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Monday, March 10, 2014

QRP Fun... Heathkit HW7 - Chapter 2 - RIT and CW Offset

Prologue:  Today is 3/10/2014.  I added this mod back in Dec 2013.  I am looking at my work to come up with a description of what I've done... and I am having a hard time remembering what some of my initial thinking was.  With that said, I will attempt to be clear to describe what I've done.

As mentioned in Chapter 1, Googling found me an article for RIT.  The article was sritten by John Grebenkemper, WA6BVA, and appeared in the July 1975 QST, simply titled RIT for the HW-7.

Here's how I think RIT is created in John's HW7:  The battery supply feeding the VFO is stabilized using a Zener diode to bias a NPN switching transistor to be a voltage regulator.  Now that the VFOs FET is stable then the FETs Biasing can be altered to predictably change the frequency.  Resistors in series with the cold side of RFC-1 (Q2 Source to GND) alter the bias to change the frequency.  On 40 meters additional biasing is needed so an extra resistor is added between the Drain and cold side of RFC-1 (via extra contacts in the 40M bandswitch)...Please read the article for more in depth info and to check my accuracy.

I did the mod as John's QST article prescribed.  It worked very well... but I didn't like the fact that the range of the RIT adj pot increased as the frequency increased.  Example: On 40M a 1.4 KHz shift occurs, On 20M there's a 2.4 KHz shift, and 15M results in a 3.4 KHz shift.  It works and the VFO is amazingly almost drift free...but there's too much RIT range for my liking.  Using the RIT Pot for fine tuning was not all that fine especially on 20M and 15M. Tic marks on the front panel for each band are needed.

Here's how I think RIT is created in kr7w's HW7:  Since transistors Q103 and Q102 switch in/out the RIT adj pot while on receive... and on 40 meters a spare set of contacts from the band switch are used to add the additional 3.9K biasing resistor... I axed myself, "Can I use additional contacts in the 20 and 15M bandswitch to operate transistors to add the appropriate resistance for preset RIT of 700 Hz for CW offset as well as a RIT adj pot?"  After some pondering... that's what I ended up doing.  

But first, do this for me:  If you are interested in modifying your HW7 like I did or want to use the concept for another project... then please read John's QST article and get a more techy description of what he did.




Comments below refer to this schematic from the QST Article.  

VFO Voltage Reguator:  The Q104 voltage regulator BJT takes the place of the original R23- 100 ohm resistor.  Using the holes in the PCB from R23 and a couple of additional drilled holes- I added Q104,the 11V Zener diode, and 4.7K.  I didn't have the exact Zener, so I soldered two 5 volt Zeners in series.  [Sidenote:  according to my super Geeky engineer ham radio friend, Bob... this type of voltage regulation is far superior to Linear regulators and/or Zener diodes by themselves].  The regulator output is the Zener value minus .7 volts from the voltage drop across the transistor, BTW.

I added transistor Q101 and R101 next to original RF Choke RFC1.  I drilled holes in the PCB... but the VFO tuning capacitor must be removed to do this. (I will have, "If I was to do this differently- this is how I'd do it" thoughts at the end of this blog entry).  A wire was run to the  12V Key Line (+12 volts applied when key is down) and to the RIT pot.  C101 bypass cap was installed under the PCB in the area of RFC1. 

The QST article shows a 3.9K ohm resistor-R104 being switched in when on 40M only.  For an unknown reason to me now, I soldered this resistor in permanently.  All 700 Hz offset resistor values were determined with the R104 in the circuit on each band.



This is my rendition (first hand drawn then annotated with Photoshop) of the RIT modification to my HW7.  Circuit description follows...  Note:  Just like there are no A, B, or C ionospheric propogation layers... there is no Q102A- there used to be... but it was removed to lessen the complexity.

Example of Operation:  40M RCV- When RIT pot is set in the center (~50 ohms) the bandswitch applies +12V (thru 10K) to the B of Q102B.  This transistor conducts and GND appears on the Collector.  This completes the path of 220 ohms in series with the RIT Pot which causes the VFO to shift its frequency ~700 Hz from the Transmit Frequency.  Note:  The 220 ohms was determined by trial and error using my Icom transceiver with my Fluke VOM.  The procedure is described below.   

40M XMiT:  When the Key is down, Q103 conducts and grounds out the Vcc on the Collector.  This causes Q102B to not conduct which opens up the RIT 100 ohm pot path.  Also, with Key Down... Q101 conducts and GNDS its Collector which places the 120 Ohm resistor in place of the RIT pot resistance path which determines the XMIT frequency.

This is the circuit board I fabricated to contain Qs102B-D and Q103.  Construction is in the Manhattan and dead bug style.  A hand held Dremmel tool with a conical shaped dentists grinding stone was used to route out some islands for wires and BJTs to be blobbed soldered to. Looks messy- but it works.  Wires to/from the board route along the vertical sides of the chassis.  No X-country wires.  All transistors are BJT MPS2N2222 acting as switches.

Regarding the RIT pot I installed... I lucked out and found an old junked out Cushman CE-3 service monitor at my radio club.  I harvested a very nice 100 ohm pot with a long enough shaft to fit through of the two front panels of the HW7.  


Here's a foto of the location of my RIT pot.  Also seen is the bandswitch where I picked up spare contacts for the RIT/offset circuitry.


Here's a foto of my HW7s inner front panel.  Hole 1, 2, and the RIT pot hole were already drilled in the inner panel (but not the front panel) before I rescued this radio.  The once pristine green front panel has been defaced with holes for the RIT control... and most recently holes 1 and 2 have been drilled in the front panel for push-button switches for the upcoming Freq-Mite and PK-4 Keyer controls. 

If I were to do this again:  
>Even though Q101s functional position is where I put it- next to RFC1, under the VFO tuning capacitor... I would put it on the homebrew PCB.  My reasoning is that since all wires are DC control (no RF signals) then having Q101 on the PCB would ease in trouble shooting and would lessen a couple of wires running along the chassis walls.  


>I will order some small 500 ohm Ten Turn pots to adjust the offset resistor value for each band. The pots should be easy enough to solder to the PCB with the transistors.  

700 Hz Offset test procedure:
Note:  When the HW7 is transmitting the RIT Pot has no affect on the output frequency.  


Pick a band and transmit a carrier with the HW7 into a dummy load.  Tune in the HW7 with a good HFT (HF transceiver).  Zero beat the HW7s signal in the HFT or make sure it is tuned in properly for a 700 Hz offset.  Do not touch the HW7s VFO dial from this point on.

Connect the HFT to a dummy load and transmit a CW carrier on the exact same frequency that the HW7 was received.  On the HW7, make sure the RIT Pot is in the middle of its range.  
Do not touch the HW7s VFO dial to tune in the HFTs signal. 

Measure the beat tone from the HW7s Audio out with an audio frequency meter- Fluke VOM or equivilent device.  This is the HW7s offset frequency.  It should be 700 Hz or the frequency you like to listen to CW with.  Adjust the resistor value in series with the RIT pot for the band you are operating on... until 700 Hz is obtained.  

End of procedure.

I was working on adding the Freq-Mite and PK-4 keyer to the HW7... and in the process of not treating sensitive semiconductors properly... I somehow blew out the PIC chip in my PK-4 keyer.  I ordered two new chips plus a complete unit with circuit board to install in my HW8 someday.

Still to come:

> Progress on the 'new ears-best it can be SA602A Mixer direct conversion receiver.
> PK-4 Keyer for paddles + SK + sidetone.
> Freq-Mite frequency Morse Code output frequency counter + its sidetone.


Sidetone is a big deal to me.  I do not like to listen to my Morse sending via square wave audio tones, like the HW7, RockMite, PFR-3, NE555 Astable oscillator and PIC based audio generators.  I've been experimenting on how to filter out the squares in square waves for more pleasant CW listening. 

End of this Blogger Entry.  Please email me for info or if you have some ideas to share regarding this project.  71r5, Rich KR7W... -30-













Sunday, March 9, 2014

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-


  

Thursday, March 6, 2014

The Lectrokit Spider Page...




Dah dit dit dit dah... Looky Here!  It's the Lectrokit Spider QRP Rig.

Info:  The content in this blog entry was originally authored on Sept 8, 2012 and posted on the old kr7w.org ham radio hobby website.  Slowly I am transferring old web pages into blogger entries.  Please read on...


I've had my Lectro-Kit Spider SP-1 for about 10 years.  Someone from my radio club gave it to me cuz I am one of those 'QRP Hams'.   I couldn't figure out what make and model it was so I've never tried to get it on the air.  Recently I stumbled across a photo of it on the world wide web... which led me to the newly archived 73 Magazines on the web where I found the SP-1 lurking in the January 1993 issue.

The SP-1s came in three flavors:  80, 40, and 20 Meters.  I can tell from the number of turns on toroids that my unit is a 40 meter model.  The 73 article says it outputs 1 watt.  There's a RIT control that changes the voltage on a varactor diode to pull the crystal oscillators output frequency in the receive mode.  The receiver appears to be a direct conversion design.  It is sort of like a RockMite without a controller IC chip.  The complete kit back in 1993 cost $39 postpaid.

I spent some time with my SP-1 and know that it does not work... and I bet it never worked.  A lot kits that are given to me by radio club members have been built incorrectly and it seems like the builder didn't delve into resolving the problems.  In this kit...  At first glance... it looks to me like the crystal oscillator has the wrong  transistor installed.  I am sure that I can find a 2N2222 around here someplace.

So for now- back to the shelf it goes... as I am currently frying up other fish from the Retirement Things to Do List.  I hope to get back to the SP-1 after the cold rainy season begins here in the Pacific Northwest.


 I am especially fond of the 'Built In Hand Key' (a piece of brass strip with a rubber foot as the key knob) on the SP-1.  But if you don't like the feel of the built in key... you can plug your own key into the 1/8" aux. key  jack.  Also there's an earphone jack for receive audio from the LM386 IC audio amp (like Rockmite, Heathkit HW-9, KX-1, and others).  It's too bad that this SP-1 has been hacked with the addition of the RCA jacks (Antenna and +12 V) just below the brown bakelite terminal strip.

This little XTAL Controlled QRP radio reminds me of the WW2 resistance fighter  Paraset Spy Radios equipped with One Tube 6V6 XTAL controlled XMTR and one tube Regen detector with a one tube audio amplifier.  Here is One Ham's Paraset Page      Use Google for others.

One day you will hear me calling CQ SOTA from a 6,000+ ft peak in my SOTA neighborhood with the SP-1.   

Thanks for reading this far.  71's, Rich KR7W