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September 1945 Radio-Craft
 [Table of Contents]
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio-Craft,
published 1929 - 1953. All copyrights are hereby acknowledged.
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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.
Sight, Hearing, Touch, Smell and Taste Are Valuable Instruments for Checking
Receivers
  By Lyle Treakle
During 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 needs rebuilding!
All information is as brief and as non-technical as possible so that the novice
may derive all possible benefit from the information given.
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 test oscillator.
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 trimmer.- Editor)
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 tapping.
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
normally.
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 connections.
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 May 4, 2021
(updated from original post on 8/20/2014)
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