Gary's Awesome Adventures in Amplitude Modulation (AM Radio Transmission & Reception) :

Prepping the Johnson Viking Ranger for Station Use, Part 3. 

 

I took the liberty of posting Greg Latta's photo of the line plug and fuse mod, because, frankly, his photo was better than mine, and also there's not really that much involved.  

 

 

 

The fuse holder is ordered from Mouser as indicated on Greg's parts list.  I also ordered the three conductor cord while I was at it.  Simply follow the original cord connects, and wire the cord's ground to...well... ground!  You can also get a partial view of the HV filter caps and equalizing resistors.  Again, I did the very same thing.  I could find no better arrangement.

 

 

I found I had a little variance in the positioning of my particular fuse holder.  I don't know if Mouser started stocking a slightly smaller base holder or not, but I was able to place mine a tad more vertically with plenty of clearance and accessibility.  Apart from this, again, I followed Greg's suggestions entirely.

Having done the re-cap, next I took on: the much feared Chernobyl Resistor!  That famed 18K resistor, also known as "R3", that strikes fear in the psyche of all Ranger owners.  If resistors could talk, this one would sound like Boris Badenov!!  

 

 

 

 

 In all fairness, it's rep is well to pay heed to.  The collapse of R3 can cause damage to the VFO, and you don't want that.  Nosir-eee.

For this, I needed to remove the keying circuit sub chassis and remove all the tubes in the area including the two rectifier tubes to have room for the screwdriver.  Any terminal strip screwed onto the plate which faces the key circuit sub chassis should be removed as well.  After removing the cover plate, what you will see is what the above photo shows.  Boris Badenov's hide-out.  There he is, sneaky little devil, in the lower right.  Don't be fooled by the brown-grey-orange tricolour, its really the colors of Pottslyvania.  Fearless Leader probably has a cam hidden in there somewhere.  Probably as stash of Molotovs as well.  Proceed with caution.

 

 

Here's a closer look.  As someone on one of the Facebook Groups pointed out, it looks brand new!  It probably would have been years before it actually broke down.  But discretion (and and ounce of prevention!) is the better part of valor, says I, and I chose to replace it with a 5-watt wire-wound.  Also from Mouser.

 

 

I opted to install the replacement resistor under the deck but near to the VFO entry port, because while I wanted the heat to be conducted away from the VFO (there is some varying points of view on this), I am not really comfortable with long wire lengths being strewn hither and yon.  So, borrowing a convenient screw already there to ground one of the new electrolytics, I installed a new terminal strip on which to place the new resistor.

 

And there it is.  Looks good, easy to access, in an un-cluttered space.  Should I have used an 8 or 10 watter instead?  Maybe... but it's better than the 2-watt resistor that was there.

 

So, what's the story behind this critter?  Let's look at the math.  (this is a cut and paste from one of the forums dealing with the subject.  It was just easier to do this than to have you think I actually  sit down with the schematic and do this stuff.  I do, when it comes to homebrewing or reverse engineering unknown mods.... but this fella already had it laid out.  Colour me lazy.)

The (in)famous "Chernobyl resistor" (R3) is used to drop the 300 volt line to 150 volts. So there is 150 volts across it.

From this we can determine the current flow:

I = E/R

E= 150 volts

R = 18,000 ohms

I = .00833 amp or 8.33 mA.

This is reasonable because the 0A2 has a minimum current of 5 mA and the oscillator tube screen draws only an mA or two.

Now for the power dissipation:

P = E I

150 volts times .00833 amp = 1.25 watts

So a 2 watter is theoretically big enough.

 

...ahh, but over the years, guess what begins to alter value with heat?  Yeah, that 18k/2 watt won't stay 18k forever. and 67 years is a long time to dissipate.  Ohms lost means increased current and increased heat.

 

Yeah, replace that.   Whether in the VFO or outside, choose your poison.  The point is to protect your historic gem, there.

 

That's it for now.  More to come, including the installation of D-Lab's K2 PTT/ SS rectifier.

 

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