and don't play with any caps in the
"front end" of the receiver as these are
rarely paper except for a few that may
be in the power supply decoupling
circuits. These, you replace.
A leaky coupling cap on the grid
of an audio preamp in a higher power
output amp can cause that tube to go
into full conduction and drive one of
the output tubes full-on to the point
that it will burn out the primary of the
output transformer — a common
problem in many old console radios.
Your best friends will be found on
the forum at www.antiqueradios
.com. These guys — if they don't own
your particular model — will know
about it, and will be able to help you
tremendously in its repair and
restoration. Don't visit the forum as a
lurker. Sign up. It's free (but they'd like
a donation, if possible). Then, you can
be set to be notified when there's
activity on your thread.
Also, don't throw away all those
tubes you replaced. There's probably
nothing wrong with them.
#2 Since you are producing "static,"
you know the power supply, final
amplifiers, and speakers are okay.
Short of having a schematic and using
a signal generator and oscilloscope to
check for proper operation, I would
replace all of the capacitors. Old
capacitors have a tendency to go bad
more so than the inductors. An old
technique involved "jumpering" a
known good capacitor around a
suspect one, but in the case of an old
unit, replacement may be the best
Also check for cracked resistors
and loose wiring (I am assuming this
unit pre-dates circuit boards), and
clean the switches, contactors, and
tuning capacitors. IF this fails, you
need a signal generator (frequencies
covering RF and IF for the unit) and
an oscilloscope to check the circuit
for the faulty stage(s).
Tim Brown PhD EE, PE
Honea Path, SC
#3 We need more information
about the receiver. What bands
(frequency range) does it cover
and what antenna are you using?
Generally, you need a long wire
type antenna that is 100' or longer
mounted as high as you can get it.
Propagation will affect what stations
you can hear. You may be listening to
a "dead" band that will come to life at
another time of day.
Also, many shortwave stations
have been relocated to different
frequencies in the last 10 years to free
up spectrum for other use. It's possible
your radio is listening in the spectrum
where stations have vacated. Finally,
the restoration of a "boat anchor" is
often more complex than just a
re-tube and clean-up can fix. A
common failure is with old electrolytic
and paper capacitors.
[#5144 - May 2014]
What Wall Wart?
While on vacation, I managed to
lose the power supply wall wart to a
cheap vintage portable no-name brand
shortwave radio. On the back of the
radio, the power jack says 9 VDC, and
has a C shaped circle and a dot in the
center with a plus sign on the dot. I’m
pretty sure this means nine volts;
positive tip. However, what it doesn't
state is the milliamp rating. If I use a
power supply that has too high or low
an amperage rating, am I in danger
of damaging the radio?
#1 The symbol you see on the radio
indicates a "coaxial" power adapter
plug with "tip positive" polarity. Go to
your local RadioShack and get a
"Universal" wall wart that will deliver
9 VDC at 1,500 (or more) mA DC.
(IMPORTANT: Take your radio to the
store so you can get the proper
"adaptaplug" to use with the wall
I suggest a 1,500 mA (i.e., 1.5A)
or larger current rating to ensure you'll
have enough power. DO NOT FEAR:
A wart with a large current rating WILL
NOT harm the radio. However, a
smaller-than-needed current rating will
quickly burn out the wall wart (or
cause overheating and/or fire hazard).
Hope this helps, and happy listening to your radio.
#2 When choosing a wall wart, you
have to check three things and look
out for a problem with the wall warts.
1. Does the voltage match?
2. Does the polarity match?
3. Does the wall wart put out
The catch is with the voltage. Wall
warts come in four different flavors:
1. Transformer — unregulated.
2. Transformer — regulated.
3. Switching — unregulated (very
4. Switching — regulated.
The belief that the general public
has about power supplies is that the
supply will drive whatever current it is
This is WRONG in capital letters.
The device will draw whatever current
it is rated at for the rated voltage. The
power supply must be able to supply
at least as much current as the device
needs. If it can't, the voltage will fall
off. If it is rated at more current than
necessary, this won't hurt anything as
long as the maximum voltage is not
exceeded. This is the catch.
This is why it’s important to check
unregulated wall warts at their rated
current. A nine volt unregulated power
supply may read as high as 18 volts
with no load on it. So, if an unregulated supply is rated at one amp at nine
volts and the device only needs 250
mA or 1/4, amp the voltage output of
>>>YOUR ELECTRONICS QUESTIONS ANSWERED HERE BY N&V READERS
Send all questions and answers by email to firstname.lastname@example.org
or via the online form at www.nutsvolts.com/tech-forum
July 2014 79