SCR-284-A Part 4      



             Section 4 contains information on the PE-104 Vibrator Supply. Vibrator info, Testing using low voltage AC, Capacitor and vibrator replacement. Filament and Bias Voltage adjustments.



             INDEX Click to Navigate         
         Vibrator info.
        PE-104 Testing using AC -
                Test Tools
        PE-104 Capacitor replacement-
        PE-104 Mods and Tips on the 1.5 Volt Filament buss.-
        PE-104 Vibrator Replac

PART 4 still incomplete- more to be added




        When the BC-654 set is powered by the dynamotor PE-103 utilizing a large vehicle battery the PE-104 vibrator supply is powered by the same battery. The PE-104 shown here will provide all the necessary receiver voltages as well as low voltages needed by the Transmitter. The PE-104 vibrator supply can be powered by 6 or 12 volts selectable by internal switch mounted on the power supply.


              Receiver power provided by the PE-104.
                              Plate 90V
                          Bias - On the receiver it is approximately -6 from the PE-104 or approximately - 45 volts from the combined BA-43 battery returns of +90 and -45. This receiver bias voltage is not critical and is used for bias on the volume/RF control circuit.

                 Transmitter Power (low voltage) supplied by PE-104 or BA-43
                          Crystal oscillator filaments -1.5 and carbon microphone bias.
                          Xtal oscillator B +
                          Bias -45V Final Power Amplifier Tubes Suppresser Grid modulation.

                                      Identify which version of the PE-104

                                   Click to enlarge
                                Early schematic of sets up to serial #12000. Red arrows indicate components that were eliminated on later sets. The heavy black line is "Radio Ground" which is not connected to the chassis. Get used to the idea of a "floating" electrical ground that is not connected to the chassis.

              Inspect the area near the rotary switch for three chokes indicating that you have a early version of the PE-104. Chokes missing ? - then you have a later model of the power supply.
                                     CLICK to enlarge
                      Later versions of the PE-104 had a Ballast Rectifier added to the -1.5 (minus 1.5) volt filament circuit to aid regulation. The heavy black line is "Radio Ground" which is not connected to the chassis.This Radio Ground is at a "Positive" potential for the lower voltages and at a Minus potential for the high voltage and is isolated from the chassis. All voltage measurement have to use the "Radio Ground".


                               PHILA-45-10 order describing the addition of the Ballast Rectifier 4A3.



           High voltages are present in the PE-104 vibrator power supply. The high voltage circuits do not have bleeder resistors and need to be discharged prior to performing maintenance.
                                                Ballast Rectifier

            The low voltage filament Ballast Rectifier location is near the "Voltage Change Switch" 4S1. Your unit may not have the Ballast installed. All the ballast rectifiers in several power supplies that I have repaired were either shorted and completly shunted the voltage to "zero" or they were deteriorated to the point where the voltage dropped was excessive and the overall filament voltage was not adequate for the radio. The 1.5 volt filament supply will be discussed later on these pages.


        Ballast Schematic. Note the polarity of the rectifier. The filament Bridge Rectifier circuit provides a -1.5 (minus one point five) voltage to pin 4 of the octal socket 4K1.

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                   This version of the schematic can be found in TM 11-310 in the rear section. It is listed in the manual index as "Power Converter Unit PE-104." Two schematic versions are contained in TM 11-310. Neither schematic in TM 11-310 have the balast depicted.


                  Download the applicable pages of TM 11-275.

                 Download the applicabale page of TM 11-310.



               Q. What was the Ballast used for?
               A. Over voltage regulation of the 1.5 volt filament voltage.

               Q. Why is over voltage regulation of the 1.5 volt filament buss important?
               A. Tube life. High voltage is bad.

               Q. How efficient was the Ballast?
               A. Not very efficient but used primarly to prevent over voltage.

               Q. How could a over voltage of the 1.5 volt buss occur?
               A. According to the TM different vibrators could produced different voltages because of the "switching" efficiency. This did not result in a problem on the High Voltage circuits as they would continue to function with a voltage varaiance of several volts but the 1.5 volt filament buss is fairly critical. Please reference TM 11-275 paragraph 35.

               Q. I've read that selenim rectifiers age and the voltage drops.

               A. That is a characterisstic of the selenium rectifier.

               Q. Can I test a seleniun rectifier with my VOM?

                A. You canít use the Diode test on a Digital Multimeter but testing each side in Mega Ohms works. Its best to get a real analog meter and use the highest resistance range.
               Q. Why was the Ballast not installed in some sets?
               A. Different Signal Corps Order Numbers has the Balast. There were tens of thousands of the PE-104s manufactured in many different locations and each under a different "Order Number".

               Q. I can't find the Ballast listed in the TM 11-275 parts list.
               A. Neither can I. It might be listed in later TM's.You can look up Bridge Rectifier 4A1 it has similiar construction.

               Q. How about using a couple of 1 Amp diodes in series as as a ballast?
               A. Experimenting with several different diodes and using 2 power supply diodes resulted in a voltage of 1.5 to 1.6 volts. Different diodes will have slight different voltage drops. Four (4) Schottly diodes (The string of four dissipates heat better) resulted in a 1.6 voltage drop. Remember that the voltage drop may increase as current increases depending on the diode. On several of my supplies I just ignored the balast and installed a better voltage regulator or varied the bridge diodes for the 1.5 volt buss on the output winding of the transformer. Since all the ballasts that were installed in my power supplies were shorted I just eliminated the wiring. Consider heat sinks for any diodes that are used to replace the Balast.

               Q. How about using a zenier?
               A. There are no standard zeniers in the 1.5 volt range.

               Q. How about using other voltage sensor circuits.
               A. Most of the voltage sensor ICs will not operate in the 1.5 volt range.

               Q. I am getting paranoid about my ballast circuit as it was orginally installed. I want every thing to be originial.

               A. If your ballast is OK then go with it. I personally don't like the ballast circuit. Read all the information on these pages.  My goal is to have a working power supply with a reliable 1.5 volt filament voltage. I like for my sets to work and I like to enhance tube life.

              Q. Why not just install a voltge regulator IC.
              A. Good idea but you will need a higher voltage input such as 6 or 12 volts and use a "Negative" voltage regulator such as a LM 337. But first you may not need an additional regulation circuit. Get your power supply running and then check your 1.5 volt voltage "under load" IAW the TM 11-275 paragraph 35 b and sketch (d).

                            More info on the 1.5 volt buss is presented later on these pages.




                                Vibrator Info

         Note all voltages are measured under load. Without a load the voltages will be higher.

            The main schematic of the PE-104 can be a little daunting to study but to get the general idea of the power supply a block diagram is presented. Please note that the vibrator has a extra set of contacts that perform a rectification cycle on the HV and Bias windings. Its PFM   The 1.5 volt supply has its own bridge rectifier and provides the filament voltage as a negative potential.  The Ballast circuit is not shown.

                The next section will discuss the vibrator followed by a section on testing the supply using low voltage AC in place of the vibrator. This "AC" procedure may come in handy and allow trouble shooting of basis power supply components.  

                                                Test Tools


     An extension cable fabricated from standard 8 pin octal plug and socket.


     Test board for the power supply. 6 or 12 volt input on the right. Pad connections at the top represent pins 1 through 8 on the octal connector. Use a 3.5 ohm resistor to test the -1.5 volt filament supply under load. Use a 100K resistor to test the bias under load. The test board is a simple project and will save a lot of time.



       Test load for the -1.5 volt filament buss. Adjusted to 3.5 (three point five) ohms.
                   There are plenty of 3.5 ohm resistors on ePay but I prefer to use an adjustble pot. A large power pot is shown above but the 1.5V circuit only consumes approximatgely a couple of watts. You should measure approximately 400 ma with all filaments active.









               As a general rule when working on older vibrator supplies you should replace the "Buffer Capacitor". Red arrow points to 4C1 a .2 uF cap. It is fairly difficult to remove 4C1 from the chassis as it is held in by a robust bracket. Additonal Info: Yellow arrow points to the original -1.5 volt bridge rectifer. The Green arrow points to the original Balast Rectifer 4A3.



               I used two .1 uF caps in parallel on this power supply to replace the buffer capacitor 4C1. Look carefully you will see that the bridge rectifier for the -1.5 volt buss has been replaced. In addtion the Balast Rectifer is MIA. 1.5 volt negative regulator circut has been installed. More info below.


           The vibrator housing can be carefully pryed opened. Don't get in a hurry.

              Synchronous Vibrator - note the dual pair of contacts and the Normally Closed pair on the bottom.

             The NC (normally closed) contacts have to be closed and make good contact in order to start the "buzzer" action of the vibrator. Note the position of the long center contact arm that will swing toward the center of the magnet when DC is applied to the coil.



Deoxit can be applied to a thin piece of card board and utilized to clean the contacts. More information on vibrator trouble shooting and testing can be found in my Electric Radio article published in August of 2014. Link below.



Read this for Vibrator Information


           CLICK here for a Vibrator Article (Word Format) published in "Electric Radio"

           CLICK here for the Original Article in PDF format.



          Do your self a favor and remove the vibrator clamp from the chassis. It is a PITA. Adjusting the clamp fingers on the side is futile and it is hard to adjust the clamp when it is installed on the chassis.


          Arrows on top of the vibrator aid in lining up the large pins for insertion. The white lanyard is attached under the vibrator to aid in extraction. Note the two "New" capicators on the left, More info on capacitor replacement below.

             During a vibrator extraction damage to the vibrator socket can occur if too much side action (wiggle) is applied to the vibrator housing. In addition if the vibrator has been seated for several years the pins may have corroded and form a tight bond to the socket pins. Do not clamp the vibrator housing with pliers during the attempt to pull out the vibrator as it may bend the housing and damage the interior. When the vibrator pins are "frozen" to the socket pins you might try a penetraing oil such as KROIL applied to the pins.  
                           Stuck vibrator?

          When attempting to pull out a stuck vibrator you can try pushing from under the socket on the large and small pins with a probe. Be careful and take your time.


          Pins 6 and 1 are larger than the other pins. View shown is looking down at the top of the socket mounted in the chassis. .



           Info: Pin numbers were missing on the early schematic figure 20. I hope I did't screw this up.



                      Socket Pins. The large pins are #6 and #1. Pin #2 is easy to identify as it connects to the buffer capacitor 4C1. The buffer wire was painted Red when I was working on the unit.



                          Q. Can I substitute a standard non Synchronous vibrator by jury rigging the vibrator plug ?
                           A. Yes but diodes will have to be added on the chassis below the vibrator socket to provide rectification for the HV and Bias circuits. One diode for the HV and one diode for Bias. The filament supply (1.5 volts) on the PE-104 has its own bridge rectifier.

                          Q. Can I use a 6 volt substitute vibrator on the PE-104 when I am using a 12 volt battery to power the radio set?

                          A. Yes the PE-104 has a voltage switch which selects approprate taps on the transformer.

                           Q. What is the operating frequency of the vibrator circuit?

                           A. 115 cycles per second.

                            Q. I hate mechanical vibrator power supplies.

                            A. Me too.

                            Q. Is there a electrical substitute for the vibrator.

                            A. I thought you would never ask. YES. See discussion later on.


                                       Low Voltage AC Testing

                      CLICK to enlarge
         Low voltage AC such as output from a 6.3 volt filament transformer may be used to provide a input voltage to transfromer 4T1 for testing of the PE-104 circuits. REMOVE VIBRATOR. Diodes will be needed to temporarily soldered on the vibrator socket for the HV and Bias windings half wave rectifiers.

                   Disconnect radio
                   Remove Vibrator
                   Install Diodes
                   Select 12 V for initial testing.

          When bench testing with an external AC voltage such as 6 volts you can expect the PE-104 power supply output voltages to approximate the published values. Do not connect the PE-104 to your radio unless you are certain the voltages are within specifications.                  

               Suggested Diodes :  1N5408 General Purpose Rectifier Diode 3A 1000V. Any general purpose power supply diode will suffice.


        Temporary diode soldered in place on pins 2 and 6 to test the high voltage winding. Low voltage AC input on pin 1 and 5 for this bench test.

                                        More to be added on solid state vibrator, filament regulation etc  

                           CLICK to enlarge.
                         The receiver should operate with filament voltages as low as 1.3 volts. The nominal voltage is 1.4 volts. Receiver operation was OK during bench testing with the filament supply as low as 1.2 volts. During the lower voltage the receiver converter/oscillator tube 2V2 would stop oscillating and the receiver became inoperative. Over voltage will result in tube life being reduced.

Testing tubes. Do not test filament (continuity) with a analog VOM unless you are sure of the voltages applied during a resistance test. Normally you will use the low ranges of X1 and X10. My Simpson 260 was OK on the lower ranges and used a 1.5 volt battery. On the higher ranges the meter utilized a 9 volt battery.             






                  Q. The bias voltage listed on the transmitter schematic is -51(minus 51 volts) for the PE-104 but for the BA-41 battery the voltage is -45 (minus 45 volts). Is this critical?

                  A. No either voltage will suffice.

                  Q. Will transmitter output be effected by the bias voltage.

                  A. Yes. A low bias voltage will result in increased RF output and modulation will decrease. During bench testing reducing the bias to approx -35 volts resulted in a increase in RF output of a couple of watts with the transmitter in the High power position. Over voltage above 50-60 volts will decrease RF output.

                  Q. How much current does the transmitter bias circuit consume?

                  A. Not very much approximately .5 mA (point five milliamps). Leaky caps will result in a higher value.

                  Q. How long will a bias battery pack consisting of 9 volt batteries last?

                  A. A long time.

                  Q. Should I remove the battery packs from the radio when the set is not in use.

                  A. I would either remove the pack or else disconnect the main connectors on each pack and remove the D cell. However the BC-654 radio has disconnect switches (1S7) installed below the battery cage that are designed to isolate all the battery outputs when the main door is closed. SEE part 2.




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                                        PE-104 Part 4 Info (Under Construction)