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This nice Paraset replica built by Dan W7OIL. Check out Dan's Website here:
http://www.w7oil.com/amateur.htm |
Chuck K7QO, the spark plug of the QRP-Tech Yahoo Group challenged the members to build some kind of QRP project during the month of November. This challenge became the stimulation to get my Paraset build started. My goal is to have my Paraset working and presentable for the Radio Club of Tacoma's Straight Key Night Celebration (Dec 31, 2016) and hopefully make QSOs with it.
To build and operate a Paraset has been on/in my bucket list for about 5 years now. At the 2015 pQRP's Salmoncon... member Steve WG0AT brought his Paraset that he was gifted from Paul W0RW. Looking and touching Steve's little radio motivated me to start collecting parts for my someday build.
A BOLO to fellow Radio Club of Tacoma members who are home builder hams (one who has previously built his own Paraset)... was initiated to obtain the parts I was missing. Hat tips to Jack's Corner Parts Store and Bob's Surplus... Still missing is the National 10009 friction drive receiver tuning dial mechanism... Construction has begun anyway.
The first order of business is to build the AC Mains Power Supply. Years ago I obtained from the "Free Table" at the radio club- a defunct CB power supply which is about the same size as the original Paraset AC Mains supply. This power supply was loosely engineered and built to fit inside of this metal box.
Reading the blogs of other Paraset builders reveals that their PS HV (also called HT in old documents) output ranged from 300 to 370 DC volts. Found in my box of 'heavy iron' was a suitable (to the voltage requirement and to the enclosure) HV power transformer. No volts will be wasted using a vacuum tube rectifier and there will be way less heat inside of the PS enclosure with a full wave solid state rectifier using 1N4007 diodes.
The next PS puzzler was HV filtration. Choke Input? Capacitor Input? Minimum HV voltage swing in the XMT mode when the key is up vs key down is the goal. Too big of swing creates a a poor sounding chirpy note from the Paraset's 6V6 transmitter. [note 1: some distinguishing chirp is OK in my book. Note 2: 4 to 5 watts output is expected; bordering on the upper edge of QRP].
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| Ha! What a mess. Prototyping the HV PS. I did shock myself by picking up a charged up Electrolytic cap by the leads. |
Goldilocks and 3 Bears method of PS testing: Using Capacitor Input HV filtering resulted in too high output voltage (over 420V). Choke input lowered the voltage down to 330V. Both methods resulted in a 80 to 95 output volts swing from Key Up DC load of 15mA vs Key Down load of 60mA.
Many Pre WW2 entertainment radios that I've restored placed the power supply filter choke (the speaker magnet field coil) in series with the transformers secondary (HV) center tap winding. This method is rarely mentioned in old ARRL or Jones radio handbooks and provided no techy pros or cons for doing this. Experimenting proved this method to be best for key up vs key down voltage regulation. Now KU vs KD swing is around 20 volts but the output voltage is 300 V instead of the 350 V originally hoped for. Now XMTR output watts will be more QRP. Baby Bear again wins.
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| Cocktail Napkin Final PS Design |
The RCVR in the Paraset is a "2 Tube Blooper" regenerative receiver. (more on this down in a future blog entry). To prevent 'tunable hum' in the Regen receiver- the filaments are powered by DC volts. The 5 V and 6 V filament transformer windings are connected in series to a bridge rectifier then fed to a switching buck-boost Voltage Regulator module: Measured VR watts IN vs watts OUT reveals 80% efficiency using a 5.6 ohm resistor = 1 amp load to emulate the Parset's 3 tubes. Also no noticeable heat present. 5 of these VRs came from Amazon for $9. See spec: https://www.amazon.com/Qunqi-
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For the more Geeky: Inside of PS... Copper clad Printed Ckt Board is used as a ground plane substrate. There's some Manhattan and Dead Bug this build.
Upper L white gizmo is a 6V incandescent light to indicate the DC filament voltage is available. Round blue gizmos are SAFETY CAPS from AC line to GND. [I recently learned that these are useful to bypass RFI to GND that might be generated within the PS circuitry. Also to bypass unwanted interference the AC power cord might be receiving plugged into house wiring.]
Upper R KOOLOHM device is the HV Bleeder Resistor- which consumes 11mA @ 300 VDC to somewhat stabilize key up vs key down voltage swing.
Front panel view:
The NE-2 bulb indicates High Volts above 80V or so.
Thanks for reading this far.
End of Part 1 of Paraset Spy Radio Build. 73 DE KR7W CL
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I began prototyping the Paraset two tube Blooper Regen Receiver per the schematics found on line... and it didn't work very well- which has been my experience with other vacuum tube regens I have attempted to build. I consulted with the ham club member who has built many regen receivers and I walked away thinking that I do not know what to look for to know if the RCVR is working or not. Further investigating was needed.
Googling took me to an in depth article about the Design of Regen Receivers from Nov/Dec 1998 QEX magazine by Charles Kitchin N1TEV. Since the Paraset uses the same tuning caps and coils as the receivers showcased in Mr Kitchin's article and I had the solid state parts on hand... I'm in progress to build a receiver that I could be more successful getting to work. When I am satisfied that 'I get it'... then I'll substitute the 6SK7 vac tubes + 300 VDC for 6 volt 2N2222s and J310 FETs.
Regards,
Rich KR7W
