SCR-511 "Pogo Stick"
        BC-745 RF Section and Chest Set
 Notes

   FLASH: K4CHE checks into the MRCA Sunday night CW net with the pogo.

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      Pogo set used for CW check in.

 

 

                 

                               

             The Pogo Stick radio was very popular with the Civil Air Patrol in the 1950's. CAP was and still is a great organization. Every kid needs a boost - a direction, they gave me one. When I wound up 2 years later as an Aviation Cadet in Navigation Training I knew all about the Air Force (I thought) , close order drill, saluting, BX, chow hall,gig lines,G.I parties etc. so all I had to do was concentrate on the academics, the PT, Officer Training and Flying. A tough year.

 
          Disclaimer: The photo and descriptions on theses k4che.com pages reflect projects that the author has either tested, repaired or fabricated. The use of hand tools such as a hack saw and a electric drill can be hazardous. Solder irons are very hot often producing temperatures in excess of 700 degrees F.  Soldering radio circuits produces noxious fumes and can be addictive. Do not inhale. Seek out therapy if necessary. Group sessions are recommended.  

                Included on these pages.

                Home brew "Ring" connector and bench socket.
               Power requirements and breakout socket.               
               Antenna info
               Tuning Units
               tube location chart      
               Coil mods
               Home brew chest set (battery and audio) (you will never find a real one.                
                        

                

 
     
                              Dave KB3ELD and Mark KD3ZK

              My NOS Pogo Stick RF unit (Receiver-Transmitter) BC-745 came from Dave's pile so now it has been "transferred from Dave's pile to my pile" (note below). Dave and Mark always have the most unusual and interesting WWII surplus items and are setup at Aberdeen in the Spring. Visit their flea market space and you can smell the MFP.

       Note: The   transfer from "My pile to your pile." Expression coined by Al, N3FRQ.

 

 

               

           First several bench problems - - how to provide power connections and PTT. The unit has a very robust PTT switch you have to push down on the antenna mast "housing" when the radio is mounted on the guidon shaft. The ON-OFF switch is buried in the chassis and is activated by the telescoping antenna when it is extended. Since I did not have an "original" antenna I left the switch in the ON position for bench testing.

 
                                      

          The main power connector consists of a very large ring connector mounted on the antenna mast with a wire cord that connects to the chest set. My supply of Pogo Stick accessories is zero and a bench "ring" connector would be handy for testing.
 
                              
                                    W3KJT photo

    I don't have any of this stuff especially the chest set but pressed on anyway with the bench testing.                  

 
                     
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      A substitute chest set which includes batteries was fabricated see page 2.Details later.
Please don't e-mail me about using portion of a telephone handset. N3FRQ did it first.
 

                    

          A connector was fabricated by cutting a piece of lexan and using #12 solid wire for the pins. A small washer and a glob of solder were used to hold the pins in place. 3 inches OD and 1 3/4 ID.
                     KISS

 
                   

                           Just a little solder and a small washer holds each pin.
 
                      

                    The fabricated ring connector can be easily inserted or removed.
 

                   

          Color coded wiring will be the same as on the schematic. The exposed terminals help with voltage measurements.

 
                       

            The chassis connector ring is stamped with color identification.
 
               

       An 8 wire colored cable was used and terminated in a standard 8 pin octal plug. The cable utilizes the same colors as presented on the schematic.
            
              
 

                         

             A break out socket was made for the octal plug on the "Ring" cable. Diodes are across the filament line to prevent those little screw ups when connected the power supplies. Instead of using my standard 1000 Volt 1 Amp Silicon Rectifier Diodes I used 1N5408 diodes as suggested by Arden Allen KB6NAX on the milsurplus list. The 1N5408 has a smaller voltage drop and its rating of 3 amps should survive those little mishaps.

                        1N5408 specs: 3 amp diode which drops 0.75 volts at 1 amp.

 

 

                  

               Power connections. Note that the "left 67 volt battery" is floating above ground.
         The "Right" 67 volt battery minus side is grounded. The 1 1/2 volt filament battery minus
is grounded.

           Remember - - The "left battery" connected to Blue + positive and Yellow - negative must be above
ground.

 

 

 

 
                  

                            Need a quick 67 volt supply? Visit the Dollar Store. Cheap batteries and
they will make it through the ham fest and military rally season.
 

                           

                  The main ON-OFF switch is buried inside the chassis below the ring connector
and is activated by the antenna. During bench testing I left the switch ON with the notched handle of the toggle switch in the UP position towards the connector ring. This switch is old and may be intermittent.   

                                         

 

                         

              Push to Talk is accomplished by pressing down on the thumb ring. The guidon staff must be on a firm surface. The leggins must be laced tight and trousers bloused properly.            

   

            

 
             

                    PTT on the thumb ring activates this multi contact sliding switch similar to the BC-611
 

                  

                In order to facilitate bench testing I disconnected the spring on the PTT assembly. Holding the thumb ring down while testing the transmitter was difficult.. Cover the disconnected spring with spaghetti tubing.

 
             

                 A "temporary coax" connector was connected to the solder terminal on
on the tuning unit chassis connector P1 to facilitate testing. Note the wooden bench mount.
 
              

            A simple wood mount support the chassis when the tuning unit is installed.
 

                   

           Voltage requirements for the Pogo. On transmit two 67 volt batteries
are connected in series via the PTT switching, on receive the receiver uses both 67
volt batteries. During PTT Filaments on V1, V2, and V3 are switched off during transmit.
One half (1/2) of V5 and V6 is switched off during receive.

 

                            
             
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                 Schematic for the BC-745 with the plus side of each 67 volt battery shown in color.  Please note that the receiver requires both 67 volt lines powered in order to function. Receiver tubes require 67 volts and V1,V2,V3(RF-IF) and well as V7 (Osc) are  powered by the "right battery". Receiver tubes V4(Det) and V5-V6 (audio) are power by the "left battery". 

 

                     
                         Tube chart and location Bottom View.
          

 
                 

                   Tube top view. (I flipped it)

 

                             

               A vs. B tuning units. Note that the B unit has a provision for multiple taps which are easily changed to vary the bias .On the A unit fixed resistors are soldered in place with a tap.

              Q. Can you use an A tuning unit on a B RF unit?
     Ans: Yes but expect transmit currents to be slightly higher and slight loss of modulation(approx 15 percent) due to bias differences. But overall perfectly useable.  An A unit can be easily rewired to copy a B unit.

WARNING: Do not transmit without a functioning tuning unit installed and providing bias as excessive plate and screen currents will occur.  On one of my units the current increased during transmit from 60 to 105 ma. Receive current went from 6 to 16.                 

          

 
          A special thanks to Pat KC2RNN who provided the A tuning unit for evaluation.  
                
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      The "A" bias unit utilizes 3 fixed resistors and tap.
 
               
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       The "B" bias is provided by a 4 resistor network for transmit and receive. The units that I examined did not have a receive tap installed.

 

                  

             A and B for comparison. Receive crystals are on the left and transmit crystals are on the right. Note that 3.9 "M" on the left is 3.9K, 1 "M" is 1K etc.
        The right unit values for transmit were R26 -15K, R27- 60K, R28- 150K and R29 -330K.
Most of the B tuning units I examined were tapped on Transmit only at position number 3 on R-28 which has a value of 150K. Operation on 75-80 meters you can convert an A unit to a B can be done by removing the resistor network on A and installing a single 150K resistor for the Transmit tap.

 
             

          B unit network with transmit tap at position 3. Taps are moved according to frequency and perhaps activity of the crystal. Several B tuning units were examined all units had a transmit tap but the receive tap was not used. I played with the tap while transmitting on 75 meters and noticed when going from tap 3 to tap 2 the total transmit current reduced by a couple of mills and the modulation increased a small amount but moving the tap through all the positions did not produce any major event event with the tap disconnected.


       
 
               

          "A" tuning unit resistor network. The tap soldered in place.
 

                            
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               BC-745 BC-611F size Comparison. Both units were manufactured by Galvin.

 
              

         The BC-745 used the capacitor- resistors "packs" extensively. The ones installed on the BC-745 are larger and have easy to read diagrams printed on the exterior.
 
                               

                        The BC-611 A through model E used the same "pack" system.
 
               
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            The BC-745 had a tube count of 9, the set has better audio stages and more RF and audio output. RF output was .75 watts Vs the BC-611 of .38 watts, barely enough to wiggle your S meter or about 3 dB which is half an S unit.
 
              

            The BC-611 utilized 4 tubes.
 
              

                The BC-745 could rapidly change frequency in field using plug in tuning units. A total of 27 channels were available. The BC-611 could not change in field and required realignment each time the frequency was changed. A total of 50 standard frequencies were available for the BC-611. Note: The BC-745 did not issue a tuning unit for 3.885 as standard equipment.
 

                         

   Antenna coils were of the same construction but slightly different ranges.

         Q. Can a BC-611 use a BC-745 antenna coil? Are the coils interchangeable?

         A, Yes. The large variable (shown above) allows a very wide tuning range. Slightly over 200 Kcs. The variable tune from 6.5 to 140 pF.

          Q. Was a 3885 Channel Tuning unit made for the Pogo Stick BC-745?

          A. No but the 3825 coil set (Ch 2) will be close enough for government work. You will probably have to remove a few turns on the receiver tank coil (reduce inductance) for 3885 operation - shown below.
 
          

 

         

                   Left Pogo Coil    Right BC-611 Coil

                 The receiver tank coils for both sets are of the same construction but slightly different frequency ranges. The receiver tank circuit has a bandwidth of approx. 200 kc. When tuning the peak is broad.       


              

 
                       
           The receiver "tank" coil is easily modified. Adding turns to a 4280 coil brought it down to 75 meters. Approx. inductance needed is 36 uH.

 

                                    

    The coil adjustment screw has a 8 Turn range. A pretty robust coil but do not force the adjustment past the stops. The plastic adjustment screw moves a small piece of ferrite core up and down inside the coil.

 
                        

                       8 Turns gives a tuning range of approximately 5 uH.
 

    A special note: Robert Downs WA5CAB has a nice selection of coils for the BC-611. He is an excellent source for military radio parts and has helped hundreds of military radio and vehicle collectors.

                        http://www.vendio.com/stores/wa5cab/

 

                                

             The antenna coil can be opened for modification. Carefully saw through the outer cover and remove.  

 
                         

                      Use caution when removing the antenna coil as the socket and coil pins are fragile.
 
                               


             Take the top winding and unsolder and remove it from the pin and pull it outside of the coil form. . Then insert a solid piece of wire into the pin and solder.
 
                            

           Feed the new solid piece of wire through the coil form and make a tab. This tab will become an anchor point for your coil modifications RF trails. Easy to solder and unsolder.
 
                           


         Add turns to bring the selected coil down to 75-80 meters. On 3885 approx. 80-85 uH is required. When the turns have been added make the connection inside the form and slide the cover back on and glue.
 

                           

    A dip meter can be used. My 3885 coils always dip "low" around 3.6 with confirmation of the dip meter frequency using a near by receiver. Expect coverage of about 400 kcs across the 80 meter band with a 3885 coil while tuning C22 in the tuning unit from minimum to maximum capacity.

 

                                    Continued on Page 2

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