Heathkit HW-16  - Adding 'Internal' VFO Functionality

                         PRELIMINARY, INCOMPLETE and SUBJECT TO CHANGE

Caveat:  Before attempting this conversion, know that there are dangerous and potentially lethal voltages involved. Unless you are absolutely certain that you know what you are doing and have taken the proper  precautions, don't perform this work.   I cannot be - nor  will  be - responsible for any injuries or damages incurred as a result of your reading this website.

   This website describes a slightly modified variation of a very clever design published in Ham Radio magazine. Unfortunately, I was not able to get my conversion working properly as the converted radio did generate some spurious signals................I'm leaving this information on the web (for w a while) should someone else want to complete it.

   If you have any doubts about the modified HW-16 signal 'purity', then you may want to check for yourself before transmitting.  As a precaution, you may use a narrow bandwidth antenna like a magnetic loop antenna.  

   I cannot nor will not be held responsible for any spurious signal emissions caused by these modifications.

Introduction
Components and Costs
Modification
Conversion Theory
Steps:
Other Interesting Modifications:



1. Introduction - HW-16 Construction / Service Manual
    The Heathkit HW-16 was a popular Novice rig.  Although I never owned one (before now), I can remember repairing one for a new ham to whom I had administered the Novice test back in Chalfont, Pa in 1972.  Since I had previously 'graduated' from an Eico 720 / 730 combination to an HW-101, I didn't really think that I'd ever have any use for one of these 'old tube radios'.

    However, like many hams I'm trying to 'recapture' the thrill of my first CW contacts on my DX-20 back in our Hatboro, Pa home.  With this 'quest' in mind, I have taken to purchasing, repairing and then ultimately reselling these 'treasures' from the past.  The HW-16 was the next radio to be 'explored'.

  I bought this HW-16 on eBay along with a DX-60 with a slightly bent front panel.  After recapping both rigs with kits purchased from the Hayseed Hamfest, I decided to modify the HW-16 with a proper power cord and with an internal VFO as described in the March, 1973 issue of Ham Radio magazine.  The DX-60 will be upgraded later with an internal antenna changeover relay, PTT circuitry and the popular audio modifications.

2. Modifying the HW-16 - From Down to Up Conversion

    The HW-16 is a tube radio with separate sensitive / selective receiver and rugged transmitter sharing same robust power supply.  The receiver is VFO controlled, while the transmitter requires either a separate VFO (like the VF-1 or HG-10), or individual crystals for the desired frequencies of operation.
  
    In the March 1973 issue of Ham Radio magazine, James Takeshima - WB6MZN - described a clever way to allow the HW-16 to utilize its receiver VFO to control the transmitter frequency.  This website article amplifies his efforts by providing a construction technique if anyone is interested in either following it or improving upon it.  After all, this is what the spirit of amateur radio is all about, right?  Someone once said that we 'all stand upon the shoulders of those who went before us.'

    A few 'tweaks' were made to WB6MZN's design:
     a) Original Design - Receiver Down Conversion - Transmitter Crystal Controlled

   The HW-16 receiver 'down converts'.  The incoming signal is mixed with the heterodyne oscillator to produce a frequency in the 5 Mhz range by passing it through a bandpass coupler (filter) that removes the extraneous mixing products.  The 5 Mhz signal is then mixed with the tunable VFO signal and then routed through the .5 khz. crystal filter set to 3396 khz.  Finally, the desired signal is 'beat' against the 3396.4 khz BFO to produce audio for the listener.

    b) Modified Design - Receiver Down Conversion - Transmitter 'VFO'  Up Conversion

    The ingenious WB6MZN improvement inverts the conversion process when transmitting.  His 'interface' circuit first combines the BFO and VFO signals (first mixer) and then sent through the same bandpass filter to remove the undesirable conversion artifacts.  A signal in the 5 Mhz range remains, tuned by the VFO.  This 5 Mhz signal is mixed (second mixer) with that of the heterodyne oscillator whereafter it is sent through the transmitters oscillator / driver / PA stages, and then to the antenna and the world.

3. Conversion Theory - How It Works

    With key down, the HW-16's sole transistor (NTE-102 is the replacement - hihi) turns off and removes the ground from V2A (the Heterodyne Mixer), preventing bandpass filter interference.  Interface board transistor QM1 turns on, supplying power to mixer transistors QM2 and QM3.  Mixer QM3 combines the BFO and VFO signals whose (5 Mhz)output is directed through the bandpass filter.  Mixer QM2 combines the output of mixer QM3 with the Heterodyne Oscillator, as shown below:  

    The output of Mixer QM2 will provide the following signals (original frequencies, sum and differences) with the VFO set to '0' khz:
   In the original Ham radio article, WB6MZN routed the output signal through the tuned circuits in the receiver's RF stage as a way to peak the desired frequency relative to the others present.  He relied on the tuned circuits within the transmitter to further amplify the desired frequency while attenuating the others.  I tried this technique two different ways and the result was always the same - the excellent sensitivity of the receiver was reduced to what I would regard as objectional because the RF amplifier circuit was 'disturbed'.

      Now, 40 meter operation works well.  However, 80 meters will not produce anything close to full output power.  To solve the 80 meter problem, another parallel resonant tuned circuit is needed between the Interface Board output and the transmitter oscillator.  Rather than including it on the Interface Board (I might do this later), I just inserted it into the VFO jack (more on this in the next update).  With it, both 80 and 40 meter transmissions are proper.  While I could not make any 15 meter tests (weak heterodyne oscillator crystal), I don't think that there will be any problems with 15 meters since the tuned circuits are far enough apart.

    QSK operation is unchanged.  

4. Conversion Steps - Check the Schematic

    Every amateur has his or her own favorite ways to perform such a conversion.  Listed below are the steps that I have taken to perform this conversion on my HW-16.  You are encouraged to make any suggestions or improvements.

    a) Preliminary Work
Note: As deascribed on the HW-16 Yahoo User's Group, the HW-16 may emit parasitic signals if the PA is not neutralized properly and if the radio is tuned for peak RF power alone.
    b) Voltage Regulator Installation
Important Note: Before starting, remove all receiver tubes lest they be damaged by the heat of your soldering iron / gun, or when resting the radio on its top.  The corner tube (audio amplifier) is a little larger than the rest and is very easily damaged - I know.
CAUTION - should the HEXFET short out to the chassis it (and other HW-16 components) will be ruined.
Note: If you decide not to complete the rest of the conversion you may leave the 'improved' radio as it is.
    c) Building and Mounting the Interface Board and Additional BFO Series Trap Circuit
Note: Be careful mounting the interface board under V4.   When repeatedly removing / reinstalling the board I accidentally grounded the filament trace which glowed brilliantly, smoked heavily and quickly burned up.  I used a piece of wire to repair it.
  
d) Connecting the Interface Board e) Testing and Alignment

f) Adding RIT