Purpose:

To permit rapid zero beating of the HX-50 transmitter frequency with a received signal without disturbing any operating controls other than the VFO tuning.

Operation:

Connect a switch to terminal #6 or #8 and ground.  Upon closing the switch contacts, the transmitter is automatically placed in “calibrate” without the necessity of operating any other control.  After zero beating, releasing the switch places the transmitter on frequency ready to operate.

When the relay is energized:  (a) One pair of contacts opens the cathode return of the speech amplifier tube thus disabling the speech and VOX amplifier circuits  (b) a second pair of contacts shunts a capacitor from one side of the balanced modulator to ground which permits a greater signal to appear on the grid of the first mixer  (c) a third set of contacts decreases the cutoff bias to the RF driver stage to allow the output to leak through to the PA  (d) a fourth set of contacts opens the VOX trip relay control coil circuit and prevents accidental transmit function during zero beating operation.

Installation instructions:

1-Remove resistor R-107 (1.5K, ½ w) from pin 4 of V101 (6C10) and ground.

2-Unsolder the GREY wire from relay K102 coil terminal and redress it to reach the ZBZ relay terminal #1 (see drawing)

3-Install R2 (6.2K 1 w) between pin #5 of V109 (OA2) and unused lug of terminal strip (see drawing)

4-Mount the ZBZ relay to the bracket.  Remove the cable clamp holding the shielded mic cable.  Mount the relay in the hole where the cable clamp was located.  Dress the mic cable under the relay.

5-Solder the 1.5K 1/2 w resistor and the 10 pf capacitor to the relay terminals. (see drawing)

6-Solder the orange wire, the brown wire and the two red wires to the relay. (see drawing)

7-solder the short bare wire in place (see drawing).

8-Check carrier balance and adjust if necessary.

9-Connect the ZBZ operating switch (not supplied) between TB101 terminal #6 or #8 and ground.

(NOTE: On the HX-50A the ZBZ switch was incorporated into a potentiometer installed on the front panel beneath the bandswitch.  The potentiometer varied the strength of the signal used for zero beating.  Details are included elsewhere in the modifications pages-ed)

HAMMARLUND BULLETIN DETAILING SEVERAL CHANGES IN HX-50 CIRCUITRY-part 1

1-R213 is changed to 1K ½ W.

2-R-116 is changed to 220 ohm 1/2W
3-R-132 has been deleted

4-A 1.8K 1/2W resistor has been installed between pins 1 and 7 of V-105 (6AU6)

5-L-119 and C-211 have been deleted

6-A 20K 10W wirewound resistor has been installed in parallel with R-218

7-CR-111 has been deleted and replaced with an OA2 regulator tube (pin 7 to ground, pins 1 and 5 to LO B+ side of R-218.  CR-111 was a zener diode from terminal strip TB-9 to ground and R-218 was a 10K wirewound resistor from LO B+ to TB-9.  The zener was prone to failure and was replaced by a second OA2 tube.

8-A trimmer capacitor (1.5-20 pf) has been installed from the junction of CR-101 and C-105 to ground.

(NOTE: In early radios there was no CR-111 and no R-218, and hence no regulation on the LO B+ feeding the low level stages.  These changes may have been included in a Hammarlund bulletin code 2 or 3.  R-218 was a 10K 10W wirewound resistor to the junction of CR-111 from the LO B+.  In my unit there are two 12K 10W resistorsin parallel from the LO B+ to the newly installed OA2 tube (the one attached to the chassis with bolts rather than rivets).  These resistors run very hot, suggesting that less than 6K total resistance is required…perhaps more like 5K or 4.7K.-ed)

HAMMARLUND BULLETIN DETAILING SEVERAL CHANGES IN HX-50 CIRCUITRY part 2

 (NOTE: several of these are also detailed in the Code 4 modifications described below-ed)

1-The RF tank shield has been modified to the new cut-down version to simplify installation of the 160M kit and lengthen life of the 6DQ5 final tube through better heat dissipation.  The RF cover or cage has also been removed for these same reasons. (My unit has a perforated cage on 3 sides around the PA-is this what Hammarlund means by “cut down version”???-ed)

2-A relay adjust potentiometer, 1 meg, has been installed on the rear chassis to permit setting the relay pull-in/dropout points.  Place the transmitter in VOX and vary the potentiometer end to end until thebrelays pull in.  Back off the potentiometer until the relays open.  Test and re-set until the proper points have been set.  The potentiometer connects to the terminal strip adjacent to the original OA2 regulator tube.  Counting the terminal numbers from the rear of the chassis (terminal 1), the potentiometer goes to terminals 2 and 5 on this strip.-ed)

3-L-107 has been changed from 2.2 uh to 2 uh to increase drive on 40M.

4-R-144 has been changed from 4.7K to 3.3K to increase the level of the calibrate signal.

5-R-179 has been deleted and replaced with wire.

6-L-114 and C-198 have been deleted

7-R-194 has been deleted to increase drive on 20M

HAMMARLUND BULLETIN DETAILING SEVERAL CHANGES IN HX-50 CIRCUITRY part 3

1-To increase VOX sensitivity, install a 47K 1/2W resistor between C-110 and the junction of T-102 and R-113.  C-110 connects to T-102 and pin #10 of the 6C10.

2-To reduce key clicks, install a 2 mfd capacitor between the junction of R-199 and R200 and ground.  If an electrolytic capacitor is used, install with positive terminal to ground. (NOTE: this was the first crude attempt to alter the time constant of the keying waveform; it was superceded by the modification noted below in the bulletin released in 1963 in which the 2 mfd capacitor is not used-ed)

HAMMARLUND HX-50 CODE 4 MODIFICATION KIT INSTALLATION

This set of modifications must be done before the Code 5 modifications

1-Locate R153 (430K 1/2W in line with the center of the microphone connector and 1” down from the top of the chassis).  Replace with a 1 meg variable resistor.

2-Locate coil L-107 (2.2 uh) on the bandswitch front wafer and replace with a 2.0 uh coil.

3-Locate R-144 (4.7K 1/2W between operations switch and ground lug on audio pot on front of chassis) and replace with a 3.3K 1/2W resistor.

4-Locate R142 (3.9K 1W between TB8 and the bias control) and add a 6.8K 1W in series (this is now referred to as R216).  Run a 7” piece of brown wire from the junction of these two resistors to the normally open contact of the VOX relay K102 (this lug will now have two brown wires attached to it).

5-Locate L109 (.68 uh, on bandswitch front wafer) and replace with a .56 uh coil.

6-Locate R-116 (820 ohm ½ W) on the terminal board of the crystal mounting bracket.  In some production runs there is an additional 820 ohm resistor in parallel with R116.  If present, remove the second resistor.

7-Locate R179 (47 ohms 2W) on antenna changeover relay and remove it.  Replace with a wire jumper.

8-Locate choke L114 (560 uh between pin 6 of V111 and nearby terminal board).  Remove this and replace with a wire jumper.

9-Locate R194 (10K 1/2Win parallel with secondary of T-120, the 20M driver coil).  Remove this resistor.

HAMMARLUND MODIFICATION INSTRUCTIONS-CW KEYING MAY, 1963

1-Remove R-119 (1K) and C-114 from T-103 and ground the terminal of T-103 from which the parts were removed.  Leave the other end of the resistor connected to the blocking bias tie point for now.

2-Unground pin #6 of the 6AW8A, V103 and install a 270 ohm resistor shunted by a .01 mfd disc capacitor from pin #6 to ground.

3-Remove R125 (10K 1/2W) from pin #7 of V104 (6BA7).  Install a 10K 1/2W resistor between pin #7 of the 6BA7 tube and the block bias tie point where R119 was left connected.  Remove R119 after the new resistor has been installed.  Install a .01 mfd disc capacitor from the tie point to ground.

4-Remove R118 (330 ohm ½ W and C217 (2 mfd capacitor).  In some early units the 2 mfd capacitor was not installed.  If this is the case, after removing R118 add a jumper between the junction of R199 (1K) and R200 (82K) and the terminal on the tie strip where the 2 mfd capacitor would have been connected.

5-In the event that the CW VOX time is too short add a .22 mfd 400V capacitor across the CW VOX delay pot R149B.  The delay will then be the RC product set by R149A/C147 plus R149B and the .22 mfd capacitor just added).

6-Setup: with the transmitter in VOX USB or LSB set the outer shaft of the delay control for the desired delay.  Switch the transmitter to VOX CW mode and key the transmitter.  Adjust the inner shaft of the delay control for the desired keying delay.