[Boatanchors] Excessive Heat in Viking Valiant VFO Compartment

[Garey Barrell]

Mike -

It's a bunch of heat confined in an essentially closed box. It gets HOT in there!  :-)

There is a 'less popular' theory that having the 18k resistor(s) inside help it to heat up faster 
and form an 'ovenized' VFO.

As for the 0A2, they DO get hot.  More important is that it is a SHUNT regulator, meaning that its 
30 mA limit is the amount of current it can draw, NOT the amount that can be carried through it.  If 
you examine the schematic, you'll see that the 0A2 is connected from the 18k resistor to ground.  If 
the VFO tube is removed, the 0A2 has to carry however much current it takes to drop the B+ to 150 V 
with 18k of series resistance. I don't have a schematic handy, but that's what happens.  A 
reasonable design goal is to calculate the resistor  so that the drop is for 15-20 mA in this case, 
or even less.  When the 5 mA current draw of the VFO tube is added, the 0A2 has to draw  _LESS_  
current to maintain the 150V regulated voltage. As the VFO tube current varies, the 0A2 current 
varies too, keeping the constant drop across the 18k resistor constant for exactly 150 VDC.

73, Garey - K4OAH
Glen Allen, VA

Drake 2-B, 2-C/2-NT, 4-A, 4-B, C-Line
and TR-4/C Service Supplement CDs
<www.k4oah.com>

Michael D. Harmon wrote:
> I have an old Valiant I transmitter and a Ranger I transmitter which
> were purchased (CHEAP) from a local CBer that I'm attempting to
> resurrect from the (almost) dead.  In addition, I have a couple of parts
> carcasses I've been cannibalizing components from.
>
> I have a total of four VFOs for these rigs.  Two of them (in the
> carcasses) appear to be in excellent condition.  Both have the original
> 18K 2-watt current limiting resistors with no signs of overheating.  The
> other two (in the transmitters) are badly overheated and charred around
> the hole in the phenolic sub-panel where the 0A2 tube comes through.
> The 18K 2-watt current limiting resistor in one of these "toasted" units
> has been replaced by two 10K 2-watt resistors in series.  In the other
> unit, it's been replaced by a 500 ohm 7 watt wirewound resistor (go figure).
>
> I've heard all the stories about the 18K resistor in the VFO compartment
> and the need to increase the wattage and move it elsewhere.  Initially,
> the fact that the charring was just around the tube hole made me wonder
> if maybe the 6AU6 VFO tube was drawing more than the 30 mA that the 0A2
> was designed to handle.  After I looked up the 6AU6 in my tube manual,
> and found the screen current to be typically less than 5 mA, I realized
> that there's no way the 6AU6 could be overloading the 0A2.  I'm guessing
> that the fact that the VFO compartment is a closed box with no external
> air circulation must have caused enough heat buildup to create the
> charring. Obviously, the resistors in both overheated VFOs had been
> changed at some time in the past.
>
> Is this charring around the hole the typical symptom of the overheating
> problem that everyone always talks about?  If, so, I should be able to
> deal with the problem.  I just wanted to make sure I didn't have a
> unique situation with my transmitter(s).
>
> Thanks for any and all advice, tips and suggestions!
>
> Mike Harmon, WB0LDJ
>