■ FIGURE 1. Defective capacitors may cause the
collateral circuit damage shown in this circuit. This
schematic is the quintessential output stage of a low
power table radio. Some components have been omitted for clarity. Refer to your own radio's schematic.
Old capacitors typically found in antique radios dry
up. Electrolytic capacitors such as C1, C2, and C6
lose their capacitance and sometimes develop shorts.
Foil-n-paper capacitors such as C3, C4, and C5 also
lose capacitance and become electrically leaky.
designs sealed in wax. You will be hard-pressed to find an original capacitor that
is within specifications. The electrolytic
capacitors will have lost much of their
capacitance causing your set to hum.
They may even be shorted causing
instant circuit damage when the radio is
energized. The coupling capacitors with
their high leakage are just as bad.
It is worthwhile to investigate the
collateral damage caused by faulty
capacitors. The types of circuit damage are outlined in Figure 1. Ideally,
you would never see this damage. But
that would mean that the radio was
never turned on. This is rarely the case.
Look on eBay where you will find the
telltale descriptions. The Philco radio
was no exception. Its eBay description
read “It works, picks up the local AM
stations. There is a hum in the sound.”
Physically replacing the capacitors
is an easy job. Modern capacitors are
much smaller than the capacitors of
yesteryear, as shown in Photo 3. The
new capacitors are easily mounted
under the chassis of the radio. If you
want to preserve the original look of the
radio, you can install the new modern
capacitors into the original paper cases.
Consider this option if you have a high-value collector’s radio or if you truly
want to maintain the original cosmetics
of the set. The before and after pictures
of the capacitor replacement are shown
in Photos 4 and 5. I chose not to
preserve the original look of the radio.
However, I did save the old capacitors
■ PHOTO 3. New capacitors are
physically smaller than their 1940’s
counterparts. It is possible to mount
them in the old cases to preserve the
in case I change my mind.
You have two options for mounting
the new capacitors. You can completely remove the old capacitor and install
the new part. Alternatively, you can cut
the old capacitor out leaving its connecting wires in place. The new part is
then soldered to the old wire. Each
method has its advantages. Removing
the old leads looks better but could
potentially damage the components
the capacitor is soldered to, such as
tube sockets or stand-off insulators.
The electrolytic capacitors used in
tube radios typically have values from 10
to 100 uF. Under the old naming convention, you will see these components
marked as MFD. Recall that electrolytic
capacitors are polarity conscious. It in
imperative that you install them correctly since they otherwise have a tendency
to explode! Axial capacitors (shown in
Photo 3) are preferred as the wire at
each end stabilizes the component.
Choose voltage ratings equal to or
higher than the original. I used 160 VDC
electrolytic capacitors in this radio. The
actual capacitance value for electrolytic