Sunday, July 23, 2017

In the Beginning........

In the beginning, Heathkit marketed the HWA-7-1 power supply that was designed to power the HW-7 and HW-8 QRP transceivers. Unfortunately this power supply  was little more than a battery eliminator with a Zener based voltage regulator. Many of us who purchased the HWA-7-1 quickly found that the PSU was not a good fit for the radios. 

Why? 

Without going into great detail lets just say that the direct conversion (DC) receivers in the 7 and 8 did not respond well to the AC fields set up within the PSU's transformer! The result was some vicious hummmmm which made for some difficult listening. Additionally the primitive voltage regulation circuitry inside the HWA-7-1 left a LOT to be desired. Voltage regulation in the HWA-7-1 is accomplished by using a Zener diode to set a threshold level of 13.8V on the regulating transistor, Q1.

When reverse biased, Zener diodes are designed to provide a stabilized voltage under fluctuating current. However their current handling abilities are very limited; ergo the addition of the NPN transistor to handle the current. Also Zeners are quite noisy and you need to be very cognizant about injecting additional noise into your receive system. 

There are definitely better voltage regulation circuits out there which is one reason to seriously consider either rebuilding your HWA-7-1 (scrapping everything but the case,switches,& power transformer) or using one of the newer switch mode PSUs with your DC receiver. My Alinco 25A switching PSU induces no additional noise or hum into my HW-8 and the cost was in line with older analog PSUs. Money well spent. 

I know that there are HW-7/8 aficionados (read that "purists") that would not seriously entertain the idea of gutting a HW-7-1 PSU. In order to use the AC mains the other inexpensive solution would be to procure a ham fest "find" of an old 2-3 amp analog AC supply (working or not) and re-purpose it by turning it into an upgraded power source for your Hot Water QRP rig. A multitude of schematics are available via the interweb, so don't let fear hold you back! Assemble your parts list for the upgraded supply and hit the hamfests in your area. Chances are you'll find most, if not all, your needed parts at the fest. So get busy!

Alternatively you could go to a battery and ditch the AC PSU altogether. The up side....no induced hummmmm. The down side....you gotta keep the battery charged, and with an intermittent operating schedule, unless you keep your battery on a trickle charge,r you may not have any "juice in the box" when it comes time to fire up the rig. OH, yeah...then there is always the chance of hydrogen buildup in the shack as the battery charges and hydrogen is quite flammable! Be extremely careful!

For several years I ran K7SZ QRP off of a sealed deep cycle marine battery connected to a "smart" charger and that configuration worked well for me. The down side was that the big battery was always getting in the way (my shack was VERY small). My ultimate goal was to use some photo voltaic panels to charge the battery. However, that idea got put on hold when we moved from PA to GA. I still have the solar panels so provided I can find a suitable location for the placement of the panels I will be solar powered QRP!


Here is my ideal HWA-7-1 rebuilt PSU design criteria: 

Full wave bridge rectifier 
IC voltage regulation 
High quality electrolytic caps (4700uF @ 25WVDC or larger)
Fused AC input and DC output
Crowbar circuit that trips at 15VDC
Decoupled AC line ahead of the power transformer (.01uF disk caps to ground)
RF shielding on DC output (copious use of ferrite beads)
Three wire AC line cord. 
Ground screw on PSU case
Anderson power poles on DC output

Which ever route you choose rest assured that your version of the HWA-7-1 will definitely work better than the Heathkit offering. Power supplies are the heart of every ham shack. Information and experience you gain by undertaking this project will enhance your ham radio experience. None of this is difficult. Circuitry exists in numerous publications, including the ARRL pubs, that will allow you to construct a very well filtered and stabilized PSU that you will proud to place on your ops bench. 

I am in the process of gutting and rebuilding a small PSU to power my HW-7 and HW-8 and will give details on the project as I progress. Don't forget the ham radio hobby is a technical adventure. There will always be things to learn in this technical hobby. 

Until next time

Vy 73 

Rich K7SZ





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