressed and formed to look good. Unfortunately I think the persons soldering technique fell a bit short of reliable solder joints. I could not prove any of the solder joints caused the intermittent failures. I could not also find a hard failing tube socket.
The three main contributing factors as I see it, in order, are:
1) Tube sockets, cracked when wiggled,
2) Candohm resistor, an old
3) Solder Joints, easy to miss when inexperienced even when consciences.
There is one electronic system left unchecked. The original speaker with the frame mounted audio output transformer. I used a test bench speaker and transformer. If this failure happens again some person will need to check the speaker assembly with the transformer.
Keep Practicing! and remember, Soldering is Fun!
This is the second 1204 chassis in for restoration.
So far all If transformers and the antenna transformer show good DC continuity. One bad 6L7 tube totally dead. The two 6V6 push pull tubes are "?" questionable. I will later see if they still work or need replacements.
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