Replacing OA2 VR Tube with Zener Diode

Ed Tanton n4xy at ATT.NET
Wed May 2 12:33:57 EDT 2001

Hi John... I've given this considerable thought. I think the best thing to
do for this is to use three 5W zeners totalling ~ 5V LESS than the VR
voltage. That way you don't have to go into the VFO compartment to remove
the VR tube. Of course, if you haven't already replaced that pesky dropping
resistor (that is also inside the VFO compartment), then I would CERTAINLY
do that AND pull the VR tube at the same time. Don't bother trying to remove
the resistor... just short it with a simple jumper wire (a WHOLE LOT EASIER)
and find the wire where it enters the VFO compartment-then trace it back to
where it connects to the power supply terminal strip and insert the new
resistor there. You can add the zener(s) there too. While the resistor
formulas are somewhat different for VR-tubes vs zeners, I think the idea is
close enough that you can just use the same value. (I THINK I once
calculated the 'right' value, using the correct formula, and it was within
1K.) By using 3 ~ equal value 5W zeners, you have 15W worth of zener...
giving yourself quite a margin. I did something similar when I replaced my
Ranger II's VR-resistor, using three 3W resistors.

73  Ed Tanton  N4XY  <n4xy at>


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