Six Digit LED Frequency Display for Heathkit SB, HW, HR Series, for the

Kenwood TS-520(S / SE), and for the

Heathkit SB-104 / 104A 

dfd2 upgrade  

1. Introduction - Now with a Bright 6 Digit LED Display  

    Earlier I replicated the AADE custom DFD-2 for the Heathkit SB line, for the HW radios, for the solid state HR-1680, for the Kenwood TS-520S / SE, and for the Drake 2 series.  

    Now, the PCB (and the software) have been redesigned to allow a 6 digit LED display (TM-1637 chip - controlled serially by the PIC processor) to be used.  Jumpers (options) on the new board allow the design to serve the Heathkit family of radios, the Kenwood TS-520S / SE, and (later) theYaesu FT-101 and the Collins radios.  It can also be used as a simple frequency counter. Thus far, the code has been written for the Heathkit and Kenwood TS-520 radios only. 

    Bare-bones boards with pre-programmed PIC processors are available for others who might like to build one.

2. Design and Schematic - Phase 4 Board (Current)

   P/C board design:
3. How It Works - This program renders an accurate 100 Hz readout on a 6 Digit LED display which is updated only when the frequency changes.  It can also be used as a simple frequency counter.
    
4. Construction Details (read on down)
Note:  Before mounting ANY components, please read through ALL of the instructions, most especially how to install the counter time base - if you opt to use it (follows).  

A 'canned' 20 Mhz crystal oscillator  (like Mouser - 520-2200BX-200) provides satisfactory accuracy.  One might also consider a 20 Mhz. TCXO as they can always be adjusted later, if necessary, although I've never had to adjust one.  (They come factory calibrated).  There are usually inexpensive TCXO's on eBay.  For example, a Vectron TCXO is shown here.  Ensure that it as an 8 pin DIP footprint, and that you mount it properly on the PCB.  Pin #1 is usually marked with a circle (and a pointed metal pin on the 'canned' oscillators).. Here are some pictures showing how to orient a 'canned' package before soldering it - pin1_1      pin1-2     pin1_3.

For an adjustable surface mount TCXO, place some solder globs on the proper pins and attach a small length of bare 30 gauge wire to each.  Route these wires thru the P/C board holes; center and snug the TCXO against the board.  While soldering the first wire, gently press the TCXO to the board.  Then, carefully solder the remaining wires, carefully trimming the excess after the solder has cooled.  I used  a Vectron 20 Mhz TCXO (surface mount),  also available on eBay.  Pin #1 is marked with a small circle on the metal just like the canned oscillators.

1) Refer to the printed circuit board layout and to the schematic as you procede.  Mount your components on the notated silkscreed side of the board:

Important Suggestion - when soldering the pin headers (next step), you might first want to first place them in a spare integrated circuit socket or in a female header pin set (usually available on eBay).  This  way, the heat of your soldering iron will not melt the plastic and cause the pins to seat unevenly.  I've soldered hundreds of pins this way and it always works.    

2) Mount the Pin Headers on the board . Note: no need to install PICKIT3 header or resistor R4 as they are used by me only for in circuit programming:
3) Install three (3) 16 pin sockets for the HFO, VFO and BFO (CD74HC4046).  The notches point towards the inside of the P/C board.  Solder carefully.  Follow the silkscreen patterns.
          4) Install the 16 pin IC socket for the CD74HC153 with the notch pointing toward the bottom of the board. 
5) Install the 18 pin IC socket for the PIC16F628A microprocessor with the notch pointing toward the bottom of the board.
6) Install the (9) .1mf ceramic capacitors. (C2, C3, C4, C5, C9, C10, C12, C13, C14 )
7) Install  C6, C7, C8 -.01 mf ceramic capacitors,  80-C324C103K3G5TA
8) Install the (6) 1N4148 input protection clamping diodes  (component names not marked on board) for the HFO, VFO and BFO
9) Install R1 (100 ohms), R2 (1K ohms) and 3 (1K ohms) - all 1/4 watt resistors). 
Note:  R3 on the PCB is not used in this design so you'll have to jumper it with a 30 gauge wire (may already have been done - please check).

10) Install the polarity reversal protective diode D7 (1N4001, or equiv), 
11) Install C1, C11- 10 mf elecrolytic capacitors (watch the polarity and soldered connection spacing),
12) Install the 5 volt voltage regulator (LM7805)  - IC-7
13) INITIAL POWER TEST - Apply 13 VDC to the power connector and verify that +5 VDC is present on pin 14 of the microprocessor socket, on pins 15 and 16 of the CD74HC153 socket, and on pins 16 and 3 of  each CD74HC4046 socket.  REMOVE THE POWER and verify that ground is present on pin 5 of the microprocessor, on pins 1, 8, 10, 11, 12, 13 of the CD 74HC153, and on pin 8 of each CD74HC4046 socket.  Leave the power off until step 17.

14) Install the 4 pin 'canned' crystal oscillator (Mouser - 520-2200BX-200).  This is a 4 pin DIP and it must be properly installed.  Pin #1 of the oscillator has a pointed edge.  Here are some pictures showing how to orient the package before soldering it - pin1_1      pin1-2     pin1_3.
15) Referring to the printed circuit board layout, install the integrated circuits by straightening the pins (rolling them on a hard surface) and then my 'rocking' them in.....noting  their orientation.  If you encounter any resistance check it out before proceeding.   New integrated circuit sockets sometimes offer insertion resistance the first time they are used  (like prom night?).
16) Wire the 6 Digit LED Display (available on eBay, on Amazon.com and very recently from Walmart).  Pick your desired color and be sure it has a TM-1637 serial controller.  They come with a 4 pin wire wrap connector that must be soldered to their backplane, as shown here.  Four (4) connections need to be made for Power VCC (+5VDC), Ground (GND), Digital I/O (DIO) and Clock (CLK) - as shown in the schematic.   Double check to ensure that you have made the right connections (especially VCC and GND) before powering it up.
17) Power up your board.  Since it will not be connected to a radio, you'll see a negative number similar to F1572.22 . until the display has been connected to your radio's HFO, BFO and VFO points.

Note:  If you find that the display is too bright for your liking (most probably only on WHITE displays), you may tone it down a bit by shorting the DIM pins on the board.  Since the unit has been programmed to update the display ONLY WHEN the received FREQUENCY HAS CHANGED, you may not see the display change immediately.
6. Interfacing with a Radio

    This  design will serve any of the supported HEATHKIT radios, viz the SB-300 / 301 / 303 / 313, the SB-100 / 101 / 102, the HW-100 / 101, the HR-1680, the HW-104, the SB-104(A), and the KENWOOD TS-520S / SE / D.  The Kenwood TS-520 can also be connected provided the builder is willing and able to do a little 'under the hood' work.

            Note: Using this device with an SB-104 (A) is overkill as another, less expensive option exists. 7. Enclosure Suggestions

    For this prototype, I used a small clamshell type of enclosure found on eBay.  Since it was a light unit, I added a couple of  flat electrical box covers (from Lowes)  to both weigh it down (so it would not move on my desk) and to provide a heat sink for the voltage regulator.  The regulator does not get all that hot, but inasmuch as I had installed the flat electrical box covers beneath the board for weight I decoded to use it as a 'sink'.  I had some spacers from another radio that I had cannibalized, so I used those.  (Simple 4-40 machine screws, washers and nuts would have worked just as well.)  

   The 6 digit LED is mounted on the front along with an OFF - ON power switch.  The power connector, 3 RCA inputs and an optional switch to activate the counter feature are located on the rear panel.  I put sticky feet on the bottom.  I'm sure that those of you with better mechanical skills could whip up something a lot nicer.

   Mouser sells some interesting enclosures that would also fit the bill.  Here's just one series: http://www.busboard.com/documents/datasheets/BPS-DAT-(BOX2-P14)-Datasheet.pdf  You'll want to get one that has removable front and rear covers.
8. 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, TS-520S (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  HR-1680 heterodyne oscillator (HFO) adjustments as the additional capacity may detune them somewhat.

Also Note:  If connecting to an SB-303 / 313, place a 47 ohm resistor across the VFO coax connection from the radio to the DFD-2.  If this resistor is not placed, the frequency display may become intermittent.

Also Note: If you are mounting the circuit board on your radio's chassis, ground each of the 4 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.

9. Heath SB-100 / 101 / 102 / HW-100 / 101 Interfaces

    CAUTION: - if you are unsure, unfamiliar or unable to work safely within a tube radio where potentially lethal voltages lurk, please don't consider this product unless you have a competent and experienced helper with you.  I will not be responsible for any harm to either you or to your radio as a result of attempting this conversion.  If you have any doubts - don't try it.
10. 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.  

Important Note:  Connections to the SB-300 / 301 / 303 are very simple as Heathkit designed these radios with suitable pick-up coils on the HFO, VFO and BFO oscillators to  signals to a matching transmitter (e.g, the SB-400, 401).  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.  The HR-1680 conversion requires a smiple HFO buffer if sensitive 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, a small 2N3904 emitter follower circuit is mounted it under the chassis (on double sided tape), as shown here.  With it, received signals are significantly improved on the higher frequencies.  

Also Note: This is the same circuit as the HFO buffer used in the Kenwood TS-520 radios.  It works!


11. Kenwood TS-520S Interface 

    The Kenwood interface is a simple as that of the Heathkit SB-300, 301, 303 (etc).  One simply plugs the HFO, VFO and BFO (Carrier) plugs into the rear mounted jacks, applies power to the unit and everything should work FB.

    Power (12VDC) can be suppiled via an inexpensive wall-wart and / or is available from the radio itself on the VFO shorting plug.  Good luck on finding the original GD-5 plug that powered the DG-5 digital display.  Small female jacks can readily be installed on the TS-520S rear panel to provide a source of fused 12VDC for your counter - just attach the lead to the DG-5 connection points.

12. SB-104(A) Interface

        Not Required! - an inexpensice 6 Digit Digital  Frequency Counter available on eBay can nicely fulfill this need 
     
13. 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 6 digit LED. Connect the digital display to your radio and apply power.  Switch on your radio and verify that some frequency is being displayed.

14. Calibration

    If you are using the 'canned' oscillator, no calibration is either required or possible  It should be smack on.  Just to verify this, (for heathkit programmed DFD-2's) 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 40 Mhz, or thereabouts.  The HFO input will record the frequency to which this lead has been connected.

    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.  All the TCXO's that I purchased on eBay and installed on pre-assembled boards did not require any tweaking - they were apparently set at the factory.  I did notice - however - that their accuracy was off a bit after they were soldered inton the circuit.  Allowing them to properly cool solved this problem.  After all, they are temperature compensated, right :-)

15 . 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.

16. Radio Service Manuals & Schematics

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