LESSON NO. FIVE - RADIO COURSE

 

DIAGNOSING BY RECEIVER OPERATION

 

There are many receiver faults that can be

identified with a fair degree of accuracy by

simply operating the set and noticing the

quality of reception. It is a great advantage to

the serviceman to be able to diagnose the cause

for complaint quickly and easily in order to

decide what service operations will probably have

to be performed. This lesson will form at least

a brief guide for the beginning serviceman in

training himself to become proficient in quick

diagnosis.

 

 

CUSTOMER COMPLAINTS

 

The general run of customer complaints are

limited to a few categories due to the average

person's difficulty In describing the symptoms.

The most definite complaint and luckily the most

frequent one Is "the set is dead" or "won't

play". As we will see, this is usually the

least difficult service call to handle as the

defect is comparatively easy to locate. Other

customer complaints such as: Intermittent

reception, noisy, poor tone quality, weak, etc.,

are not so easily found and corrected in many

cases.

For reasons of simplification we will take a

series of typical complaints and outline the

reasoning or actual tests to be applied by the

serviceman in diagnosing the cause of the

trouble.

 

 

AVERAGE RECEPTION

 

Before anyone can start to analyse receiver

operation it is absolutely essential that he be-

come familiar with the local reception

conditions for a set of the general type at hand.

In some communities there are a number of

powerful broadcast stations that provide clear, un-

interrupted reception day and night. Even the

most insensitive or smallest receiver may be

expected to bring in these locals satisfactorily.

In other sections of the country there are

many localities that must depend on reception

from more distant or less powerful transmitters

that cannot be received without some fading,

atmospheric interference or static or other

deficiencies except on larger receivers in good

operating condition. The first step, then, in

training to diagnose radio trouble is to become

familiar with the quality of local and distant

reception in the service territory.

 

 

LOCAL DISTURBANCES

 

After gaining experience in judging average

reception in the territory as a whole, the

serviceman should learn to recognize purely

local causes for poor reception. There are two

general effects caused by conditions close to

the customer's receiver. The first is weak

reception caused by the presence of large metallic

masses (steel or reinforced concrete buildings,

mineral deposits, etc.) Receivers in steel

buildings usually require outside aerials to

overcome the absorption or shielding effect of

the metal framework.

Static or popping, crackling, buzzing or snap-

ping sounds may be caused by local electrical

equipment and is therefore called "man-made

static". Properly speaking, static is

atmospheric interference caused by electrical storms,

lightning and other weather disturbances. In

the course of the day's work on radio service

calls the serviceman finds out by direct

comparison what the weather conditions affecting

radio reception are by listening to several

receivers. Naturally, if all receivers experience

the same difficulty in bringing in stations

usually heard without interference the cause is

adverse atmospheric conditions. Very little can

be done to eliminate this trouble. However, the

receiver should be properly installed as to

aerial, ground and electrical connections to

minimize the effect by providing maximum signal

strength from the transmitter to the receiver.

 

 

MAN-MADE STATIC

 

Noises heard along with otherwise normal local

broadcasts are often caused by nearby electrical

equipment in operation.  It is a fundamental

fact that, almost invariably the source of

electrical interference (man-made static) is

some piece of equipment that "makes and breaks"

the current either by design or due to a defect.

There are a multitude of appliance^ found in

homes or offices that create man-made static

during normal operation. All universal (brush

type, AC-DC) motors used on vacuum cleaners,

sewing machines, food mixers, variable speed

fans, electric drills, shavers, etc. cause

electrical interference when running. Induction

motors used on electric clocks, small fans,

phonographs, etc., do not cause Interference as

t here is no interruption of current Involved

except in starting and stopping.

Automatic electrical appliances having

thermostats or time switches like automatic

irons, toasters, waffle Irons, heating pads,

roasters, sign flashers, oil burners,

refrigerators, electric range ovens, etc. may cause

snapping or popping sounds each time they turn

on or off automatically. The time interval

between "snaps" is a guide to the kind of

equipment causing the trouble.

Medical and dental equipment such as X-ray,

violet ray, carbon are sun lamps and therapeutic

machines, sometimes cause a constant buzzing or

whirring type of interference.

 

 

DETERMINING SOURCE OF INTERFERENCE

 

When dealing with a "noise" complaint the

first step is to determine whether the interference

originates within the set or outside the

set. Upon this test depends the decision as to

whether the receiver requires service work or

some local electrical equipment needs attention.

A word or two concerning the means by which

Interference is introduced in or to the receiver

will make further explanations clearer. There

are two Important paths for interference to

follow into a receiver, through the antenna and

through the power line. By far the most frequent

means of entry is the antenna.  Only

rarely does interference reach the receiver

through the actual power line wires.

Electrical Interference caused by any of the

equipment or appliances mentioned above is

virtually "broadcast" by the power line feeding

the equipment unless a filter is used to sup-

press the "noise". The noise impulses are

generated by the arcing during make and break

operation of the appliance. A spark or arc is

a high frequency discharge that creates a whole

series of Instantaneous "signals" . These

Impulses are radiated by the power line near the

appliance. The intensity of the noise signals

depends on the character of the arcing, the

power line and the proximity of the receiver

antenna to the Interfering line.

From the above it follows that the best and

easiest way to find out whether the source of

the noise is Inside or outside of the receiver

is to "short out" the antenna while the set is

operating. If a separate or outside aerial is

used this can be done by shorting a screwdriver

across the "ant." and "gnd." terminals. If an

enclosed loop antenna is used it may be

necessary to use a jumper wire between the end

terminals of the loop in the receiver. If, after

shorting out the antenna, the noise is reduced

sharply or entirely eliminated the source is

outside of the receiver. If the noise continues

undiminished the receiver itself is at fault.

In this lesson we are mainly concerned with

diagnosing troubles quickly rather than giving a

complete procedure for finding and repairing.

We can suggest a few of the most common sources

for noise at this point in the hope that the be-

ginner will be able to correct a certain

percentage of the noise complaints he encounters.

 

 

NOISE ORIGINATING IN RECEIVER

 

If the simple test described above indicates

that the noisy reception is caused by trouble in

the receiver the following equally simple tests

may locate the cause.

 

The tubes are a common source of set interference.

In the lesson on Radio Tubes and Testing we

discussed shorted tubes and described the

mechanical reasons in tube construction for this

defect. An intermittent or partial short between

the tube elements Is the usual cause for

noisy reproduction.

 

The easiest and usually most effective test

for noisy tubes is made by tapping each tube In

turn while the set is operating. This "tapping"

should be done gently with a finger tip or a

light rubber Instrument. Tap each tube lightly

on top and around the sides while listening

attentively for a snapping or popping sound from

the speaker. If one tube Is found that causes a

"crackle" when tapped it is probably the source

of the noise. You cannot be sure until a new

tube Is substituted and the test repeated. The

noise has been corrected if the set operates

smoothly with the new tube.

 

Tubes can cause a constant frying or crackling

sound due to internal defects. Such tubes will

usually be indicated as "shorted" in a sensitive

tube checker. Replacing the suspected tube is

again the only conclusive test.

 

In the process of tapping tubes be careful not

to be misled by the change in volume or the

sound that occurs when the top grid contact on

some tubes is touched with a finger or metal

instrument. In tapping, strike the glass (or

metal) shell of each tube only.

 

It often happens that a noisy receiver will

have a "super-sensitive" loose connection or

defective part that is difficult to locate be-

cause the noise will result when any tube or

portion of the set is rapped or Jostled. It Is

necessary to go over each part in the set, care-

fully tapping QJ- almost "patting" each in turn

to locate the one which is most sensitive to

light Impact.  It may be a tube or some part,

such as a condenser, resistor or coil.

 

A simple loose connection in the wiring of the

set may be the cause. By moving wires and leads

in the receiver while listening will ordinarily

show up a poorly made connection. As all

connections are soldered the correction is to re-

solder the defective joint. Whenever a soldered

connection falls (without a good mechanical

reason like vibration or flexing) it means the

previous soldering was not done properly. A

"cold solder joint" is one in which the wires or

lugs were not heated sufficiently to "flow the

solder." Such a connection may look all right

as the joint is covered with solder but actually

the solder is not in firm contact with the wire

or soldering lug. If the wire is looped through

a terminal and "cold soldered" the connection

will not pull apart but will still cause trouble

eventually. Use a hot soldering iron and heal-

the entire connection thoroughly so that the

solder Is melted by the wires or lug and not

directly by the iron,

 

Look for "shorts" in a receiver that is noisy

when jostled. Bare wires touching metal parts

or other wires to which they are not Intention-

ally connected cause noisy operation. Correct

by separating such wires and if necessary

Insulate or support them to prevent future con-

tact ,

 

Look for corroded connection or charred

insulation on wires, condensers, resistors, or

other parts. Corrosion is a condition often

found in older receivers. In certain locations

1t is a frequent cause for partial shorts.

Charred or burned insulation or other material

sometimes is a sign of active trouble. The

cause of the overheating condition must be found

and corrected or at least it must be determined

that the condition was temporary and has not

affected the operation.

 

The conditions described are only a few of the

possible causes for excessively noisy receiver

operation. For a more thorough check a future

lesson will cover "Trouble Shooting" in the set.

 

 

INTERMITTENT RECEPTION

 

Another common customer complaint Is: "Set

cuts off" or "Cuts in and out" or "Intermittent

reception". There are sometimes other statements

in connection with the above, such as,

"Set cuts in or out when reading lamp is turned

on or off," or Set stops, will start when

jarred".

 

"Intermittent" complaints can sometimes be the

most time-consuming and troublesome to handle If

the set operates normally for long periods of

time. Before a diagnosis can be made it is al-

most essential that the serviceman should

actually hear the set cut in or out.

 

All of the possible causes for noisy operation

already given should be considered and the

entire chassis Inspected for loose or corroded

connections and shorts especially if the set

cuts oat when jarred.

 

Defective tubes are a frequent cause of

intermittent reception. A "thermal break" in the

filament or another element will cause the tube

to cease operating after getting warm or hot.

It may cut in again after a minute or two of

"rest".

 

If turning lights on or off in the house

causes the set to change volume drastically it

is probable that the set is an old one without

automatic volume control (AVC) and furthermore

the antenna and/or ground is not effective. A

newer receiver equipped with AVC may react in a

similar manner but the trouble is more likely to

be found in the receiver. In the latter case

the outside Influence (turning light on) merely

acts as a "trigger" to set off the actual

condition in the receiver.

 

Intermittent reception complaints can be

caused by defects within components of the

receiver like condenser, coils, and resistors.

Later lessons will give details of localizing

such defects. For the beginning serviceman it

is enough to test for defective tubes, bad or

shorted connections (visible) or installation

defects (antenna and/or ground connections). If

the cause for the trouble is more deep-seated an

experienced serviceman should get the cal1.

 

 

TONE QUALITY COMPLAINTS

 

A customer's complaint on tone quality alone

is not a very common one. Usually it is in

combination with other complaints like "Set weak"

or "No power". This is to be expected as it

often is a fault in the power output stage of

the receiver that causes the poor tone or

distortion.

 

Tone quality complaints can be divided into

two general classifications, (1) distortion and

(2) extraneous noises (buzzing, rattling or

"fuzzyness"). Distortion is usually caused by

some component in the receiver having failed or

changed in value. A weak or partially shorted

tube (especially the power output or 2nd

detector tubes) may cause lack of fidelity. If

the tubes are O.K. the next most frequent fault

is in by-pass or coupling condensers or voltage

dropping resistors in the audio or output stage.

The first test to make is a voltage measurement

at the tube socket terminals. The details of

this test will comprize a later lesson.

 

Buzzes, rattles and fuzzy sounds in reproduction

are most often caused by a fault in or

near the speaker. The commonest cause in an

off-center voice coil in the speaker. Dynamic

speakers have a field magnet (either an electro-

magnet or a permanent magnet) that is designed

to concentrate the maximum magnetic field at the

end of a round bar or pole. The voice coil is a

few turns of wire wound on a thin ring attached

to the apex of the speaker cone and supported in

such a way that the coil surrounds but does not

touch the magnet pole piece. The voice coil is

held in position by the "spider" which is a

flexible assembly either inside or outside of

the voice coil. An inside spider is attached

to the cone and voice coil and is adjusted by

means of a screw in the center of the pole

piece. Simply stated, the screw is loosened and

the voice coil spider assembly is moved carefully

until the voice coil is centered exactly

about the pole piece and does not touch at any

point. The same final result is attained with

an external spider but the adjusting screws

(usually two or three) are found between the

cone and the magnet assembly.

 

A loose or torn speaker cone can cause a

rattle or fuzzy sound. The cone must be securely

attached to the speaker rim all around

the flexible edge. More involved speaker

troubles will be covered in a special lesson or

speakers.

 

A listener is sometimes deceived in blaming

the speaker or receiver for a buzz or rattle

during certain types of reproduction when the

actual source is a loose part of the cabinet or

chassis vibrating in sympathy with the normal

speaker tones. Careful observation and a sense

of touch and hearing will locate the offending

part.

 

Receiver Dead (Set Won't Play)

 

We have saved this complaint for the last in

this lesson because it is the most common one

and usually the easiest to handle. There can be

no question of personal "opinion or preference in

this type of complaint as there may be in "tone

quality" or "noise" calls. The set is "dead"

and it is up to the serviceman to make it work

again.

 

A systematic routine to be followed in diagnosing

inoperative receivers will save time and

effort. More servicemen have waited hours of

time by jumping to conclusions and assuming that

certain parts are "all right" without actually

testing than are willing to admit it. There is

nothing to compare with the foolish feeling one

has when, after spending a great deal of time on

a "tough job", one finds that some absurdly

simple fault was overlooked at the very first.

 

 

The first observations to be made are:  (1) Is

the switch turned "on"? (2) Is current reaching

the set? If the dial light or tube filaments

are lit it is safe to assume the house current

is available and the switch is "on". Note. Most

midget and AC-DC sets have the tube filament

connected in series. If one tube is blown none

will light until the "open" filament tube is re-

placed.

 

The next step is to test the tubes on a reliable,

accurate tube checker. Replace any

found to be defective but do not assume that the

others are positively operating unless a new set

of tubes known to be good are substituted. This

tube substitution step is usually not taken

until other more common faults are looked for.

 

If the receiver still falls to function the

next step is to measure the voltage at the tube

socket terminals using a sensitive voltmeter.

The service notes and circuit diagram for that

model of receiver will be found in the Standard

Nomenclature or Repair List books. The voltages

Indicated at the various socket terminals of the

circuit diagram should be found in testing with

a high resistance (1000 ohms per volt) volt-

meter. Any deviation greater than about 10%

usually Indicates where the fault lies.

 

If no plate or screen voltage is found at any

tube the rectifier and filter section should be

suspected. Measure the rectifier AC plate volt-

age (using an AC meter) . If no input (AC) to

the rectifier is present the power transformer

is at fault. When AC voltage is present at the

rectifier plate (or plates) but no DC output

between the rectifier filament (or cathode) and

the plate winding of the transformer the fault

is usually a dead short in the high voltage

circuits of the receiver.

 

The most common causes of a short in the high

(or "B") voltage lines are shorted filter

condensers or by-pass condensers.  In order

to locate the faulty one, each filter and by-pass

condenser must be disconnected and tested

Individually using a continuity or ohmmeter.

Another more indirect method Involves disconnecting

each condenser in turn and again measuring the

voltage at the rectifier output. When

the shorted part is disconnected the voltage

will become normal.

 

When all voltages are normal at the tube

sockets but no signal is heard the speaker

should be tested. A quick but somewhat

Inconclusive test can be made by:  (l) listening for

the slight hum always present in an operating

speaker if the rectifier and output power stage

are operating or (2) pull and push  the output

tube out of and into its socket several times

while listening for a "click" or snap from the

speaker Indicating it is "alive". If both tests

give negative results the speaker and attached

output transformer should be suspected and given

more thorough tests as outlined in a later les-

son.

 

Before "trouble shooting" further let us re-

view the procedure so far and see what has been

accomplished.

 

1. The receiver is getting current from the

wall outlet.

 

2. The tubes are probably O.K. because they

have been tested and bad ones replaced.  (Test

the "new" tubes also before replacing) .

 

3. The power transformer is O.K. because the

filament voltages and plate voltage to the

rectifier have been measured.

 

 

4. The filter and plate by-pass condensers

are not shorted as the plate and screen voltages

at the tubes have been measured.

 

5. The speaker is operating because it reproduces

the characteristic hum or responds with a

distinct "click" when the power output tube is

pulled out and in.

 

All of the tests made so far involve the tube

operating voltage circuits in the receiver

(except the speaker test). The remaining tests

are concerned with the "signal carrying" circuits

which usually include some part of the

voltage circuits. Signal voltages are not

measurable using an ordinary 1000 ohms per volt

meter. A number of more or less involved and

expensive types of test equipment have been

introduced to permit "signal tracing" through the

receiver circuits from the antenna to the

speaker. Without intentionally minmizing the

value of these analysers it can be claimed that

they are not necessary to the competent service-

man.

 

We can now proceed with a few more checks or

tests that should locate the actual trouble with

the receiver or at least indicate the circuit in

which the fault lies.

 

From the circuit diagram determine which is

the 2nd detector tube (in a superhet) . If this

tube is one having a top grid cap, place one

finger on the cap without touching part of the

tube shield or chassis. A whistle or squeal at

the speaker will show at least that -the audio

amplifier and output end of the set is functioning.

 

The course to follow then is to test each previous

stage (or tube circuit) back to the antenna to

find which one is failing to pass the

incoming signal.

 

A signal generator can be used for this purpose.

Tune the signal generator to the Inter-

mediate frequency for the particular receiver

and feed the signal into the grid of the final

I.F. tube. The generator signal should be heard

in the speaker. Repeat this on each I.F. tube

and finally on the grid of* the 1st detector or

translator tube. Logically if a signal Impressed

on the grid of any I.F. tube does not

produce a sound in the speaker, that stage is at

fault if all succeeding stages have passed the

signal in previous tests.

 

Assuming that all tests described up to this

point have given satisfactory results we can say

the trouble with the receiver lies in the oscillator,

radio frequency (R.F.) stage or in the

antenna circuit. The signal generator can be'

turned to some frequency in the broadcast band

(550 to 1600 K.C.) and the signal impressed on

the grid of the R.F. tube (if one is present).

The dial of the set must then be tuned In to the

selected signal. If the signal is heard in the

speaker then the R.F. stage and the oscillator

circuit are O.K. and the trouble must lie in the

antenna circuit. If no signal is heard in the

last test given above then the fault may be in

either the R.F. or oscillator circuit. Further

analysis becomes too involved for discussion in

this lesson. The serviceman who is just gaining

his first experience had better turn the

inoperative receiver over to a more competent man, if

possible. It has been the purpose in this les-

son to present a guide for the beginner to follow

in diagnosing receivers without describing

in detail me final service procedure in re-

storing it to operation. If we succeed in pre-

paring the new man to take care of service calls

involving the simpler troubles covered here,

such as replacing defective tubes, locating and

correcting shorts, opens and burn outs, then we

have prepared him to handle about eighty percent

of the usual run of complaints without help.