JerryIf the intent is to limit current by controlling voltage then there are
much better and safer ways to do this.
The typical B+ power supply is a low impedance circuit. By this I mean that a
relatively small change in the applied voltage will cause a great change in
input current. It is difficult to control voltage steps by turning the knob on
a VARIAC, especially if you monitor neither voltage nor current. Using this
method is like crossing one's fingers and hoping for the best outcome.
Typically, reforming electrolytic capacitors is the reason stated for using a
VARIAC. If the capacitor has not been in use for years then the chance of a
low resistance in the dielectric barrier between the etched aluminum foil and
the electrolyte is quite high. If a voltage is applied to an electrolytic
capacitor in this state then a high current will flow. This will cause the
capacitor's internal IR losses to produce heat which can cause the remaining
electrolyte to expand and burst the container. If the capacitor reforms
quickly then the internal heat produced is low. If the reforming process takes
some time then much more internal heat is produced.
One method to control the current supplied to a radio that has been out of
service for a while is a dim bulb tester. A fine tutorial can be found here:
https://www.antiqueradio.org/dimbulb.htm . The incandescent bulb's resistance
is nonlinear. At low current the bulb resistance is near zero. As the current
increases, the bulb's resistance also increases thus limiting the current to
the radio under test. So a problem in the power supply of the radio will
result in a large current and a brightly shining bulb. While the dim bulb
tester is great for testing a reworked radio it does not limit the electrolytic
capacitor reforming current to a level that allows the capacitor to reform
without producing a lot of internal heat.
An easy method to reform an electrolytic capacitor is to pull the rectifier
tube and substitute solid state rectifiers in series with a power resistor to
limit the current to a level that will not cause a lot of internal heat in the
electrolytic capacitor. One can use a salvaged tube base to mount this
rectifier and resistor circuit.
The resistor value can be calculated by using the secondary voltage and the
limiting current needed to safely reform the electrolytic capacitor. If the
power transformer B+ winding produces 300 volts between the center tap and one
of the rectifier plates and you want to limit the reforming current to say 50
mA then the resistor's value would be 6k. The wattage would be 300 X 0.05 or
15 watts. This would allow a shorted electrolytic to stay cool or one to
slowly reform without getting too hot in the process.
As the capacitor reforms, more of the applied voltage is dropped across the
electrolytic capacitor and less across the series resistor. Once the voltage
across the resistor drops to about 15% of the applied voltage, the capacitor is
sufficiently reformed to allow for normal operation. This can take 15 minutes
(or less) or dozens of hours (or more), it all depends on the initial condition
of the electrolytic capacitor. Remove the resistor and diode network and
reinstall the rectifier. At this point the dim bulb tester will let you know
if all is well or additional troubleshooting is needed.
This rectifier substitute can also be used to test all coupling and bypass
capacitors it the radio. Just measure the voltage across each capacitor to see
if the capacitor is leaky and a much lower then expected voltage is
measured.Regards,Jim Logic: Method used to arrive at the wrong conclusion, with
confidence. Murphy
On Monday, June 3, 2024 at 07:37:31 AM CDT, [email protected] <[email protected]>
wrote:
MY R390 which was restored by Don Hayward about 12 years ago but has been
idle for past 7 years after moving to new QTH. I've read mixed opinions on
the forum about slowly bring up the R390A with a variac. is there any major
risk with the R390 (non-A)? I will be monitoring current draw with an
ammeter.
Jerry
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