RCA Radiola 17 Operating and maintenance manual

RCA
Radiola 17
SERVICE NOTES
RCA Radiola 17
First Edition—Nov. 1927 R17—1
Radio Corporation of America
SERVICE DIVISION OF THE PRODUCTION AND SERVICE DEPARTMENT
233 BROADWAY, NEW YORK CITY
DISTRICT SERVICE STATIONS
326 Broadway 1412 Monroe St., N. W. 2001 West Pershing Road 274 Brannan St.
New York City Washington, D. C. Chicago, 111. San Francisco, Cal.

A WORD OR TWO ABOUT SERVICE
Service goes hand in hand with sales. The well informed RCA Dealer renders service at time of
sale in affording information as to proper installation and upkeep. Subsequent service and repair may
be required by reason of wear and tear and mishandling, to the end that Radiola owners may be
entirely satisfied.
Obviously this service can best be rendered at point of contact and therefore Dealers and
Distributors who are properly equipped with a knowledge of the design and operation of Radiolas
occupy a favorable position to contract for this work.
To assist in promoting this phase of the Dealers' business the Service Division of the RCA has
prepared a series of Service Notes—of which this booklet is a part—containing technical information
and practical helps in servicing Radiolas.
This information has been compiled from experience with RCA Dealers' service problems, and
presents the best practice in dealing with them. A careful reading of these Service Notes will establish
their value to Dealer and Distributor, and it is suggested they be preserved for ready reference.
In addition to supplying the Service Notes the RCA, through its Service Stations, has available
to Dealer and Distributor the services of engineers who are qualified to render valuable help in solving
service problems.
Property of Radio Corporation of America. Confidential and to be used only by its authorized distributors
and dealers in furnishing service in connection with its apparatus.
Copyright 1927—Radio Corporation of America.

CONTENTS
A Word or Two About Service ………
Introduction …………..........................
Service Data Chart................................
PART I—SERVICE DATA
PART II—MAKING REPLACEMENTS
ILLUSTRATIONS
Page
RCA Radiola 17 ...............„.............................:........ 1
Top View of Chassis Assembly................................. 4
Socket Power Unit Showing Parts.................................. 5
Radiotron Sequence ................................................... 5
Schematic Circuit Diagram of Receiver………………. 6
Schematic Circuit Diagram of S.P.U.............................. 7
Radiotron Socket Contacts ........................................ 8
Method of Cleaning Radiotron Prongs.......................... 9
Releasing Volume Control ......................................... 10
Adjusting Contact Arm of Volume Control ................ 11
Turning Cable Adjusting Screw...... …………… .. 12
Condenser Cable and Drum Mechanism……………. 13
Schematic Circuit for Resistance Measurement.......... 14
Adjusting Potentiometer tor Minimum Hum ............. 16
Page
View of Pilot Light Socket and Canopy. …….….... 17
Wiring Diagram of Sub-Chassis Assembly ……....... 18
Continuity Wiring Diagram of S.P.U.......................... 19
Internal Connections of Filter Condensers ............... 20
Schematic Circuit for Securing Grid Voltages.......... 21
Removing Receiver Assembly from Cabinet…… ….22
emoving Receiver Assembly from Cab-22
Page
2
5
32
Page
Radiotron Sequence .................................................... 6
Circuit Characteristics ................................................ 6
Radiotrons.....;..................................................... 7
Antenna Installation (Outdoor)........................ 8
Antenna Installation (Indoor)........................... 9
Ground ........................................;.............................. 9
Antenna System Failures ....................................... 10
Radiotron Sockets...............:.............................. 10
Radiotron Prongs........................................................ 10
Loose Volume Control Contact Arm.......... 11
Adjustment for Slack Drum Control........ 11
Mechanical Hum......................................................... 12
Broken Condenser Drive Cable........................ 13
Loudspeaker Polarity .
Uncontrolled Oscillation
Distorted Reproduction
Audio Howl ......................
Acoustic Howl ........
Hum ............................
Line Control Switch ............
Wiring Cable .............................
Pilot Lamp and Canopy
Filter Condensers ..................
Voltage Readings ..................
Grid and Plate Voltages
Heating of Cabinet ...............
Continuity Tests .....................
Page
13
. 14
.... 15
.. I5
.... 16
... .16
......18
..... 19
..... 19
..... 20
..... 20
..... 21
..... 22
..... 23
Page
Replacing Volume Control ........................................... 24
Replacing Radio Frequency Coils................................... 25
Replacing Radiotron Gang Sockets................................. 26
Replacing Main Tuning Condensers and Drive …...... . 26
Replacing Large By-Pass Condensers............................. 27
Replacing Audio Transformers ....................................... 28
Replacing Output Transformer........................................ 28
Page
28
29
30
30
31
31
31
Replacing Condenser Drive Cable.........
Replacing Dial Scales............................................
Replacing Power Cable................................
Replacing Filter Condenser Assembly....
Replacing Either Power Transformer or Filter Reactor
Replacing Terminal Strip
Replacing Miscellaneous Parts in S.P.U....
R F. Transformer Connections...................................24
By-Pass Condenser Connections................................25
A. P. Transformer Connections..................................26
Output Transformer Connections...............................26
Replacing Dial Scales ...............................................27
Color Scheme of Power Cable Connections………. 29

POWER CABLE TUNING CONDENSERS OUTPUT TRANSFORMER
Figure 1—Top view of chassis assembly showing the principal parts.
ANTENNA AND
GROUND
CONNECTIONS

RCA RADIOLA 17
SERVICE NOTES PREPARED BY RCA SERVICE
DIVISION
INTRODUCTION
RCA Radiola 17 is a six-tube tuned radio frequency receiver (Figure 1), utilizing RCA Radiotrons
UX-226, UY-227, UX-171 and the Radiotron full wave rectifier UX-280 in the socket power unit
(Figure 2). The use of Radiotrons UX-226, UY-227, and UX-171, using raw alternating current for
filament supply,
FILTER CONDENSERS FILTER REACTOR POWER TRANSFORMER
Figure 2—Socket Power Unit showing various parts.
and Radiotron UX-280 in a plate and grid supply unit makes Radiola 17 a complete socket power
receiver operating on 105-125 volts, 50 to 60 cycle A. C. lines.
Very little service work should be required on Radiola 17. However, the following notes are
published for the guidance of those called upon to locate and remedy any trouble that may occur.
To
Ant
6 5 4 3 2 1
2nd 1st Detector 3rd 2nd 1st
A.F. A.F. R.F. R.F. R.F.
Radio Frequency
To
Loudspeaker
Figure 3—Radiola sequence in RCA Radiola 17
TERMINAL
STRIP

PART I—SERVICE DATA
(1) RADIOTRON SEQUENCE
Figure 3 illustrates the sequence of the Radiotrons in the receiver proper, omitting Radiotron UX-
280 in the socket power unit. From right to left, when facing the front of the Radiola, the Radiotron
sequence is as follows:
Radiotron No. 1 is an untuned stage of radio frequency amplification. It is coupled directly to the
antenna and ground and is not tuned in any way.
Radiotron No. 2 is a stage of tuned R. F. amplification employing a grid resistance to prevent
oscillation. It is tuned by the first gang condenser.
Radiotron No. 3 is the second stage of tuned E. F. amplification. It also
Figure 4—Schematic circuit diagram of receiver assembly.
employs a grid resistance for the purpose of stabilizing or preventing self oscillation in the circuit. It is
tuned by the second of the main tuning condensers. Radiotron No. 4 is the detector tuned by the third-
gang condenser. Radiotrons No. 5 and No. 6 are respectively the first and second stages of audio
frequency amplification. The last stage, Radiotron No. 6, employs power amplifier Radiotron UX-171.
(2) CIRCUIT CHARACTERISTICS
The following principles are incorporated in the circuit design of Radiola 17
(Figure 4 and 4A.)
1. A three-gang condenser, employed to tune two of the radio frequency circuits and the detector
circuit, provides one tuning control.
2. An aperiodic antenna, or first E. F. circuit, eliminates the necessity for a separate antenna tuning
control.
3. The volume control regulates the input grid voltage to the first R. F. amplifier stage. This is the most
practical method of volume control for use
6

with A. C. Radiotrons and gives a smooth control of volume without distortion.
4. No neutralizing condensers are employed. Grid resistances in the two tuned radio frequency stages
effectively prevent any tendency to self oscillation.
5. Raw A. C. of the correct voltage is used for filament heating of all Radiotrons. This eliminates the
use of "A" batteries.
6. The three B. F. stages and the first audio stage receive a plate voltage of 135 volts in conjunction
with a negative grid bias of 9 volts. The detector receives 45 volts plate supply without grid bias. The last
audio stage receives a, plate supply sufficient to provide ample loudspeaker output. The plate and grid
voltages are supplied by means of a built-in " B " and " C " supply using Radiotron UX-280 as the
rectifying device.
7. Radiotron UX-171 in the last audio stage provides ample volume without distortion in loudspeaker
reproduction.
Figure 4A—Schematic circuit diagram of socket power unit.
The various circuit characteristics of Radiola 17 provide for easy installation and simple operation
coupled with quality reproduction delivered to the loudspeaker.
(3) RADIOTRONS
Radiotrons UX-226 are used in all radio frequency amplifying stages and in the first audio
amplifying stage. It has an oxide coated filament consuming 1.05 amperes at 1.5 volts.
Radiotron TJY-227 is used for the detector. It operates on raw A. C. for filament supply, making
use of an indirectly heated cathode. This Radiotron has five prongs, the extra prong being connected to
the oxide coated cathode. Under normal conditions Radiotron UY-227 should give little trouble.
However, in some cases a slight howl may develop in the detector circuit which will necessitate
substituting another UY-227 Radiotron. Although a howl may develop in a receiver with one UY-227,
in another the same tube may prove 0. K. On examining a Radiotron UY-227 in operation, a slight
flickering of the heater
7

element, incased in its insulating material, may be noticed. This condition in no way affects the normal
operation of the Radiotron. The lag in the transference of heat from the heater element to the cathode, as
evidenced when starting and stopping the operation of the tube, takes care of any variations indicated by
this flicker, which supposedly might affect the normal operation of Radiotron UY-227. In Radiola 17
there is a positive potential of 9 volts applied to the cathode of Radiotron "UY-227 with the negative side
of this potential connected to the center connection of the potentiometer across the heater winding for this
Radiotron. This prevents a possibility of the cathode emitting any electrons back to the heater instead of
to the plate.
The power-amplifier Radiotron UX-171 in the last audio stage is interchangeable with the new
EGA Radiotron UX-171A. An output transformer protects the loudspeaker windings from the high plate
voltage used with this Radiotron.
Radiotron UX-280 is a full wave rectifying Radiotron used to rectify the alternating current into
pulsating direct current, which is smoothed out by means of a filtering system, and used to provide all
plate and biasing voltages.
Figure 5—Radiotron socket contacts
ANTENNA INSTALLATION (Outdoor Type)
Due to the high sensitivity of Radiola 17 the most efficient antenna system is one of
approximately 25 feet in length—depending upon local conditions—measured from the far end
of the antenna to the ground connection. It should be erected as high as can be conveniently arranged
and as far removed from all obstructions as possible. The lead-in should preferably be a continuation
of the antenna itself, thus avoiding all splices which introduce additional resistance to the antenna
system and which may in time corrode sufficiently to seriously affect reception. If, however, it is
absolutely necessary to splice the lead-in to the antenna, the joint must be carefully soldered to insure a
good electrical contact. Excess flux should be cleaned off and the connection carefully covered with
rubber tape to protect it from the oxidization effects of the atmosphere.
The antenna and lead-in should be supported by high grade glass or porcelain insulators. At no
point should the antenna or lead-in wire come in contact with any part of the building. The lead-in wire
should be brought through the wall or window frame and insulated therefrom by a porcelain tube. The
use of a flat "window-strip" type of lead-in is not recommended.
8
(4)

The antenna should not cross either over or under any electric light, traction or power line and
should be at right angles to these lines and other antenna. It is desirable to keep the lead-in a foot or
more from the building where possible. When an outdoor antenna is used it should be protected by
means of an approved lightning arrester, in accordance with the requirements of the National Fire
Underwriters' Code.
(5) ANTENNA INSTALLATION (Indoor Type)
"Where the installation of an outdoor antenna is not practical, satisfactory results may generally
be obtained by using an indoor antenna consisting of about 25 feet of insulated wire strung around the
picture molding or placed under a rug. In buildings where metal lathing is employed satisfactory
results are not always possible with this type of antenna. Under such conditions various arrangements
of the indoor antenna should be tried to secure satisfactory results. An indoor antenna is not as
efficient as a properly installed outdoor antenna.
Figure 6—Method used to clean Radiotron prongs.
(6) GROUND
A good ground is quite as important as the antenna. No specific recommendations can be given
in this matter as conditions vary in different locations. Water and steam pipes usually make good
grounds. G-as pipes usually make poor grounds and, as a rule, are to be avoided. If neither water nor
steam pipes are available, a pipe or metal rod may be driven into the ground to a depth of several feet.
The success of this type of ground depends upon the moisture present in the soil. The ground lead
should be. connected by means of an approved ground clamp to a section of pipe that has been scraped
and thoroughly cleaned. The connection should be inspected from time to time to make certain that a
clean and tight electrical contact exists between the clamp and pipe.
It is recommended that the service man experiment with various grounds, and employ the one
giving the best results. Radiola 17 is capable of receiving over good distances when connected to an
efficient antenna and a low resistance ground.
9

If the results of experiments seem to indicate that a good ground is not possible, the use of a
counterpoise is suggested if local conditions permit. A counterpoise is in effect a second antenna. It
should be about six feet above ground, well insulated, of the same dimensions as the antenna and
located directly under it. The counterpoise is connected to the Radiola in place of the ground
connection.
(7) ANTENNA SYSTEM FAILURES
Complaints of swinging signals, or of intermittent reception with probable grating noises, as
distinguished from fading effects, are generally the result of antenna and ground system failures and to
this, therefore, the service man should give his first attention. A grating noise may be caused by a poor
lead-in connection to the antenna, or antenna touching some metallic surface, such as the edge of tin
roof, drain pipe, etc. By disconnecting the antenna and ground
Figure 7—Releasing mounting screws holding volume control.
leads from Radiola 17 and noting whether or not the grating continues, the service man can soon
determine whether or not the cause of complaint is within or external to the receiver and plan his
service work accordingly.
(8) RADIOTRON SOCKETS
The sockets in Radiola 17 are of the standard gang UX and UY type (Figure 5). The three-gang
socket is for the radio frequency amplifiers; the single socket—a five-prong detector socket is for
Radiotron UY-227 and the two-gang socket is for the audio frequency amplifiers. Care must be
exercised when inserting Radiotrons in the sockets. A socket contact may not be in its correct position
and forced insertion of a tube will bend or break it. If care is exercised and the Radiotron inserted
gently, little trouble will be experienced with socket contacts. A bent one will be noticeable on
inspection and may be corrected by inserting a narrow instrument in the socket hole and pushing the
contact into its correct position. A badly bent or broken socket contact must be replaced.
(9) RADIOTRON PRONGS
Dirty Radiotron prongs may cause noisy operation or change the resistance of the filament circuit
sufficiently to cause a hum in the loudspeaker. They should therefore be cleaned periodically to insure
good contact.
10

The potentiometers (Part I, Section 19) should be readjusted for the position of minimum hum
whenever the Radiotron prongs are cleaned.
The prongs should be cleaned by using a piece of fine sandpaper (Figure 6). The use of emery
cloth or steel wool is not recommended. Before re-inserting Radiotrons in their sockets wipe the
prongs and base carefully to make certain that all particles of sand are removed.
In placing Radiotrons in the UX sockets care should be exercised to make certain that the two
large pins and two small pins of the Radiotrons match the socket holes. The UY-227 Radiotron has
five prongs all of the same size and will fit in the socket only one way. If a Radiotron will not fit into
a socket without considerable pressure being applied, look for excessive solder on one or more of the
prongs. Excessive solder on prongs may be removed with a file or knife.
Figure 8—Adjusting the contact arm to secure improved contact
with resistor strip.
(10) LOOSE VOLUME CONTROL CONTACT ARM
A loose volume control contact may cause noisy or intermittent operation and should be
remedied. If the contact arm is loose, the remedy is to bend it slightly so that it makes firm contact
against the resistance strip. In order to do this it is necessary to remove the chassis from the cabinet as
described in Part II, Section 1. The volume control is then readily accessible. By removing the two
screws (Figure 7), that hold it to the metal frame it may be completely removed. The small U-shaped
washer is removed from the shaft and the spring contact arm is pulled out to clear the resistor strip.
The spring contact arm may now be bent sufficiently to make a good contact. Figure 8 illustrates the
bending of this contact arm. After adjusting the spring contact arm, replace the mounting screws and
return the chassis to the cabinet and replace screws and control knobs.
(11) ADJUSTMENT FOR SLACK DRUM CONTROL
The main tuning condensers are controlled by a cable and drum arrangement giving a smoothly
acting vernier movement that has no back lash. (See Figure 10.)
11

After considerable wear, or extreme changes of temperature the cable may become slack. To take
up this slack open lid of cabinet and turn the cable adjusting screw with clamp until the cable is taut.
(See Figure 9.) In extreme cases as might occur after considerable use and several adjustments this
screw may become seated thus allowing no further tightening of the cable. When this condition occurs
it will be necessary to slip the cable a half turn on the grooved drum. To make this adjustment it is
necessary to remove the chassis from the cabinet as described in Part II, Section 1. Remove the cable
adjusting screw and clamp (see Figure 10). The cable will then have approximately one inch slack. By
removing the tapered pin holding the front grooved drum to its shaft and replacing it on the opposite
side (180 degrees) the one-inch slack in the cable can be taken up by using the new position of the pin
for anchoring the cable. Figure 10 illustrates this operation. It will be noted that the tapered pin in the
Figure 9—Turning cable adjusting screw to take up slack in tuning drum cable.
new position cannot be inserted as far as originally. However, it can be inserted far enough to lock the
grooved drum to the control shaft and clear the metal housing. If the cable again is stretched to the
maximum adjustment of the cable adjusting screw the tapered pin can be returned to its original
position and a half turn slipped on the drum which will provide for taking up all slack. Sufficient
grooves are provided on the drum for this purpose.
(12) MECHANICAL HUM
A mechanical hum caused by vibration of loose laminations in the power transformer may be
corrected by removing the power transformer from the S. P. XL as described in Part II, Section 12, and
heating it in a slow oven. The open end of the transformer should be kept up and the wax heated
sufficiently to allow it to adhere to the laminations of the transformer. After heating, the transformer
should be allowed to cool for at least 24 hours and then returned to the S. P. U.
12

(13) BROKEN CONDENSER DRIVE CABLE
Should a cable become broken, due to considerable use or excessive tightening, the proper
remedy is to replace the cable. The procedure for making this replacement is described in Part II,
Section 8. However if a new cable is not immediately available a temporary repair may be made in the
following manner provided the break in the cable is not in that section that passes over the small
grooved drums.
The two ends should be spliced together and then soldered. Splicing consists of interweaving the
strands as with rope and not just twisting the cable
Figure 10—Radiola 17 three-gang condenser cable and
drum operating mechanism.
ends together as in an electrical wiring splice. Splicing gives greater strength and results in a smaller
body being formed on the cable. When soldering, use plenty of flux and a small amount of solder.
Heat sufficiently long for the solder to adhere to all the small strands of the cable.. Placing the splice
in an alcohol or bunsen flame affords sufficient heat and allows any excess solder to drip away. It is to
be understood that this is but a temporary repair and should be used only until a new cable can be
procured and installed.
(14) LOUDSPEAKER POLARITY
Due to Radiola 17 using an output transformer, there is no polarity to the output current of the
receiver. Consequently, when connecting any type of loudspeaker (either horn type or cone type) the
speaker should be connected in the manner that gives the most pleasing reproduction.
13
MAXIMUM
ADJUST OF CABLE
ADJUSTING SCREW
CABLE ADJUSTIJNG SCREQ
REMOVED TO ALLOW
HALF-TURN EXTENSION
OF CABLE ON DRUM
DOTTED LINES INDICATE
REFERSAL OF PIN AND HALF-
TURN EXTENSION OF CABLE
ON DRUM

(15) UNCONTROLLED OSCILLATION
Should Radiola 17 oscillate or regenerate at any point in the tuning range the trouble is probably
caused by:
1. Defective grid resistor in second or third E. F. stages.
2. Excessive filament voltage.
3. Antenna lead too close to third E. F. transformer.
4. Detector tube howling.
In the case of No. 1 the grid resistance of Radiola 17 may be checked by means of a resistance
bridge. If' a resistance bridge is not available the voltmeter-ammeter method gives accurate results
provided the meters used are calibrated accurately. This method makes use of a milliammeter with a
scale of 0-500 and a voltmeter of 0-7 volts. A voltage is then applied that will give a substantial
reading. A circuit diagram of this method is shown in Figure 12.
E VOLTS
ACROSS RESISTANCE R= ______ OR 1000 ____________
TO BE MEAUSERED I MILLIAMPERES
V 0-7 6 VOLTS
0 - 500
Figure 12—Schematic circuit for resistance measurement.
The resistance may then be calculated by the use of Ohms law.
E
R ==—(Where R equals ohms, E equals volts and I equals amperes)
I
Volts
or 1000 ___________
Milliamperes 1
Since the current reading is taken in milliamperes (or —— ampere) it is necessary
to multiply by 1000 to get the resistance value in ohms. 1000
In the case of No. 2 the line switch should be placed at the position of minimum brilliancy for
the pilot lamp. Excessive filament voltage on the E. F. amplifiers may cause these tubes to oscillate at
some point on the tuning scale.
In No. 3 the remedy is to place the antenna lead at a greater distance from the third E. F.
transformer than that normally used. This can be done by slipping the antenna lead from under the
chassis and taking it out of the cabinet close to its connection at the volume control.
A detector tube may cause oscillation or a howl, very similar to a microphonic howl. The remedy
in this case is to interchange the detector Radiotron with another UY-227 Radiotron. A tube may howl
in one Radiola 17 and perform normally in another.
14
MA

In some cases with certain antennas, the Radiola may oscillate even though everything is 0. K.
The remedy is to change the antenna length or interchange the UX-226 Radiotrons in the R. F. stages.
(16) DISTORTED REPRODUCTION
Under normal conditions Radiola 17 will deliver a strong signal of good quality to the loudspeaker.
If the loudspeaker production is poor test the loud speaker output from the receiver. A pair of phones or
a loudspeaker of known quality may be used for this purpose. Poor quality or distortion may be due to
any of the following causes:
1 High or low plate and grid voltages from socket power unit This may be due to a defective
Radiotron UX-280 or tapped resistance unit. The remedy is to replace the Radiotron UX-280 with one of
known quality or check the various resistances of the tapped resistor for a possible short or open. _
2 Defective Radiotrons. Though the Radiola may be in operating condition a defective Radiotron in
any stage will cause distortion. This is especially true of the detector, 1st and 2nd audio stages and the
rectifier tube.
3.. Potentiometers not properly adjusted. Unless the potentiometers are correctly adjusted sufficient
hum may be present to cause distortion. The correct adjustment of these potentiometers is described in
Part I, Section 19
Should Radiola 17 become noisy in operation or signals come in and die out abruptly with periods
of hum or no reception, test in the following manner:
(a) Disconnect antenna and ground leads. If the Radiola becomes quiet and signals from local
stations, though weak, are received it would be an indication that the trouble is either in the antenna
system or is caused by nearby interfering electrical apparatus. The remedy in the first case is to repair
the antenna system and in the second place Radio Frequency chokes on any offending nearby
apparatus. The location of interfering electrical machinery and the cure will require patience, skill and
experimenting.
(b) If disconnecting the antenna and ground system does not eliminate the noise the trouble is in
the Radiola. A defective tube, one having poorly welded elements would cause a disturbance of this kind
and this point should be checked by interchanging the Radiotrons in the Radiola with others of the
same type. If it is definitely established that the Radiotrons are ok then the contact between the
Radiotron prongs and the socket contacts should be examined for a dirty or poor contact. The three
potentiometers in the Socket Power Unit and the Volume Control should be examined for a dirty or poor
contact between the contact arm and the resistor strip.
(17) AUDIO HOWL
Radiola 17 may have a tendency to howl when first installed. This can usually be remedied by
interchanging the detector Radiotron with another UY-227 Radiotron. If this does not remedy the
trouble try the following:
(a) Put the line control switch in the position that gives the least light at the pilot lamp. This
reduces the filament voltage on all the Radiotrons which may be high, causing oscillation on the part of
the R. F. amplifiers.
15

(b) Place antenna lead at a greater distance from third R. F. transformer than that normally used by
slipping it from under the chassis and taking it out of the cabinet close to its connection at the volume
control.
If the Radiola has been in operation for a considerable time and a howl develops, the following
points should be checked for possible defect:
(a) Defective Radiotrons. A Radiotron after considerable use may cause a howl. Substituting a
Radiotron of known condition will isolate the defective one.
(b) Open audio by-pass condensers.
Figure 13—Using a long screwdriver in adjusting the
potentiometer for minimum hum.
(c) Defective grid leak or open grid connection of any tube in the Radiola
except Radiotron UX-280.
(d) Open E. F. grid resistor.
Any part found defective should be replaced and any open or poor connection should be repaired.
(18) ACOUSTIC HOWL
Generally speaking, Radiola 17 is much less susceptible to acoustic howling
due to microphonic tubes than receiving sets using other than A. G. tubes. However on some occasions
acoustic howling may be experienced and the loudspeaker location must be chosen with care. This
howl is somewhat different from the usual microphonic howl in that it disappears when a station is
tuned in, but still causes some distortion in the received signal. The remedy is to interchange the UY-
227. detector tube with another of a similar type or change the position of the loudspeaker. In extreme
cases both remedies may be necessary.
Three potentiometers are provided in Radiola 17 for the suppression of any AC hum These
potentiometers are adjusted for the correct electrical center of the filaments of Radiotrons UX-226, UY-
227 and UX-171. The following procedure should be used in eliminating hum:
16

(a) Place set in normal operation with loudspeaker connected.
(b) Remove Radiotrons 2 and 3, counting from left to right (first audio and detector stages facing
front of Radiola).
(c) Locate position of three potentiometers in power unit.
(d) Adjust potentiometer, located at extreme right when facing front of Radiola (Figure 13), for
position of minimum hum.
(e) Now replace Radiotron No. 2 (UX-226), previously removed, and
Figure 14—Detailed view of the pilot light socket and canopy.
adjust the potentiometer located at the extreme left (facing the front of the Radiola) for position of
minimum hum.
(f) Replace Radiotron UY-227 and with the Radiotron in normal operation
adjust the center potentiometer for minimum hum.
Under normal conditions these three adjustments will suppress any noticeable hum in the
loudspeaker.
If the foregoing procedure does not reduce hum, try the following:
(a) In some cases when adjusting the potentiometers there may he no apparent point of minimum
hum. This is due to low line voltage and may be remedied by throwing the line switch to the position
that gives maximum brilliancy of the pilot lamp. If this does not remedy the trouble, try changing the
position of the line switch to each location several times. There may be a dirty contact in this switch,
making a high resistance connection which may cause the filaments to glow below normal brilliancy.
When this condition is present it will be impossible to adjust the potentiometers for minimum hum
until the filament
temperature of all Radiotrons is normal.
(b) When adjusting the UX-226 potentiometer (at the left) it may be noted that the position of
minimum hum is at one extreme of the potentiometer. When this is encountered the potentiometer
should be arbitrarily placed at its center position and then the UY-227 Radiotron placed in its socket
and the center potentiometer adjusted for minimum hum. The left potentiometer may now be re-ad-
17

justed for a further minimum value, which will not now be located at one of
the extreme positions.
(c) After making any potentiometer adjustment, further reduction of hum may be attempted by
reversing the input plug.
When adjusting the potentiometer at the extreme right with a metallic screw driver a flash will occur
from the screw driver to any part of the frame that the screw driver may touch. This is normal and does no
harm. If it is desired to avoid this condition, an insulated shaft screw driver or a metallic screw driver
wrapped with insulating tape should be used.
(.d) If these various adjustments suppress the A. C. hum correctly, but after a short time the hum
reappears it is a good indication that some of the
Figure 15—Wiring diagram of sub-chassis assembly showing color scheme
and connections with relative location of parts.
Radiotrons are making poor socket contacts, thus destroying the electrical center of the filament
potentiometers. These prongs should be cleaned as described in Part I, Section 9.
If at any time the Radiola is changed from one electrical outlet to another outlet or Radiotrons are
interchanged or replaced with others it may be necessary to readjust one or more of the potentiometers.
(20) LINE CONTROL SWITCH
The line switch should be kept at the position of least brilliancy for the pilot lamp, provided there is
no change in signal strength. This will materially increase the life of all the Radiotrons used in Radiola
17 and in many
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cases give better loudspeaker reproduction. If however an A. C. voltmeter is obtainable or the line
voltage can be obtained from the power company -the following adjustment should be made at this
switch:—For lines from 105 to 115 volts keep the switch at the 110-volt position. If the line voltage is
from 115 to 125 the switch should be placed at the 120-volt position.
(21) WIRING CABLE
On examination of the chassis wiring in some models of Radiola 17 there will be noticed a green dead end
wire, about 6 inches long, connected to the third
Figure 16—Continuity wiring diagram of socket power wit and color
scheme of wiring.
R. F. coil. This is normal and no attention should be paid to the presence of this lead.
(22) PILOT LAMP AND CANOPY
Radiola 17 is equipped with a small pilot lamp (Figure 14), operating from the UX-171 filament
winding for illuminating the dial and indicating that the Radiola is in operation. The latter use is quite
important because when starting Radiola 17 approximately 30 seconds are required to bring the detector
UY-227 into operating condition. The lamp and canopy are packed separately and must be installed
when the Radiola is first placed in operation. The pilot lamp
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is a standard T-3 Mazda, miniature base, 6 volt, 0.15 ampere light and is screwed into its base directly
over the tuning dial. The canopy has three projections which fit three holes directly over this light.
Should this lamp be damaged or burn out a new one can be obtained on the open market.
(23) FILTER CONDENSERS

In general a defective filter condenser will be indicated by the plates of Radiotron UX-280
heating excessively, with the set giving weak, distorted or no reproduction and a loud hum. When this
condition is experienced, the condenser bank should be disconnected from the circuit and the
condensers tested with a reasonably high voltage, not over 200 volts. The correct way to test filter
condensers is to charge and discharge them, being careful not to come in contact with the terminals.
Figure 17 illustrates filter condenser connections.
3.5 MFD
Figure 17—Internal connections of filter condensers
(24) VOLTAGE READINGS
When checking a Radiola 17 for possible defects it is good practice to check the voltage of the
various sources of current. To do this a service man will need both an A. C. and a D. C. voltmeter, the
D. C. meter to be of at least 600 ohms per volt in resistance. The following voltages should be obtained
at the terminal strip when the set is in operation with full load on the socket power unit (Figure 4A).
The terminal strip numbers are from front to rear, No. 1 being closest to the front of the Radiola and
No. 11 closest to the rear.
Terminals Correct Voltage
1 to 2 1.5 A. C.
3 to 4 2.5 A. C.
5 to 6 5.0 A. C.
7 to 8 45 D. C.
7 to 9 135 D. C.
Gnd. to 10 ' 165 D. C. (Approx.)
7 to 11 9 D. C.
11 to adjusting screw of TTX-171 potentiometer 30 D. C.
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3.5 MFD
3.5MFD
1MFD
3MFD
1MFD
2MFD