At this point, with the
major caps replaced, and all of the obvious issues cleaned up, I should have had
better results. But the set remained essentially quiet.
Now with the 100W removed, I again applied power. After warm up, I
checked the voltages, recording the readings in a log book.
The rectifier voltage
was now at ~290V, about 10% low. Most other voltages were 10 to 20% low.
A quick way to check out the audio amplifier is to introduce a signal at the
input to the audio amplifier stage. This should reproduce that signal at
the speaker. By touching the center tap of the volume control with a
screwdriver, or just touching the cap on the 1st audio amplifier tube, a 60
Hz signal is introduced, which should result in a loud hum at the speaker.
this section of the schematic, the input to the audio section is outlined
Although there was some
hum from the speaker, you had to put your ear right at it to hear it, even at
full volume. Not good.
At this point I
suspected either a bad tube, or two, or some other component failure.
Since I hadn't yet found a tube tester, and I had planned a full electrical
rebuild in any event, that was my next step.
finds More Trouble
In preparation for the rebuild, I again ran through both a visual inspection and
an electrical check. This time, though, I was much more methodical, carefully
inspecting each and every part.
capacitor has a slightly bent fin. When the radio dial is below
around 600Khz, the fins short out. Careful work with a needle-nose
pliers took care of that problem.
In walking through the
schematic part-by-part, I discovered a disturbing problem. The schematic
shows that the antenna transformer for the broadcast band should have a
resistance of around 0.1 ohm, but my meter showed that it was open-circuit.
I carefully unsoldered and removed the antenna transformer.
the culprit! This photo, taken through a magnifying glass, shows
that the coil had somehow gotten a bit fried! There should be a fine
wire where the yellow arrow shows open air!
patience, and luck, I was able to catch some solder on the remaining wire
and bridge the gap with a strand of fine wire.
reinstalling the transformer in the chassis, I recalled the shop keeper's
promise that the radio had been working, but maybe just needed a tube.
Maybe in 1938 it was working, but never in the months the shop had it!
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