September 1945 Radio-Craft
[Table of Contents]
People old and young enjoy waxing nostalgic about and learning some of the history of early electronics.
Radio-Craft was published from 1929 through 1953. All copyrights are hereby acknowledged. See all articles
Anyone who has done a lot of electronics troubleshooting - especially
on a wide variety of hardware - knows that proficiency depends on
acquiring a 'feel' for how things ought to be. An oft-given example
is that bank tellers and store clerks are taught to spot counterfeit
bills not by showing them what all the different phony currency
looks like, but what real money looks like. That way, anything that
does not look familiar is readily apparent. The same philosophy
has served technicians and engineers well since the beginning of
electrical and electronic circuits. The more senses that can be
drawn upon to facilitate troubleshooting, the more quickly you are
likely to pinpoint the problem. Depending on the symptoms of the
failure, sniffing the assembly for toasted or cracked components
and looking for discolored areas is a good first step for catastrophic
scenarios. Broken wires and failed solder joints are very common
causes, as are partially plugged-in or contaminated connectors (cables,
board-to-board, plug-in components); I have fixed many problems
in the past simply by exercising connector interfaces a few times
to clean contacts. A well-trained ear can also be an invaluable
troubleshooting aid. A recent news story reported on experiments
by medical MRI researchers converting image details into audio signals
to listen for recognizable sound signatures. Some electronic test
equipment has an audio output jack to enable listening to waveforms
for troubleshooting or even tuning. Although obviously not real,
in the movie Contact, the extraterrestrial signal was recognized
by ear rather than relying on software algorithms. A sense of touch
can reveal a loose connection or an overheated component. While
I do not necessarily endorse it, even taste can come in handy for
something like testing the status of a 9 V transistor radio
battery (try sticking your tongue across the + and - terminals of
a 9 V battery sometime... it probably won't kill you, but just
to be safe, please add me to your will first). Of course sight plays
a dominant roll in troubleshooting both from a need to visually
inspect the failed unit and for reading test equipment displays.
Radios Serviced by Observation
Sight, Hearing, Touch, Smell and Taste Are Valuable Instruments
for Checking Receivers
By Lyle Treakle
about fifteen years of radio servicing I have noticed many beginners
(and some not beginners!) tinkering with radios and getting nowhere.
I have worked with a few so-called "engineers" and have seen them
search for many hours to discover trouble that would have been apparent
at once if they had but used their knowledge and observed some things
that are quite plain to see.
Careful observation will locate at least seventy-five percent
of all radio troubles. The following system is one I use all the
time, and it leads me to the trouble quickly, in most cases. Old-timers
will agree that observation is well worth while, but beginners will
find the system something they have wished for since they first
became interested in "fixing" radios. These instructions are not
likely to be of much use to the man who has so much confidence in
his native luck that he plunges into a radio chassis with screwdriver,
pliers and soldering iron and really "fixes" the set - so that it
All information is as brief and as non-technical as possible
so that the novice may derive all possible benefit from the information
Let us suppose that we have a six- to ten-tube superhet on the
bench and that we are preparing to analyze the trouble. However,
this system may be adapted to any other type of circuit also, with
proper consideration given to certain differences of circuit action.
1 - First, see that all tubes are in their proper sockets. Often
the owner has removed the tubes for testing (for free) and frequently
replaces them in the wrong sockets.
2 - Next, turn on the set. If tubes do not light, check line
cord for breaks. On portables especially, check the switch. 3 -
Watch the rectifier tube for signs of over-heating, plates turning
red, etc. Check for shorted filter condensers, shorted sockets or
shorted transformer windings. 4 - Turn chassis over and look for
wires touching, burned-out resistors, etc.
5 - Have the dial set on a strong local station and the volume
control set at maximum position. 6 - Touch the grid cap of the first
audio tube, or the grid terminal. This usually can be easily located
as the grid lead comes from beneath the chassis. In the case of
the single-ended tubes, touch a test prod to the center of the volume
control to get the same results as though your finger were placed
there. A loud clear buzz should be heard if all is well in the audio
end. 7 - If not, pull out the power tube. It should make a thump
in the speaker if there is voltage on the plate of the tube. 8 -
If not, check the voice coil, 9 - On midgets, make sure the pilot
lamp is O.K.
10 - Feel the output transformer. These often become warm when
excess current is flowing through the plate winding. 11 - The small
tone-compensating condenser connected from plate to cathode (or
ground) may be shorted. Disconnect it and see. Or the coupling condenser
may be leaking a positive voltage to the grid, causing the tube
to draw excessive current.
12 - Listen closely to the speaker. There should be some hum
if there is any voltage at all on the power tube. If it is entirely
quiet look for an open voice coil or broken leads to the voice coil.
13 - Listen to the output transformer. You can hear it singing if
the voice coil circuit is broken.
14 - Watch any tuning indicator that may be present. If it indicates
a signal the R.F. end is probably O.K. Electron-ray indicator tubes
appear to burn red when no voltage is supplied to their anodes.
15 - Have a test prod on the lead-in from a long antenna. Touch
the grid of the I.F. tubes. Noise coming through will indicate the
stage is in passable condition. Work back toward the antenna post.
16 - Turn the wave-band switch to be sure it is set on the broadcast
band. If the noise still comes through, but no signal, the oscillator
is perhaps not functioning. 17 - 0ccasionally a strong signal will
force its way through the I.F. Section when the oscillator has stopped.
You can double check this by connecting the test oscillator to the
grid of the first detector tube and setting it at a frequency of
a local station plus the LF. frequency of the receiver. The signal
will come through if that is your only trouble.
18 -Try adjusting the I.F. compensating condensers to be sure
some home mechanic hasn't discovered they were loose and screwed
them down tight. Mark the original setting and don't turn them far
off without returning to the original - especially if you have no
This procedure should not have taken over five minutes, and the
service man should, with a little reasoning, have a good idea as
to where the trouble lies - at least, in which stage it lies.
19 - If you are without the test oscillator, you still can do
a fair job of alignment on a receiver by using the noise pickup
of your antenna. If you should be so (un) fortunate as to have your
shop in an interference-free location, generate noise with a buzzer
or spark coil.
Set the dial at a point where no station is heard. Turn up the
volume control and adjust the I.F. trimmers for the highest noise
level. The noise has very little effect on the AVC action and accurate
adjustment can be made in this manner. 20 - Next, tune in a station
on the high frequency end of the dial and adjust the oscillator
trimmer until the station is received best. Move the dial off the
station and adjust the R.F. trimmers for maximum noise level. Lastly,
set dial at the low frequency and adjust padder for maximum noise.
The broadcast band is now aligned.
(This system will work only on sets with fixed padders in which
no accident has caused the oscillator frequency to be "off." Where
the padder has been screwed down so that the intermediate frequency
generated by the oscillator is - say - 300 kilocycles, an attempt
to align will leave the I.F. tuned to 300 Kc instead of the normal
450-465 used on most radios. The result is that stations will come
in only on that part of the dial at which the receiver has been
"aligned." Attempts to "align" using the high-frequency end of the
dial are equally dependent on the correctness of the oscillator
Shortwave bands can be aligned also in this manner by using the
government monitor station at 2.5, 5, 10 and 15 megacycles to set
the oscillator trimmers, and the noise level to adjust the R.F.
trimmers. Generally, the short wave bands should be .aligned first.
A word or two on cut-out cases. These are in no way difficult.
Locate the section giving the trouble. Then concentrate on that
section. 21 - A pair of headphones clipped in through a small, condenser
to the grid of the first audio tube will indicate whether the trouble
is in the audio end of the receiver. If the signal is still coming
through the phones, connect them to the grid of the second audio
tube, if the set has one. If signal is also in the phones, go back
to the detector. If signal is still heard, the trouble is not in
the I.F. or R.F. sections of the set:
22 - If a test oscillator is available, connect it to the antenna
and tune in the signal. Turn off the modulation. Turn up the volume
control. Any loose connections can easily be heard by probing and
23 - When a suspected open condenser is to be bridged with another
one on a cut-out job, touch one side in the usual manner, then holding
the other lead with the forefinger and thumb, touch the other terminal
with the little finger, thereby charging the condenser slowly through
the fingers before completing the connection. This will not cause
sudden shock which will often make an intermittent radio start operating
24 - If the set is full of birdies, an R.F. or I.F. stage may
be oscillating. This can be located best by touching the lead of
a lead-pencil to the plate lug of the tube socket. A loud click
will be heard when you have located the right one. If it proves
to be an I.F. stage, everything apparently normal, put a resistor
of as high a value as possible across the primary winding of the
I.F. transformer. This will stop the oscillation. Usually 50,000
ohms will take care of it.
Distortion is the cause of many complaints so I will include
some information which may be of some aid in locating the trouble,
especially for the beginning serviceman.
Let us go back to locating the defective section, then the defective
stage, and finally the defective part. After a little practice the
serviceman will be able to distinguish by ear whether the trouble
is in the R.F., audio, or speaker. However, touching the first audio
tube grid may tell the story. A rattle will indicate speaker trouble.
A distorted buzz proves the trouble is in the audio, a clear buzz
indicates it is probably in the R.F. section.
If the distortion is traced to the audio amplifier, check all
voltages carefully, especially the bias. Be sure it is correct before
leaving it. 25 - Check the voltage, grid to cathode. If it is resistance-coupled
you won't get much indication of voltage, but must have some indication
on the output stage. 26 - The grid must have some negative voltage.
If not, check the coupling condenser. If it is shorted or leaky
the power tube may become very hot.
27 - A shorted output transformer will cause poor tone. If it has
been replaced with another, be sure the load matches the characteristics
of the power tube.
28 - If you are using the usual 1000 - ohm-per-volt test meter,
place your leads from grid to ground on all stages, as the grid
may be floating. This may clear the tone. If so, replace the grid
resistor. If the first audio tube is of the pentode type with a
series screen resistor you may not read much voltage. However, turn
your, volt meter scales down. This may not increase the deflection,
but it will lower the supply voltage to the screen. A high voltage
here will cause distortion.
If the distortion trouble has shown up since you have been working
on the set, you have probably caused it yourself. Check over the
work you have done for defective parts, poor soldering - or wrong
R.F. distortion may be due to misalignment or to the wrong bias
voltage. Check for both.
If distortion is only on strong signals, disconnect the antenna.
If this clears up distortion you can be sure it is due to wrong
bias voltage. On the older sets using 24's, etc., voltage under
25 volts on the screen or over 12 volts on the grid will cause the
tube to be unstable. It will be necessary to install super-control
tubes or a local-distance switch to lessen pickup.
Again check the tubes as to their right positions in sockets.
A sharp cut-off type like the 6J7 will not replace a super-control
tube of the 6K7 type, where the volume is, controlled by the C-bias
either manually or with automatic volume control, which includes
almost all sets in use today.
Be thorough. Don't skip a stage until you have checked everything.
Particularly, don't take it for granted that the tubes are in the
right places, even though you may have replaced them in the sockets
yourself. Check them again.
Posted August 20, 2014