70 I just found a (very old tube) radio in a (barn, attic, junk sale, etc.). It's complete. Can I plug it in and see if it works?
Description
This article is from the Antique Radios And Phonographs FAQ, by Hank van Cleef vancleef@netcom with numerous contributions by others.
70 I just found a (very old tube) radio in a (barn, attic, junk sale, etc.). It's complete. Can I plug it in and see if it works?
If you didn't hear the radio playing, it would be very wise to do
some resistance checking first.
a. What is the condition of the line cord? Replace it if it is
frayed or the rubber is petrified.
b. Condition of filter capacitors. Wet electrolytics, which
were used in the 1930's, should be replaced without question before
applying any power. These are identifiable by the metal cans with vent
holes on them. Dry electrolytics (which aren't really dry inside) can
also lose their film and be low resistance. If DC resistance between
the B+ line and circuit ground (this may not be chassis ground) is not
500K or more, find out why. Make sure the speaker is included in this
check if it has a field coil or has the output transformer mounted on it.
With electrolytics and any voltage divider resistors out of the B+
circuit, DC resistance should be several meghohms.
c. If it's an AC-DC set, check to see if one side of the line is
wired to the chassis. Many of them were. If so, keep the set away from
any metal objects to avoid shock hazard. Some of the early AC-DC sets
would hum like crazy if they were plugged in with the chassis "hot."
d. If it's an AC set, consider installing a fuse in the line
circuit. 2 amps 250 volts for sets with 80/5Y3, 4 amps 250 volts for
sets with 5Z3/5U4.
e. Do a cosmetic inspection. You'll want to vacuum off any old
dust, dirt, cobwebs, etc. first. Look for things like charred
resistors, melted wax from capacitors, coils, and transformers, and any
indications that the radio go put in the (barn, attic, etc.) because
something was wrong with it.
f. Take a look at the bias circuit for the power output stage.
See below for discussion of typical bias circuits. If there is an
electrolytic in the circuit, make sure it isn't "low ohms." If your
output stage is 6L6's, or if it is filament tubes like 2A3, 6A3, 45, or
47, take a very good look at things.
g. Condition of old wiring is important. Don't fool around
with petrified insulation that is breaking off the wires.
A few hours spent doing a good visual inspection and some ohmmeter
checks can pay off handsomely. If you've got to replace a charred
resistor, find out what burned it out and fix that too, before applying
power to the set.
Remember that 99% of vacuum tube failures are due to open heaters or
filaments. The other 1% are due to gas or interelectrode shorts. This
leaves the item that tube testers have a big BAD-?-GOOD meter to
measure, emission, down in the mud as a tube fault that makes a radio
play poorly. Except for rectifier tubes that have been "sucked dry" by
a gassy output tube or shorted filter cap, most of the tubes I have
diagnosed as causing problems because of low emission would not exhibit
that low emission in a tube tester. Example: a 6SQ7 diode that quit
conducting after 15-20 minutes of playing. Diagnosis was confirmed by
soldering a 1N34 diode across the terminals.
If the getter material (you can see it on glass tubes) is white instead of
silver, the tube is probably gassy----most common on power output and
rectifier tubes. A few sets used gas-filled rectifiers. The 0Z4 is
most common in auto radios, but you may find and old set with an 82 or 83
mercury vapor rectifier.
Also remember that with tube equipment, DANGER, HIGH VOLTAGE. applies.
In home entertainment transformer sets, we are talking about as much as
500 volts, and most smaller transformer sets used somewhere between 250
and 350 volts as the main B+ voltage. The transformerless sets
generally provide 135 volts, and have the mains power (to use the British
term) hooked directly to various circuits and often the chassis as
well.
Continue to:
My Books
