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Thursday, December 8, 2016

WW2 Paraset Spy Radio... Part 2

Construction of the radio has begun.  The front panel- made from a layer of .040" PCB material sandwiched with a layer of .040 aluminum- left over from the small camping trailer I built.  These materials were chosen because they were on hand... as well as- I didn't think the trailer aluminium was thick enough to be sturdy enough by itself- and the copper PCB allows for dead-bug and/or Manhattan component installation.  Holes were drilled per one of the many scale drawings found on the web. 

Sandwich of PCB, Aluminum sheeting and drilling guide.  The front panel is cut to be 1/4" wider on each side to provide a lip for the chassis to fit in its wooden box.
Aluminum front with many components installed.  [The annotation is 'photo-shopped'- not actually stenciled on the panel.  Note error:  RX Band switch should read 'TX' Band switch. ] 

Some components shown from the back.  

All holes drilled with .013" drill bit for #4 screws.  Holes for capacitors and other devices with a shaft were enlarged with a step bit.  The tube socket holes were drilled with a 1-1/8" hole saw.  

The two major challenges for me to create the Paraset replica like I have in mind are: 

1. The National model 10009 Tuning Dial with 'pinch wheel' gear reduction tuning for the receiver.  Some builders don't bother with the pinch wheel mechanism- instead they install a small variable tuning capacitor in place of the pinch wheel shaft.  The large tuning knob is course tuning; the small knob provides vernier tuning.  This method is a good disguise and probably functions quite well.  

2. The built in Telegraph Key.  

A very nice replica shows the tuning dial I desire to have on my set.  Underneath of the large tuning knob is a brass wheel almost the same size as the knob skirt.  The small tuning knobs pinch wheel mates with the large wheel which creates an approx 3.33 to 1 ratio.

1. No pinch wheel dials were found scouring the web and radio club builders didn't have exactly what I was hoping for.  I found that some builders had built their own dials.  So I set out to do same- with some help.   .026" brass sheet (for hobby use) was obtained from Amazon.  pQRPer- PJ N7PXY generously offered to fabricate the brass wheels for the tuning dial in his metal working shop.

Here we are in PJs shop.  The brass disk was cut using a 3" hole saw.  The mandrel in the chuck is a 1/4" bolt.  The disk was cut down to 2.5" to fit behind the 2.75" tuning knob skirt.  

The drawing called for .75" wheels with .15" center hole.  The small wheels were cut using a smaller hole saw and sanded smooth.  The end result size was .79".  Close enough.

Back in my workshop... the large brass wheel was drilled to attach to the large tuning knob skirt- using the same screws that hold it to the bakelite knob.  The small pinch wheels were assembled to their shaft and inserted into its bushing through the chassis.  

Curses!! It did not work. Somehow the bushing or capacitor hole is in the wrong place by 1/8".  The pinch wheels were just touching the large wheel- but not over lapping it.  Recovery was accomplished by making new pinch wheels using the next larger hole saw size.  It's a good thing that the pinch wheels do not need to be precisely round- as there's no metal lathe here.

The larger sized pinch wheels [ .97" dia and no spacer between them] hug the larger brass wheel with about 1/16 inch of meshing surface.   The pinch wheel shaft is 1/4" diameter aluminium stand-off material with 6-32 threads.  The shaft bushing is from an old junk box bakelite bodied 5 watt potentiometer that was easy to harvest with a few hammer blows.     

 2.  The Telegraph Key- As a CW straight key user... I am hoping for a key with a firm feel- not flimsy that feels like it is about to fall apart or a key that goes click-clack when the knob is pushed down.  It's noted that some builders have adapted industrial electrical switches and others used PCB material to create their key.   

On line info reveals that there were two styles of keys used in the original sets-

Paxolin version key was included in the wooden box radio sets- which were built first.  The metal "Cash Box" sets had the plastic body key.  

The plastic key was attempted- but using hard maple in lieu of lexan or plexiglass.  But the key body parts couldn't be cut precisely enough without putting my fingers in danger.  A cutting jig assembly needs to be built.  The Paxolin version was attempted. [Paxolin is an electrical insulating material with sandwiches of plastic and paper molded into sheets- sorta like PCB but no copper face].  Since the diagram came with a scale in MM- the drawing was imported into a CAD app, enlarged to scale, and printed to be a cutting guide.  The keys parts were cut from PCB material with a diamond wheel in a Dremmel tool.  

Ver 1 prototype of Paxolin design PCB key.  The spring/lever is .040" thick and .5" wide.   It feels sorta flimsey.  I am toying around with using a metal lever- but harvested from what?  Hacksaw blade with brass screw contacts? 

Whilst looking for a toggle switch... This switch was discovered.  Perhaps the lever with contacts can be harvested and melded into the PCB Paxolin key.

Some useful Paraset links that are inspiring to me:

End of Part 2.  Thanks for reading this far.  For Blog entry Part 3- I hope to have either the RX or TX working.  QHH es 73 DE KR7W CL

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