A step-by-step approach
to vintage radio repairs
As with all items of electronic equipment,
repairs to old valve radios require a
logical commonsense approach. The
f ollowi:qg step-by-step procedure should
yield results in most cases.
While the following suggests a suitable procedure for "dead " receivers,
many of its points are equally applicable to radios with weak reception.
It is not suggested that the procedure outlined should be used followed
to the letter. No-one who smells a
burnt-out power transformer would
persist in making tests on an aerial
coil for example, and would have
enough sense to detour to the appropriate section.
(1) . Examine the set for signs of
overheated components such as power
transformers and resistors, or charring of the rectifier or speaker sockets.
Do not switch the set on for this check.
Check that all valves are tight in their
sockets, the grid clips are on, the
speaker plug is making good contact,
there are no leads broken away, and
that the set is switched to "Broadcast"
and not "Gram" or "Shortwave".
(2). Before turning on the set, check
the condition of the power cord and
plug. If the cord is damaged or stiff
and brittle, or if the plug is cracked or
chipped, replace the cord and/or plug.
Replace with a 3-core cord where possible (except on AC/DC sets) and wire
according to SAA mles.
Switch on the set, watching the rectifier valve to see if it arcs or has a
heavy blue glow. If OK, proceed with
This old Radiola 5 is a popular receiver from the mid-1950s. Although not
really a set to appeal lo the vintage radio enthusiast, it was quite a reasonable
receiver & you should have little difficulty in obtaining parts.
14
SILICON C1i11'
section 3. If arcing is present, a short
circuit is indicated, or maybe a heavy
load. This may be a shorted or leaky
filter capacitor or a short between the
high side of the choke and chassis.
A heavy blue glow may indicate a
leaky filter capacitor or some other
leakage or short on the high tension
line. Some rectifiers may become gassy
w ith age and exhibit a blue glow.
Switch the set off in all cases and
take a reading with an ohmmeter between the cathode (indirectly heated
rectifier) or heater and the chassis.
The reading should not be below 1 QkQ.
If it is, move the ohmmeter to the
output side of the filter choke or drop ping resistor to see if the reading is
higher or lower. If higher, the short or
leakage is on the rectifier side - if
lower it is on the output side. Having
found on which side the fault occurs,
unsolder one component at a time on
that side of the line until the ohmmeter reads lOkQ or higher.
(3). Visually examine the screen of
the output valve. With some typ es of
valves, it may be necessary to make
the observation by looking downward
from above the valve. If the screen is
not glowing, proceed with step (4). If
it is glowing, it indicates that the
screen is receiving voltage while the
plate is not.
Most likely the speaker transformer
has an open circuit primary winding.
If the speaker transformer is mounted
on the speaker, it is possible that there
is a high resistance or open circuit at
the speaker socket or on the wires to
the speaker.
(4). Visually examine all valves to
ascertain if their heaters are lit and
feel the valves to see if they are warm.
If OK, proceed with step 5. Where
heaters are not glowing, take an ohmmeter reading between each valve
heater pin and its socket contact in
turn to ensure that the contact resistance is less than 0. lQ. If the contacts
are OK , check the valve heaters with
an ohmmeter after withdrawing the
suspect valves from their sockets.
(5). Listen for hum in the loudspeaker. If it is present, proceed with
step 6. If not, proceed with step 7.
(6). Turn up the volume control and
touch a finger to the pick-up terminal
(remove the shorting link if necessary) or to the grid cap of the audio
valve (warning: don't do this with a
transformerless set). A loud buzz or
hum in the speaker indicates that the
audio section is functional. If so, proceed to step 8; if not, go to step 7 .
(7). Make a voltage check between
the plate of the output stage and the
chassis, ignoring the screen voltage. If
no click i's heard even though normal
voltage is present,,, and if no hum is
heard in the speaker (see step 5),
switch off and check the voice coil
winding and loudspeaker leads for
shorts or an open circuit. Desolder
one side of the voice coil winding for
this test. If no voltage is present at the
plate, check at the screen. If absent
here also, check at the rectifier cathode and on either side of the choke
and/or field coil.
Make a voltage reading between the
grid of the output valve and the chassis. Any positive reading indicates a
faulty coupling capacitor and it may
also be necessary to replace the valve.
For self-biased sets, check the bias on
the output stage cathode. If absent,
check the bias resistor for continuity
and the cathode bypass capacitor for
a short. Also, check any capacitors (if
present) that connect between the
output valve plate and chassis or plate
and screen, and any capacitors that
form part of any plate circuit tone
control. If the audio fault persists proceed with step 9. If cleared, proceed
with step 8.
(8). Check all plate, screen and bias
voltages on valves prior to the detector stage. If OK, switch off the set and
make resistance checks on all coils. If
OK, proceed with step 9.
(9) . If a valve tester is available, test
the valves for emission and shorts.
Temporarily replace doubtful ones,
particularly the mixer, for faults ahead
of the detector, and the detector for
faults that exist from that stage on. If
OK, proceed with step 10.
(10). Turn the volume control full
on. Switch the set to the broadcast
band and inject a signal from an RF
signal generator at the correct intermediate frequency (IF) to the primary
of the first IF transformer. If no sound
is heard, the fault is after the first IF
transformer. Go to step 13 for further
checks. If OK, proceed with step 11.
(11). Inject a signal at broadcast frequency to the antenna terminal (note:
turn the signal generator output well
up to allow for mistuning). Use a high
frequency (1200--1500kHz) to avoid
misleading results if the variable capacitor plates are shorted. Tune the
set to the approximate frequency. If
no signal is present, go to step 12. If
the signal is weak, check the aerial
coil for shorts. It should not be open,
as the resistance test in step 8 would
have shown this fault.
(12 ). Connect a high resistance multimeter or VTVM (AC range) to the
fixed plates of the oscillator gang. A
reading of between 5 and 30 volts
should be obtained. If not, check the
oscillator grid leak, capacitor and oscillator plate voltage. When the oscillator stage is OK, go to step 13.
(13). Check the AVC and other bypass capacitors, then the plate, screen
and bias resistors. Test the IF and
other trimmers for short circuits. If
OK proceed with step 14.
(14). Connect a meter or CRO to the
voice coil winding and turn the signal
generator off. There should be no reading or pattern (except a straight line).
If there is , oscillation is occuring, causing possible grid blocking or overloading. Short the plate of each valve
from the output stage back in succession with a capacitor to earth to localise the fault. If OK, inject in turn an
AF signal at the pick-up terminals, an
IF signal at the detector and each IF
stage, and a broadcast signal at the
aerial and RF stages, to locate the
faulty stage. There will of course be
no pattern or reading if the voice coil
is shorted but this fault should have
been cleared by check 7.
It is highly probable that most faults
will have been located well before
check 14.
Footnote: this article was contributed by Resurrection Radio, 51 Chapel
St, Windsor, Victoria 3181.
SC
WARNING
Dangerous voltages are present
in valve radio sets. In particular,
stay away from transformerless
AC/DC sets unless you know exactly what you are doing.
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15
