On Apr 14, 10:47 pm, Fred McKenzie wrote:
In article . com,

"wb5kcm" wrote:
My Ranger has all the mods in the power
supply, solid state both HV and LV. The LV capacitor is 330 mfd and
the HV cap is 165 mfd.

Randy-

Suppose both the RF section and the modulator used 100 watts of power
each, and filaments used another 50. That would be roughly 2 Amps at
120 VAC. You should use a fuse about twice normal current, so 5 Amps
would be reasonable.

If that blows, it very well could be due to inrush current related to
the solid state rectifier conversion. Vacuum tube rectifiers would have
required a warm up, which would bring their output voltage up gradually.
Using something like a 47 Ohm 10 Watt resistor in series with the input
of each set of rectifiers, might be all it takes to tame it.

What is the voltage across each filter capacitor while in standby or
key-up? I would worry that voltage might exceed the capacitor ratings
after conversion to solid state rectifiers, especially under no-load
conditions.

Fred
K4DII

Hey Fred,
The new electrolytics that I put in are much larger than the original.
The low voltage power supply now has 330mfd @ 450 volts and the high
voltage now has 165mfd @ 900 volt rating. With the solid state
rectifiers, I am getting low load, unkeyed voltage, LV PS 305 VDC
(regulated with 2 OD3 tubes). The HV PS is now about 625 VDC. Last
Friday, I picked up several inrush current limiting thermistors from
Allied Electronics. These have a 2 amp rating. I put one in the Ranger
in series with the hot AC power lead, down stream from the fuse. This
made a huge difference in the inrush current. This thermistor has a
high resistance cold and as soon as you apply the power it starts
heating up and the resistance drops to a very low level. It still has
some resistance and gets pretty warm, about the same as a 5 watt power
resistor. With this added resistance in the AC line, I notice that the
power supply voltages have dropped somewhat. The LV supply is still
about 300 volts due to the OD3 regulators. The HV side is down to
about 580 volts under full load. This should make everything run even
cooler and last longer. Pretty happy with this thermistor so far. My
only concern is how long will it hold up with the current hitting
close to its max rating of 2 amps at times. Anyway, I will watch it
and see what happens. I also put one in my Hammarlund HQ-170 and it
seems to be doing very well there. When you turn on the power it
slowly comes on. That will be very nice on the tube filamants and
everything. Thanks for your input. 73, Randy, WB5KCM