SCR-284-A Part 4      



             Section 4 contains information on the PE-104 Vibrator Supply. Vibrator info. Testing using low voltage AC and Capacitor replacement.



             INDEX Click to Navigate         
         Version ID
        Ballast Info.
        Vibrator and Test Tools
        PE-104 Testing using low voltage AC         
        PE-104 Capacitor replacement





                                           Version ID

        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 (red arrow) 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     RED arrows indicate early set components.
              Shown above 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.
                                      Ballast Info
                                     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 measurements use the "Radio Ground" as reference.



                               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 capacitors 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 completely 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 and in part 5 of this series.



        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.

                         Click to enlarge       TM 11-310 schematic.
           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 ballast depicted.



                  Download the applicable pages of TM 11-275.

                 Download the applicable 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 primarily 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 variance 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 selenium rectifiers age and the voltage drops.

               A. That is a characteristic of the selenium rectifier.

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

                A. You canít use the Diode test on a Digital Multimeter but testing each side using the Mega Ohms range 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 Ballast. There were thousands of the PE-104s manufactured in many different locations under a different "Order Numbers".

               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 similar 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 ballast and installed a better voltage regulator or varied the transformer circuit bridge diodes for the 1.5 volt buss on the output winding of the transformer. Since all the ballast's 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 Ballast.

               Q. How about using a zenier?
               A. There are no standard zenier diodes 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 originally installed. I want every thing to be original.

               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 and in part 5.  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 voltage 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. And in Part 5.

                                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 at 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 basic power supply components. Vibrator replacement with a solid state unit and additional filament regulation will be discussed in Part 5 of this series.  

                                                Test Tools


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


                When wiring your plug add extra bare wires to be used for meter probe tabs.

     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.



       Resistor 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 adjustable pot and monitor current on my Simpson 260. A large power pot is shown above but the 1.5V circuit only consumes approximately a couple of watts. You should measure approximately 400 ma with all filaments active.








                                                   Buffer Capacitor
                                RED arrow points to the buffer capacitor.

               As a general rule when working on older vibrator supplies you should replace the "Buffer Capacitor". Red arrow points to 4C1 a .2(point two) uF cap. It is fairly difficult to remove 4C1 from the chassis as it is held in by a robust bracket. Additional Info: Yellow arrow points to the original -1.5 volt bridge rectifier. The Green arrow points to the original Ballast Rectifier 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 addition the Ballast Rectifier is MIA. A 1.5 volt negative regulator circuit has been installed in the upper right of the chassis. More info below.


           The vibrator housing can be carefully pried 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. This will help in identifying the NC contacts.



     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.



          Links provided below for additional Vibrator Information


           CLICK here for a Vibrator Article published in "Electric Radio"       


           CLICK here for an excellent "QST" article by W1JPE written in 1941 on "Vibrator Power Supplies" presented here with permission of the American Radio Relay League.





          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" capacitors 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 penetrating 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 above 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 didn'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 appropriate 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 in Part 5.


                                       Low Voltage AC Testing. Vibrator not needed.

                                           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 transformer 4T1 for testing of the PE-104 circuits. REMOVE VIBRATOR. Temporary diodes will be soldered on the vibrator socket pins for the HV and Bias windings half wave rectifiers.

                   Disconnect radio
                   Remove Vibrator
                   Install Diodes
                   Select 12 V on the PE-104 for initial testing.
                   Apply 6.3 volts


          When bench testing with an external AC voltage such as 6 volts you can expect the PE-104 power supply output voltages to only approximate the published values. Do not connect the PE-104 to your radio unless you are certain the voltages are within specifications and tested under load.  A variac on the filament or similar transformer input can come in handy.                  

               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 and output. Low voltage AC will be wired to pins 1 and 5 for this bench test in order to check out major components.



       A solid state vibrator board can be substituted for the vibrator. More info in Part 5 of this series.

                                  Capacitor Replacement


                    Most of the large value capacitors are easily replaced if necessary.


    Hum on the receiver? Check 4C15. It was very leaky on several of my power supplies. The newer capacitor (blue) is slightly smaller. 4C15 is a 4500 uF and is the main filter for the 1.5 volt filament buss.                     Note the arrows on the vibrator to aid in alignment during insertion.

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


 A preliminary check can be made on the capacitors using a ohm meter. A more detailed evaluation of a capacitor can be made using a capacitor analyzer.

First discharge the capacitor by applying a short across the terminals. Disconnect one wire. Using an ohm meter the cap will initially show a low resistance and then gradually increases toward the infinity. This indication means that Capacitor passes the preliminary test. An Intermediate reading of thousands of ohms indicates a leaky cap. Try this resistance test on a good capacitor for practice.




         As you explore the wiring using the "Connection Diagrams" - label the caps in case you have to come back to a section to trouble shoot.


                      CLICK to enlarge

      Multiple capacitors may not be arranged in numerical order. The 4C10 through 4C13 bank are mounted out of order. The order goes 11-10-12-14-13 with 13 being at the bottom next to the chassis. Consult the "Connection Diagrams".



             Label as you go through the circuits it will help with trouble shooting.


                         Possible suspect capacitors to be replaced.

       In several power supplies the 4C15 and 4C2 were leaky and replaced.


                 Part 5 will discuss the solid state vibrator replacement, Filament voltage and Bias voltage adjustment

                                       UNDER CONSTRUCTION  Continue with Part 5

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