QRP Fun... Heathkit HW7 - Chapter 1 - New Ears

Prologue:  I have a complete Heathkit HW9 Station- the transceiver, antenna tuner, SWR meter, and the matching rare speaker.  My HW9 is a Work In Progress and am currently performing mods on the transmitter to make it more stable

For a long time I've wanted to add a HW8 QRP transceiver to my collection of unique radios that I take off the shelf, operate from the shack table, or set up on a picnic table in the park, or out back packing to make some QRP Morse QSOs. Hamfest and Ebay HW8 prices were way too high for my liking.  One day I was surfing Ebay and found the HW8's older brother- an orphaned HW7.  The seller advertised as "Guaranteed Not To Work (instead of UNTESTED)  Buy it Now!".  The 'not to work' grabbed my attention- just like the street person with the sign that says, "I need money for beer".

Here's someone else's HW7, circa 1972-75, that is better looking than mine.  After reading about the disappointing receiver performance and lack of 700 Hz offset and RIT, this has got to be the second worse Amateur Radio product that Heathkit produced- Note: this is my humble opinion.  

CHAPTER 1.

Sure enough, the HW7 did not work.  Armed with the schematic, VOM, and O'scope I found two burnt traces on the PCB and the RF Choke in the output PA burned out. I thought that maybe lightning was the cause- but no solid state devices were damaged.  I soldered jumper wires around the burnt traces and rewound the RF Choke.  This brought the radio back from the dead- Abracadabra!

I had great fun troubleshooting the radio and making some QSOs on 20 and 40 meters.  The stations I had QSOs with were hard to copy because they were 'Zero Beat' in the receiver.  If I adjusted the VFO to copy better then my XMIT frequency changed and the far end Op couldn't copy me some times.

Other issues I encountered:
>I was being QRMed by couple of nearby 50KW AM BC and strong SW stations like Radio Habana and WWV- received along with CW signals. 
>AC hum in the audio (when on AC supply).
>Selectivity seems about 2 KHz wide.
>Microphonics in the audio.

!! But not is all is crappy- the 2.5 watt transmitter part of the radio seemed to work pretty well.  I became determined to improve the HW7s receiver. 

While having those QSOs... I discovered that the HW7 has no means to offset the transmit vs receive frequencies.

I could live with the BC + SW QRM but not the lack of 700 Hz CW offset, wide bandwidth, nor the microphonics.  I didn't find a lot of published modifications for this radio on the web or in my HW8 Handbook- which covers mods for the HW7, HW8, and HW9.  Then it dawned on me- maybe there just wasn't enough time for geeky hams to come up with a lot of mods- as Heathkit came out the HW8, only four years later in 1976.

Googling deeper, I found a mod from a 1975 QST that added a RIT pot for the user to offset the receive frequency separate from the transmit frequency but it did not produce a predictable 700 Hz offset like the RockMite, HW9, or modern transceivers.  

Producing a 700 Hz offset on each band seemed difficult due to the design of the VFO.  The 40M VFO operates on 3.5 MHz and is doubled.  On 20M, the VFO also operates on 7 Mhz and is doubled.  15M is 3 x 7 Mhz. RIT on 15M is 4X as effective as on 40M.  Using the principle of the original RIT mod, I came up with a more elaborate method of obtaining a 700 Hz offset for each band and a plus/minus 100 Hz RIT pot for fine tuning.  This mod will be described in a future blog entry.

Regarding the AM+SW QRM in the detector... I read that balanced and doubly balanced Mixers are the solution to eliminating unwanted QRM. I found two more promising modifications.  One used a CA3028A balanced mixer IC (remember when ICs were round?) and a mod that added a SBL-1 passive mixer chip.  I harvested both mixers from junk found at my radio club.  I wasn't able to get the SBL-1 to work for me. Then it dawned on me... the popular RockMite and other simple QRP radios have a Direct Conversion receiver like the HW7.  I had two RockMites and made lots of QSOs... and the receivers worked pretty well aside from the BCI (broadcast interference).  

I began to pursue a receiver design that uses a SA602A Gilbert Cell mixer IC that includes an on board RF input amplifier. I've noticed that the Elecraft KX1 and K1 radios also use the SA602A mixer IC (these radios are not Direct Conversion- they have an IF).  Now I have the wheel without reinventing it very much.

On the left is the 700 Hz offset / RIT control circuitry.   I added a pot on the front panel for + / - RIT- which works quite well as a Fine Tuning control.  The lightened area on the right side of the PCB is where approx 70 components were removed from the receiver and audio circuitry.  On the right rear panel is the HI-Per-Mite 50 dB gain audio amp / 700 Hz audio filter.  I purchased this kit from the Four State QRP club.  It works exceptionally well, BTW.

Before I began the SA602A receiver prototype... I removed all of the receiver components from the HW7s PCB to see how much real estate I have to work with.  My goal is to build the new Direct Conversion receiver on a piece of PCB using Manhattan and/or Dead Bug style construction and mount it on top of the HW7s PCB. 

Here's RCVR Prototype # 1.  A PC Board like this will fit in the open space where the receiver components used to reside. The Pre-selector tuning cap will still reside on the front panel.  

I've plagiarized a composite receiver design from web sources that use a pre-selector parallel tuned circuit (much like the original HW7 receiver) that doubles as a Z matching transformer  to provide balanced input to the SA602A at approx  3K ohm Z.   T50-6 toroids  and the www.kitsandparts.com toroid app were used to calculate the # of turns and tuning capacitor size for 7-21 Mhz.  

I'm not sure that the Z match between the Preselector and the SA602A mixer is the best it can be.  Further experimentation is needed.

This sketch shows the mixer output and the Hy Per Mite Audio Filter/Amp.  Pin 6 is the external signal input from the HW7s VFO.   

I wanted good audio filtering at ~700 Hz.  So, I've added a 4 States QRp Club's Hi-Per-Mite active audio filter with 50 dB audio amplifier was obtained and installed.

Side note:  Working along side Geeks in my professional life.. I've noticed that they have a tendency to "Gold Plate" a project.  Something tells me that I will be going down that path here.

At this point in my experimentation- these issues do not make my HW7 not ready for prime time:
>RF Pre-selector not sensitive enough of 40 and 15 M vs 20M.
>Lack of audio gain.
>Loud BC and SW stations still being detected when propagation is good.

Future blog 'Chapter #' entries planned:
>Techy info on the 700 Hz offset and RIT.
>Addition of PK-4 Keyer for Iambic + Straight Key + side tone.
>Addition of Freq-Mite frequency meter.
>Making the 'best it can be' Direct Conversion Receiver' for this rig... or knowing when to stop.

End of this Blogger entry.  Please email me for info or if you have ideas for my project.  71r5, Rich KR7W...  -30-