Digital Frequency Display for Heathkit SB, HW and HR Radios  

(Grandfathered Due to Lack of Continued Interest)

dfd2 picture 

hr1680 picture

1. Introduction

    Neil Hecht founded the Almost All Digital Equipment (AADE) enterprise which produced many innovative electronic adjuncts for radio amateurs.  Unfortunately, Neil passed away last year and the status of his excellent product line is uncertain at this point.  You are encouraged to view his preserved site on the internet Wayback Machine.

    Inasmuch as I had built several of his DFD2 units and found them quite well done, I was disappointed that I could not order another unit.  So, I replicated his custom DFD-2 for the Heathkit SB line, for the HW radios and for the solid state HR-1680.  This project has been designed for the experienced homebrewer.

   I will make the bare-bones boards and pre-programmed PIC processors available for others who might like to build one up.  I will not have the time to coach anyone through the building process nor assist with the installation within any particular radio.  For this, you should probably refer Neil's website on the Wayback Machine.

    You'll notice in the picture that this particular display has been installed in a very large enclosure, mainly because I wanted to use a large, 1x16, one line display.  Since the P/C board itself is just 2.5 by 4 inches, it may be readily mounted in a smaller enclosure (providing a smaller LCD is used).  The LCD itself can be 'wired' to plug directly into the P/C board, or connected to the board by either a ribbon cable or individual 30 gauge wires - your choice!

2. Design and Schematic - Phase 2 Board

   My P/C board design is very similar to the one that Neil developed except that:
3. How It Works - Phase 2 Design - Either 100 Hz (Faster) or 10Hz (Slower) Frequency Accuracy (Read Below) May be Selected

    For the Heathkit and other similar transceivers, the software first reads and stores the HFO and BFO values, subtracting the BFO from the HFO and placing the result in a holding register.  This value is updated every 2 or 3 seconds.  Then the VFO is sampled and subtracted from the holding register.

 Note: For processors shipped after 3/12/2017, two versions of the program will reside on the same PIC16F628A.  The original program can be accessed by leaving the strap OFF the SPARE pins.  This program will check the frequency between 3 to 4 times a seconds and will provide a 10 Hz readout.  A faster version is available by strapping the SPARE pins.  This program will make 10 samples per second and provide a 100 Hz readout, and is more suited for rapid QSY activity.  You can check them both out to see which better suits your application and preference.  Just be sure to power down the chip before making the changes or the display may not be correct.   If you want to cycle between the two program versions, you could install a miniature SPST switch somewhere on the enclosure.

In either version, the LCD is updated only when the frequency changes, thus eliminating the repetitive 'chirps'.

4. Building It - Read the CAUTIONS Below

 Important Notes: When populating the P/C board, it's vital that the components be installed on the proper side as you'll pay hell to remove them if they have been incorrectly installed on this board which has neither a component or solder mask.

CAUTION  - When making the connections, do not use any more solder than necessary, especially in the area of the 16 pin LCD connector.  If too much solder is used on either pin #4 or pin #5, then there's a very real possibility that the +5VDC line will short to ground - the topside trace that connects pin #2 to VCC will be the one impaired. It's also possible that the trace going to pin #12 will be affected.  If this happens, the LM78L05 will become very hot when power is applied and / or the LCD will not display properly  It's probably a good idea to see if  pin #2 is grounded after the 16 pin terminal strip has been soldered.  Here's the area of the possible fault.

Now, if you find that pin #2 is grounded, it's easily fixed by using a razor blade of Xacto knife to sever it in 2 places (topside), as shown here - which isolates pin #2.  
Then a small 30 gauge jumper can be used (on the bottom of the board) to reconnect pin #2 to the VCC.

If pin 12 is affected, you'll need to isolate this lead, as well, and a small 30 gauge wire can be used to connect pin 12 to pin 11 of the PIC16F628A, as shown here.

This unfortunate manufacturing defect was was caused by an excess of solder on pin #5 of the LCD connector which - in a few cases - leeched into the adjacent thru traces.  The remaining boards shipped from 2/16/17 onwards will have these pins checked and - if necessary - corrected.  The P/C board has been redesigned to eliminate this possibility in the future.

Sorry about this, but this is the first time I've had a problem with this particular PCB 'house'.

    If you are building this board, you first have to determine if you want to mount the LCD display directly to the P/C board, or just mount the P/C board on the chassis and then use either 30 gauge wire or ribbon cable to interconnect the two.  You also have to decide upon a suitable enclosure if the display is to be mounted externally.
    If you plan to mount the LCD on the P/C board, then the TEST and LCD two wire pins must be mounted on the foil side of the board along with R8 (the LCD contrast adjustment).

5. LCD Backlight

    Some back lit LCD displays may be powered directly from the on-board LM78L05 regulator by selecting a suitable dropping resistor (R10).  The voltage drop across the LEDs is usually 4VDC, with the current ranging  from 20ma to 150ma. The dropping resistor required is R = (V-4)/I where I is the desired current and V is the supply voltage. If the backlight on your LCD will not draw more than 30 ma, then you may use the display board's LM78L05 regulator.  I used a 47 ohm resistor on mine which drew just 12 ma and provided adequate brightness.
Important Note: If your LCD display draws more current and the LM78L05 gets very hot, then power both the board and the display with an external LM7805 heat sinked to the chassis of the radio to which the digital display will be connected.
6. Enclosure Suggestions

    One of the few worthwhile items you can still find at Radio Shack is a good plastic enclosure.  I used the smallest I could find (5 x 2.5 x 2 inches) that would hold the electronics which is stock number 270-1803 ($5.49).  The plastic in these boxes is easily cut with an Exacto (or similar) knife.  Some suggestions:
7. Connecting It

    Using 2 to 3 foot lengths of RG-174 (or equivalent), install PHONO plugs on one end and and solder the other end to the HFO, VFO and BFO P/C board connection points.  Either tag or color code these connections and insert them into the SB-300, SB-301, SB-303, SB-310, SB-313 (etc) radio of your choice and the appropriate jacks will have already been placed on the radio's rear panel.  These are the easiest interfaces.   Some minor additional work will be required for the HW-100, HW-101, SB-100, SB-101, SB-102 (see below), and to the HR-1680.

Note: You may need to repeak the heterodyne oscillator adjustments as the additional capacity may detune them somewhat.

Also Note: If you are mounting the circuit board on your radio's chassis, ground each of the 3 mounting holes by soldering a small gauge wire from them to the ground that 'runs' around the side of the board.  Use spacers so that the board does not 'ground out' when mounted.

8. HW-101 Interfaces
9. HR-1680 Interfaces

    As in the case of the Heathkit SB / HW series, connections are required to the HFO, VFO and BFO points within the radio.  These connections - made with miniature coax (RG-174 or equivalent) - are sent through a small capacitor (from 30 to 60 pf).  The HFO and VFO connections are made on the underside of the chassis, whereas the BFO connection is made on the AUD / REG circuit board (D).  The BFO connection is made at the top of the board with the shield of the coaxial cable terminated on the rear of the board.  

Note: When making the BFO connection, be sure to leave enough coax so that the AUD / REG circuit board may be placed on an extender should service ever be required.

  Suitable RCA phono jacks are mounted on the radio's rear panel where there is also a convenient source of 13 VDC.  Radio Shack sells a suitable connector (#2740222).

Important Note:  Connections to the SB-300 / 301 / 303 are very simple as Heathkit designed these radios with suitable pick-up coils on the Heterodyne Oscillator specifically to send the HFO signals to a matching transmitter.  With them, it's just plug 'n play'.  Connections to the SB - 100 / 102 / 102 and to the HW-100 / 101 are a bit more involved in that the HFO, VFO and BFO signals can be picked off the cathodes of the respective tubes.  However, the HR-1680 conversion may be a bit more involved, especially if operation on 15 and 10 meters is contemplated.  While the display will work with direct connection (via a small capacitor) to the HFO output, the additional capacity on these overtone crystals may attenuate the HFO output and the received signals.

To solve this problem, I built a small 2N3904 emitter follower circuit and mounted it under the chassis (on double sided tape), as shown here.  With it in the circuit, received signals (as measured by the calibrator) are significantly improved on the higher frequencies.  

This is the same circuit as the HFO buffer used in the Kenwood TS-520 radios and also displayed on the inactive AADE website.  It works!

I have a few extra parts ( sets) if anyone has an HR-1680 they'd like to convert.

10. Testing It

    The digital display needs a permanent source of 13 VDC power.  This can be accomplished by using an appropriate wall-wart, or by deriving the power from the radio itself.  If the radio is driven by a 13 VDC source (like the SB-303 or the HR-1680), then a suitable power source can be found within the radio.   If you want to power the display from a tube type radio, then you'll have to derive the power from the filament circuit by using a simple voltage doubler and rectifier combination (google it).

   Once you have verified that the proper voltages are present on the IC's, you may install the integrated circuits and make the connections to the LCD. Connect the digital display to your radio and apply power.  Adjust R8 for the proper contrast.  Switch on your radio and verify that some frequency is being displayed.

11. Calibration

    If you are using the 'canned' oscillator (Mouser - MXO45HS-3C-20M0000), no calibration is required.  It should be smack on.  Just to verify this, you might want to place a temporary jumper across the TEST pins which will make the DFD-2 work as a general purpose frequency counter up to 50 Mhz, or thereabouts.  The HFO input will record the frequency to which this lead has been connected - for example - the crystal oscillator's output.

    If you opt to use a mechanically adjustable TCXO (wired with 30 gauge wire the the canned oscillator's connection points), connect the output of the HFO to the oscillator's output and adjust the display for 20 Mhz - or just monitor it on a very accurate receiver.

12. Birdies

    Most every radio equipped with a digital frequency display has its share of internally generated 'birdies'.  Anyone who has used rigs like the Ten-Tec Digital Century 21, the Ten-Tec Omni series or even the Drake TR-7 will confirm this phenomenon. Therefore, it should come as no surprise that the DFD-2 Clone will generate some low level 'birdies' - here and there.  In most cases though, normal band atmospheric noise will render them largely unnoticeable.

a) SB-300 Series

    There are some very minor 'birdies' that I noticed on my SB-300. Whether or not they will appear on your radio is anyone's guess.  Normal band noise should mask them out, and grounding the P/C board directly to your radio's chassis will further attenuate them (none caused the S meter to 'twitch'):
b) HR-1680 Series

    The unmodified radio - with no antenna connected - will exhibit 'birdies' that are strong enough to move the S Meter at 3652 Khz, 3738 Khz, 7030 Khz and at 21200 Khz.

13. YAESU FT-101 - Software Not Written Yet!

Cabling Options:

1)  You may use the remote VFO plug (octal) if:

2)  You may bring cables out of the rear of the rig and terminate them with RCA or BNC connectors at the Digital Frequency Display unit if: 

3)  You may install RCA or BNC chassis mount jacks on the rear panel if:

Yaesu FT-101 Signals

   The BFO signal (3.1793 MHz) is approximately a 3 volt (peak-to-peak) signal that may be tapped in either of two different places.  It appears on pin 6 of board 1184A and is carried by a short piece of coaxial cable to pin 5 of board 1183A.  Each board has a convenient grounded pin for shield connection, but board 1183A is easier to reach with a small soldering iron.

   After routing the new cable (or mini-coax, which ever you have chosen) from the rear panel to the area of  the board chosen,  solder a .01 disk ceramic capacitor to the center conductor of the cable.  Then solder the free lead of the capacitor to the chosen pin of the board edge connector and the shield to the nearest grounded pin on the edge connector.

   The Local Oscillator signal (approximately 6MHz above the displayed signal, or 8 to 36 MHz) is approximately a 3 volt (peak-to-peak) signal that is available at the test point near the top edge of board 1181A.

   You have 2 cabling options here.  The first option is to route the cable from the rear panel toward the front of the transceiver, and the openings around the tuning dial.  Use these openings to pass the cable to the top of the chassis.  While viewing the FT-101 from the normal operating position in front of the rig, route this cable up over the tuning shaft and to the right of the chassis.  Solder a .01 disk ceramic capacitor to the center conductor and a small ground lug to the shield.  Solder the free lead of the capacitor to the test point at the top of board 1181A,  and attach the ground lug under the adjacent control’s mounting nut and lock-washer.

CAUTION. This method of installation means that you must disconnect the ground lug in order to remove the board from its edge connector.

AN ALTERNATIVE installation method is to install the blocking capacitor (.01 disk ceramic) on the board between the test point and unused pin 15.  This will allow easy removal of  board 1181A.  However,  it requires soldering the coax to the edge connector for board 1181A in a very congested area.  In this alternative installation the cable stays on the underside of the chassis and is soldered to pin 15, with the shield is soldered to pin 18 of the edge connector for board 1181A.

The VFO signal (approximately 9 MHz) is about a 1 volt (peak-to-peak) signal available at pin 11 of board 1180A.  After routing the cable from the rear of the chassis, solder a .01 disk ceramic capacitor to the center conductor.  Solder the free capacitor lead to pin 11 and the shield to pin 10 of board 1180A.

(The VFO signal also appears on the remote VFO adapter (octal) plug.  In the beta unit this signal appeared to be a little dirty causing erratic operation until the unit had warmed up for awhile. This may be peculiar to that one unit.)

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