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RCA Radiola 17, Service Notes

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RCA Radiola 17 Service Notes Download Page 21

 

 

Any  serious  variation  from  these  voltages  would  indicate  a  defective  resistance  strip  or  power 

transformer.  An  easy  method  to  determine  whether  the  defect  is  in  the  power  transformer  or  the 
resistance strip would be as follows: 

Defective power transformer. 

(a) Any A. G. voltages off the correct value. 
(b) All D. C. voltages, high or low, their differences remaining constant. Defective resistance unit.                                       

.  (a)  Any  D.  G.  voltages  being  either  high  or  low,  but  not  all  consistently  high  or  low  with  all  A.  C. 
voltages correct. 

(25) GRID AND PLATE VOLTAGES

 

In order to intelligently service Radiola 17 it is well to have a good understanding of how the various 
circuits function.. The plate supply and filament 
 

 

Figure

 18

—

Schematic circuit for securing grid biasing voltages.

 

supply systems present no special features not used in similar circuits. However the grid biasing voltages 
are obtained in a slightly different manner from that usually employed to obtain "G" bias voltage. Also 
the —9 volt "C" used to bias the Radiotron UX-226 is also used to keep the heater element of the detec-
tor Radiotron at a negative potential. 

Figure 18 illustrates the grid and plate supply circuit. 

(a)  The  three  plate  voltages  and  the  —9  volt  "C"  potential  are  obtained  from  a  series  resistance 

unit  in  the  regular  manner,  using  the  drop  of  voltage  through  the  resistance  unit  to  obtain  the  desired 
voltage.  The  —9  volt  "  C  "  supply  is  used  as  a  bias  voltage  for  all  Radiotrons  UX-226  and  is  also 
impressed  on  the  UY-227  heater  through  the  center  connection  to  the  UY-227  potentiometer.  The  +9 
volt " C'' is connected to the cathode of this Radiotron. The net result of such an arrangement is to keep 
the heater element of the detector tube at a sufficiently high negative potential to eliminate any tendency 
of the cathode to emit electrons back to the heater rather than to the plate of the tube. 

(b) Referring to Figure 18 we note that the —9 "C" is also marked —30 "C" and connected to the 

grid of Radiotron UX-171. Also a series resistance is placed in the grid return from this resistance and 
connected to the center tap of 

 

 

 

 

 

 

 

 

 

 

21

 

Summary of Contents for Radiola 17

Page 1: ... 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 ...

Page 2: ...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 the...

Page 3: ...tenna 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 V...

Page 4: ...POWER CABLE TUNING CONDENSERS OUTPUT TRANSFORMER Figure 1 Top view of chassis assembly showing the principal parts ANTENNA AND GROUND CONNECTIONS ...

Page 5: ...RS 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 a...

Page 6: ... 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 CHARAC...

Page 7: ...tortion 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...

Page 8: ...tem 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...

Page 9: ...oor 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 nei...

Page 10: ...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...

Page 11: ...ed 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 vol...

Page 12: ...ing 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 t...

Page 13: ...esults 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 un...

Page 14: ...uit 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 positio...

Page 15: ...owing 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 ...

Page 16: ...e 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 m...

Page 17: ...he 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 thi...

Page 18: ... 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 Radi...

Page 19: ...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 operatin...

Page 20: ...ondensers 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 s...

Page 21: ...btain G bias voltage Also the 9 volt C used to bias the Radiotron UX 226 is also used to keep the heater element of the detec tor Radiotron at a negative potential Figure 18 illustrates the grid and plate supply circuit a The three plate voltages and the 9 volt C potential are obtained from a series resistance unit in the regular manner using the drop of voltage through the resistance unit to obta...

Page 22: ...ve drop depending on Figure 19 Removing the receiver assembly from the cabinet where the point of connection is made By connecting point D Figure 18 as a source of positive potential any point toward C will have an increasingly negative potential The value of this negative potential will depend on the resistance connected between C and D In Radiola 17 this negative potential is 30 volts and as sho...

Page 23: ...er Closed Open connection contacts of socket No 4 Lug No 5 to one side of filament Closed Open Connection contact of socket No 6 Lug No 6 to other side of filament Closed Open connection contact of socket No 6 Lug No 7 to cathode contact of Closed Open connection socket No 4 Lug No 8 to plate contact of socket Closed Open primary of first audio trans No 4 former or connection Lug No 9 to plate con...

Page 24: ...primary of power transformer or line switch If open throw switch to other position and test If both open test switch separately Figure 20 R F transformer connections and color scheme of wiring PART II MAKING REPLACEMENTS 1 REPLACING VOLUME CONTROL The following procedure should be used when replacing the volume control 1 Be move the seven screws holding the wooden back to the cabinet 2 Remove knob...

Page 25: ...rt II Section 1 2 Unsolder and tag all connections to the three transformers and the two pin jacks 3 Remove four screws that hold mounting strip to metal chassis The entire assembly is now released and may be removed The new assembly should be placed in the same position occupied by the one just removed 4 Replace the four screws that hold the mounting strip to the metal chassis 5 Replace and resol...

Page 26: ...tion 1 2 Remove and tag all leads to the terminals of the sockets Figure 22 Detail of A F transformer connections and color scheme of wiring F Figure 23 Output transformer connections 3 Drill out rivets holding sockets to metal chassis frame 4 The socket assembly may now be removed and the new one placed in the position occupied by the old one 5 Fasten new socket in place by using small head machi...

Page 27: ...1 2 Remove condenser assembly as described in Part II Section 4 3 The tabs holding the condensers to the chassis may now be bent up by using a screw driver 4 The three fixed condensers are now released as a unit from the chassis frame The defective condenser may be released by bending the tabs that hold it to the other condensers 5 Unsolder the leads of the condenser that are to be replaced Insert...

Page 28: ...et in the reverse order of that used to remove it 7 REPLACING OUTPUT TRANSFORMER The output transformer of Radiola 17 is held in place by means of four tabs which hold the output transformer to the vertical part of the chassis frame A step by step procedure for replacing this unit is as follows 1 Remove receiver chassis assembly from cabinet as described in Part II Section 1 2 Unsolder and tag the...

Page 29: ...p that were previously removed to allow the cable to pass along the groove are replaced By slipping the clamp over the cable and gradually turning up on the cable adjusting screw the cable may be tightened until there is no lost motion in any of the controls Care should be taken not to take up too much as the cable may be stretched or possibly broken 8 Return receiver assembly to cabinet in the re...

Page 30: ...ar on operating switch at front of Radiola 3 Release the cable connecting the socket power unit to the chassis assembly and the two leads to the pilot lamp This is done by loosening the screws holding them to the terminal strip on the socket power unit 4 Remove four screws at bottom of cabinet holding Socket Power Unit in place The Socket Power Unit may now be removed by slipping it out of the bac...

Page 31: ...re should be used 1 Remove S P U from cabinet as described in Part II Section 11 2 Unsolder and tag all leads soldered to terminal strip 3 Release two screws holding strip to S P U base 4 The strip may now be removed and replaced by a new one 5 Fasten new strip in position occupied by old strip by means of two machine screws lock washers and nuts previously removed 6 Solder all leads to terminal s...

Page 32: ...ith high resistance D C voltmeter and A C voltmeter Check socket power unit by means of continuity test and make any repairs or replacements necessary Defective A F transformer Defective output transformer Defective By pass condenser Dirty contact arm of volume control Potentiometers not properly adjusted Dirty prongs on Radiotrons Replace A F transformer assembly Replace output transformer Replac...

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