Vintage
Radio & Test
Instrument
Restoration 2
Updated 04/11/2021
The Following
Information Published Here Is For The Noncommercial Use Of Radio
Hobbyists
Author credit is given
to:
Radio Wrinkles
Copyright 1995-2004 (C) Philip I.
Nelson
Improving
Safety and Reliability of AA5 Radios
https://antiqueradio.org/safety.htm
The following comments are for those of
you that restore old vintage radios, especially those of you that restore "All
American Five" 5-tube AC/DC radios. These
radios present some serious safety hazards, the worst of these safety hazards is
the potential for a "Hot Chasis", which can result in injury or possibly death.
I'm including the text of the original
article and providing a link to "Radio Wrinkles", link below:
I highly recommend that you take the
time to read the article before attempting to restore an AA5 radio.
The article is very thorough and complete with
schematics.
The "All American Five," or 5-tube AC/DC radio, was manufactured in
vast numbers from the 1940s until the late 1950s or early
1960s.The
"All American Five," or 5-tube AC/DC radio, was manufactured in vast numbers
from the 1940s until the late 1950s or early 1960s. Many models and styles were
produced. Certain "classic" case styles have become collector's items and bring
high prices at auctions and flea markets. Chances are the average radio
collector will have several of this type in his or her
collection.
All of these radios
had very similar circuitry, and alas, also had certain safety and reliability
problems. The worst of these is the "hot chassis" problem. Since there is no
power transformer, the circuitry is powered directly from the AC line. In many
of these sets, one side of the AC line is connected directly to the chassis.
(See Fig. A or B.) That means contact with the chassis could result in a nasty
shock at best, and electrocution at worst.
Many sets
relied on plastic knobs and cases as the only user isolation from the chassis,
yet the screws holding the chassis were exposed underneath the cabinet, or the
back of the chassis could be touched through openings in the rear cover. Many
models employed a "floating" common bus, but since it was impractical to float
the ground on the tuning condenser, it was necessary to have a fixed capacitor
between the floating bus and the chassis for RF purposes. (See Fig. A or B.)
While not a direct connection, typical values of .05-.25 Mfd used for this
capacitor would still allow substantial AC current to pass, so the danger was
not eliminated.
The final
blow to any semblance of safety was the manufacturers' almost universal
placement of the power switch in the negative or "ground" side of the circuit.
This means that you are in danger no matter which way you happen to insert the
plug. In one case, you are in danger when the radio is on, and in the other case
you are in danger when it is off, since the "ground" is now connected to the hot
side of the line through the resistance of the tube heaters. This is still more
than enough energy to kill you. The manufacturers probably placed the switch in
this location to keep it close to the potential of the volume control (on which
the switch is usually located) to minimize hum.
With
reference to Fig. A, assume that a human body is connected to an external
ground. If the cord is plugged in so that L1 ends up the "hot" side of the line
and L2 ends up the "neutral" side of the line, with SW1 in the closed position,
the potential between the chassis ground and the external "real" ground will be
close to zero, and you would be safe.
But, when SW1 is in
the open position, the chassis ground will be at the potential of the "hot" side
of the line, because of the relatively low resistance of the tube filaments.
There is more than enough current through this path to kill a
person.
If we reverse the
plug, putting the hot side of the line on L2, we have a similar result. With SW1
in the closed position, we have the full line voltage on the chassis. The only
thing limiting the current flow through a human body would be the resistance of
the body and the size of the fuse or breaker associated with the wall socket.
With the switch in the open position, the hot side of the line is disconnected
from the circuit, so we are safe as long as we leave the radio
off
The same
condition exists with the "floating ground" circuit. The only difference is the
amount of current that will flow through the R1 C1 combination. (R1 may not be
present in all models, but is usually somewhere around 220K.) With C1 at .05Mfd,
and assuming 60Hz, the available current is about 2.6ma. This may not be enough
to kill you, but it is more than enough to cause heart fibrillation. With C1 at
.22Mfd (not uncommon) the current available rises to about 9ma, enough to
kill.
On the assumption
that the restored radio is going to be played, and not just sit on a shelf
somewhere, it is advisable to address these inherent problems along with the
usual electrical restoration.
It is well
known that the wax/paper capacitors are the most common components to fail. It
is suggested that all of these be replaced with
modern poly dielectric capacitors of at least 400V ratings. These are available
from many sources.
The capacitor that is
across the line should be replaced with a capacitor specifically designed for
this use. (Shown as C2.) You can find these in the Digi-Key and Newark catalogs. Panasonic
ECQ series capacitors are typical. They usually say 125VAC or 250VAC on them,
rather than a DC voltage rating, and have the safety agency mark(s) imprinted on
the capacitor body. This capacitor is critical because it is the one that must
handle all the crap that exists on modern power lines. Light dimmers, electric
shavers and the like may introduce spikes as high as 1500V on standard AC lines.
The agency approved (UL, CSA, VDE) line bypass capacitors (called "X"
capacitors) are designed to survive this kind of punishment. When the AA5's were
made, such transients did not exist on the power lines. If you are making the
polarized cord change, the capacitor shown as C1 in the figures does not have to
meet this requirement.
The
suggested safety modification is to use a polarized line cord, and rewire the AC
switch in the radio so that the "hot" side of the line is switched. (See Fig.
B.) If you are replacing the cord (which is often the case), you can replace it
with a cord set that includes a polarized plug.
If you wish to keep
the existing cord and plug, you can "polarize" the existing plug by cutting from
the end of one prong into the hole in the prong with some heavy diagonal
cutters. This will usually spread the prong enough so that it will fit in a
standard socket only one way. If not, you can spread the prong apart a bit
further. (See Fig. C.)
For new,
polarized cords, use this procedure.
1. Disconnect the existing cord and
remove it, leaving a "stub" or some mark on the connection point for the wire
from the line that does NOT go to the switch. (This is usually the socket of the
rectifier.)
2. Disconnect the remaining wire on
the AC switch. This connects either to a "floating ground" tie point or to the
chassis.
3. Connect the "neutral" wire from
the line cord (the one connecting to the wide pin of the plug) to the chassis or
ground bus where the switch wire was previously connected. Remove the piece of
wire that formerly went to the switch, if necessary.
4. Connect the "other" wire from the
line cord (this will be the "hot" lead) to one lug of the AC
switch.
5. Run a piece of hookup wire from
the other lug of the AC switch to the point you marked or left a stub in step 1.
When replacing a cord, remember to include some kind of strain relief such as a
knot tied inside the chassis.
If you are
keeping the original cord and plug, and doing your own "polarization" as
described earlier, use this procedure.
1. Trace or use an ohmmeter to
determine which prong on the plug end matches the wire that is connected to the
AC switch. This will become the "hot" wire, so use the diagonal cutters as
described above on the OTHER prong.
2. Disconnect the wire on the cord
that is NOT going to the switch. Call the point to which it connected "A."
3. Disconnect the remaining wire from
the AC switch. Call the point to which it connected "B."
4. Run a piece of hookup wire from
the lug on the AC switch that is now open to point "A."
5. Connect the remaining line cord
wire to point "B."
What you are doing is rewiring the circuit so that the hot side of the
line is being switched. (See Fig. B) The neutral side always stays connected,
and thus keeps the chassis at a safe potential regardless of whether the radio
is on or off.
The only
hitch to this scheme is that the outlet into which the radio is plugged must be
wired correctly. I suggest you purchase an inexpensive outlet tester at your
local hardware store and go over all your outlets, making sure they are wired
correctly. The suggested modifications to the radio will not interfere with the
operation of any Ground Fault Interrupters, if they are present. (Schemes using
a 3-wire cord will.)
To verify
your work, set your meter to AC on a scale to read 120V. Check your outlet by
measuring from the hole for the round ground pin or the screw holding the cover
to, the slot for the wide blade. You should measure zero volts. (If you don't,
the outlet is wired wrong, so do not proceed until that problem is corrected.)
Leaving the ground lead in place, switch the other test lead to the slot for the
narrow blade. You should read approximately 120V.
Now plug in
your radio and measure from your ground to the chassis of the radio. You should
still have zero volts. Try this with the radio both off and on. There should be
no difference in the readings. (For an eye opener, try this measurement before
you modify the radio, with the plug inserted both possible ways, and turning the
switch on and off. Be sure YOU are not grounded during this
test.)
With this
modification, the switch and volume control are now at different potentials, so
there is a possibility of hum being introduced. In all the sets I have modified,
I have only had that problem once, which was solved by re-routing one wire.
Careful lead dress during the restoration process should prevent any problems
like this. (In other words, keep all leads as short as
possible.)
If your
radio has no back, there is a further hazard of contact with hot tubes or live
connections. While most adults will have sense enough not to stick their hands
into the back of an open radio, children may not. I've had good luck fabricating
backs out of clear Lucite or PETG. Make a paper pattern, trace it onto the
protective paper on the plastic, and cut the basic shape with a jigsaw. To
insure adequate ventilation, drill a series of 1/4-inch holes about 3/4 inch
from the top edge and the bottom edge. These are not critical, but the idea is
to let air in at the bottom and out at the top.
You will
also need a "mouse hole" for egress of the line cord, which you can make with a
rattail file. If there were no "T" pins to hold the back, you can find a
self-tapping screw that will thread snugly into the back mounting holes, but not
so snugly that it might crack the case.
Remove the
protective plastic from your new back, de-burr the edges and holes if necessary,
and screw in place. The nice thing about a clear back is that you can see the
tubes light up when the radio is on.
Following
these suggestions will result in a radio that will play for many years, and be
as safe as any other modern household appliance.
Bob Krueger,
AB7CQ
Web
Administrator
RPTR 1:
146.920/146.320 PL 123.0 (WIRES-X Room 28187)
RPTR 2:
444.600 / 449.600 PL 100 (LAN Linked)
Simplex IRLP
Node: 7515 PL114.8 (146.540)
Email: ab7cqradio@ebidpal.com
You are here: Home-Restoration 2
Previous Topic: Restoration 1 Next Topic: Restoration 3
|