Notes on W3CRK/WB6SSW HBR-16 As Built Schematic

Notes on WB6SSW HBR-16 As Built Schematic

1. Schematic Used. Except where otherwise noted below, the receiver was wired in accordance with the HBR-16 schematic published in the original HBR-16 QST article.

2. Antenna Coupling. As originally published, the antenna was connected to L1 in a balanced configuration through a three screw terminal strip on the rear chassis. The input impedance of the receiver was measured at approximately 75 ohms (actually, it varied from 68 to 105 ohms, depending upon the band). Since the receiver would be used with typical unbalanced 50 ohm coax, a BNC connector was added to the rear chassis connected through RG-58 coax directly to the base of the L1 socket.

3. Calibration Oscillator Adjustment. The 82 uuF fixed capacitor replaced the original 47 uuF and 50 a uuF variable capacitor was added to be able to adjust the nominal 3.5 mHz crystal to within 100 Hz. A different fixed capacitor value may be need to bring other 3.5 mHz crystals to a similar tolerance.

4. Calibration Oscillator Test Point. A 250 uuF fixed capacitor, C6 and chassis test point was added to provide for a means of testing transmit crystals, using the calibration oscillator (the C6 in the original schematic was a gimmick capacitor to couple the calibrator oscillator’s output to the receiver front end). A frequency counter connected to the test point will allow accurate measurement a crystal’s fundamental frequency when the crystal is plugged into the calibration oscillator’s crystal socket.

5. HF Oscillator. Ted Crosby suggests that the C10 “set” or “vernier” control can be eliminated as unnecessary. However, since this particular receiver was to be used for CW, it was retained in order to insure in band operation in the Extra Class CW portion of the bands. Additionally, in Ed Kent’s Builder’s Notes, Ted suggested that the 10 uuF N750 capacitor be added in parallel to C10 (and the L3 secondary) in order to provide overall, non coil specific, temperature compensation.

6. C8 Trimming. Ted Crosby called out for the 2nd oscillator “gimmick” capacitor to be “3 to 4 turns of #18” hookup wire. In order to achieve the ability to kill the receiver when the second mixer gain was fully counterclockwise, it was necessary to reduce the original 4 turns to 2 ½ turns.

7. BFO. The 100 uuF capacitor across the BFO pitch control was suggested in the Builder’s Notes to allow the BFO coil to be pulled down to 100 kHz. The 150K grid resistor was changed to 100K per the 6-25-62 “HBR-16 Corrected Schematic.” 8. R.F. Amp Screen Resistor. The original 68K resistor was changed to 56K per the 6-25-62 “HBR-16 Corrected Schematic.”

9. T1 Alignment. Two test points were added to facilitate alignment of the two trimmer capacitors of the 1600 kHz IF transformer, C13 and C14. The alignment procedure is to:

a. With the receiver mounted in its case and all shields in place, pull the HF Oscillator tube, V4.

b. Using a test oscillator, inject an approximately 1600 kHz signal into the test point connected to the grid (pin 1) of the first mixer, V3.

c. Connect a scope and frequency counter to the test point connected to the plate of the second mixer, V5. Note that this test point has approximately 200 vdc, so appropriate precautions should be taken so as not to exceed the ratings of the scope probes or DC input to the frequency counter. If possible, connect the frequency meter to the scope’s vertical output jack.

d. Adjust the test oscillator for a 100 kHz signal at the frequency counter (that is, the product of a 1600 kHz injected signal and the 1700 kHz second oscillator. This permits correct adjustment even if the 2nd oscillator crystal is off frequency).

e. Adjust C13 and C14 for the maximum signal indicated on the scope.

10. T2 and T3 Output Bypass Capacitors.

11. 1 st IF Screen Bypass Capacitor. Per Ed Kent’s “Builder’s Notes” the original QST article had this erroneously labeled as .01, rather than the correct .02.

12. IF Gain Control. Subsequent to publishing his original article, Ted suggested that the dynamic range of the IF chain may be increased by reducing the original 75K 2 watt voltage dropping resistor connected to the IF Gain control to 68 ohms, “or even lower.” In this receiver it was necessary to reduce this resistor to 33K in order to keep the receiver’s IF chain from overloading.

13. 6BY6 Product Detector. In a June 1962 QST article, Ted presented in improved product detector using a 6BY6 rather than the original 6BE6.

14. Product Detector Improvements. Subsequent to the June 1962 QST article, Ted discovered that changing the coupling capacitor to the grid of the 6BY6 from 5pf to 2pf SM and the 6BY6 grid shunting resistor from 22k to 100k resolved a loss of high frequency response problem. Ted also recommended changing the 6BY6 cathode resistor from 330 ohms to 390 ohms, the screen resistor from 22K to 33K and the screen bypass cap from 0.02 mfd to 0.05 mfd. All these recommended changes were implemented.

15. V10 Detector/AVC Substitution. Ted suggested in Ed Kent’s Builder’s Notes that a 12AT7 could be substituted for the 12AU7 originally spec’d for V10. This substitution was made on the completed receiver.

16. V10 Detector Cathode Circuit. The original 330 uuF/82K cathode circuit was changed to 390 uuF/100K 56K per the 6-25-62 “HBR-16 Corrected Schematic.”

17. Detector to AF Amp Coupling. Subsequent to completion of the receiver, it was discovered that significant 100 kHz signal was present at the input of the 1st audio amplifier to drive it to overload, even with no signal present. By installing the circuit used in the HBR-11, substituting a 56K resistor for the 2.5 mh choke and a 470 uuF for the 330 uuF plate bypass capacitor the problem was resolved.

18. Audio Output. Various schematics show either a 390 or 330 ohm 2W cathode resistor and either a 470K or 390K grid resistor on the 6AQ5 AF output tube. I used 390 and 390K resistors.

19. AGC. The original AGC circuit was split into full and partial AGC line. In a June 1961 article by Alex Stewart, a single AGC line with adjustable timing capacitors was suggested (a change endorsed by Ted Crosby) and this revised circuit was included in this receiver. In addition, the AGC diode cathode timing circuit (V10B) was changed to agree with the AGC improvements Ted made in the HBR-11, changing the resistors from 2200 to 2700 and from 330K to 270K and the capacitor from .02 to .01. Finally, in a letter from Ted to Alex Stewart, et. al. on February 25, 1969, Ted suggests that the coupling capacitor from the secondary of T4 to the AGC diode “be no more than 100 pf,” due to the loading affects of this capacitor to T4.

20. S Meter Changes. Value changes in the screen and cathode S Meter circuit are per the 6-25-62 “HBR-16 Corrected Schematic.”

21. Receiver Mute. The original design muted the receiver by breaking the connection between the detector and the 1st audio amp. This required two shielded cables to and from the transmitter, possibly introduction hum to the receiver’s audio. Instead, the IF amplifier cathodes are lifted from ground to affect the mute function.

22. 700 Hz Filter. Ed Wetherhold’s (W3NQN [email protected]) passive audio bandpass filter was installed to provide a ring free CW filter for the receiver. The 1500 ohm resistor in the high impedance side of the input audio matching transformer was hand selected to provide a 200-230 ohm input impedance to the filter. The 18 ohm resistor in the filter bypass line was selected to maintain the same audio level with the filter in or out. 23. Noise Limiter. The included noise limiter circuit was adapted from a circuit published in Radio Communication Choices (1961, GRB Thornley, Radio Society of Great Britain). The 120K resistor was hand selected to give complete blanking when the 50K pot is in its full clockwise position. The original HBR-16 .005 coupling capacitor between the detector and the audio amplifier was changed to a .02 noise limiter input capacitor and a .01 output coupling capacitor to agree with Thornley’s design.

24. 4 uF Filter Capacitor Retention. Ted suggested that the 4 uF 450 vdc electrolytic filter capacitor could be eliminated if the Stancor PC-8405 power transformer was used. Even though this specific transformer was used in the finished receiver, it was necessary to retain this capacitor in order to achieve the specified 250 vdc on the A and B B+ lines.

25. B+ Set Variable Resistor. The 1 k ohm 25 watt resistor ahead of the first B+ filter choke and the test point after the second filter choke allows adjustment of the B+ lines to Ted Crosby’s recommended +250 vdc. Though the test point is on top of the chassis, adjustment requires access below the chassis. There is about a 10- volt differential between the A and B B+ lines. The 1k resistor is therefore adjusted for +245 vdc at the test point to keep both B+ lines within the +/- 10% of 250 vdc recommended by Ted.

26. Low Impedance Headphone Modification. Due to the absence of high impedance headphones on the current marketplace, a more traditional approach to headphone provisions was adopted. The S5, Headphone/Speaker switch was eliminated and the phone jack wired into the lo-Z side of the audio transformer. The front panel space formerly occupied by the S5 switch is now used for a noise blanker/bandwidth selector switch.

27. Tuning Capacitor. The original HBR articles called for a Miller 1461 variable 23 pF/23 pF/23 pF capacitor. This was a “straight line frequency” capacitor (linear frequency at the designed frequency range) originally designed for FM broadcast receiver applications. In the HBR receivers, the linearity of the frequency dial was skewed, causing the CW portion of the ham bands to be compressed and the phone portion to be expanded. This capacitor was replaced with a Cardwell “straight line capacitance” (linear capacitance) 100 pF/100 pF/50 pF capacitor. All but two stator plate were removed from each section to bring each section down to approximately 25 pF. This resulted in fairly linear frequency readout.

Rev 6-10-10