Garey Barrell <[EMAIL PROTECTED]> made an utterance to the drakelist gang
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Frank -
This is just my opinion......
A Variac is essentially a variable transformer with a rotating slider to
vary the AC line voltage from zero to as much as 20% OVER the input
voltage. It is NOT an isolation transformer.
A Variac is used primarily to test a unit (actually it's power supply)
to determine if it will work over a range of line voltage. In other
words, turn the Variac down to lower the line voltage ten or fifteen
percent and see if the tested unit's voltages stay within range, then
turn the Variac up to increase the line voltage the same amount and
again check the unit's voltages (and heat sink temperature for
regulators) for proper operation.
Variacs are also useful to bring up the filament power on big
transmitting tubes to minimize the shock of heavy current to a low
"cold" resistance filament.
Generally speaking, it is NOT a good idea to bring a piece of
equipment's line voltage up from zero, slowly or otherwise, since it can
result in all sorts of strange behavior. A good example was posted here
just a few days ago where someone was bringing up a PS-7 power supply
"slowly" and could not get it to work at all. ALL "consumer" equipment
is designed to have the line voltage applied instantly!
The best procedure I have found, and use, for "first time" use of an
unknown piece of equipment is to put a standard 40 - 100W light bulb in
series with the line voltage. Then turn on the power. The bulb should
glow brightly for a second or so and then dim. Depending upon the power
drawn by the equipment being tested vs the light bulb chosen, the bulb
should go very dim and the device will most likely operate at the 15 -
20% low line voltage being applied through the bulb. If this is the
case, merely short out the bulb and proceed to check voltages and heat
for any abnormal indications.
On the other hand, should the bulb glow brightly, and continue to do so,
it indicates a shorted filter capacitor, defective rectifier, bad
transformer, or ... In this case, unplug the rectifier tube (if
present) and see if the bulb dims. If it does, then either the tube or
filter capacitor(s) are defective, or some other fault such as a shorted
power bus is present. Ohmmeter checks (with power OFF!) should show the
fault. If the bulb stays bright with the rectifier tube out, then
either the transformer is shorted, or some wiring or component fault in
the primary circuit is present. If the rectifiers are silicon diodes,
then you would have to disconnect them to perform the above test. In
any event, even if there is a direct short somewhere in the device,
nothing is damaged further, the light bulb just glows brighter!!
One additional step I use is to check the electrolytic filter capacitors
before ever applying ANY power with an ESR meter. In fact, for simple
units such as a receiver or transmitter I check ALL electrolytics with
the ESR meter before applying power.
I own several Variacs, and never use them to "bring equipment up
slowly". For initial testing of vintage (or other) equipment, I would
suggest investing in a light bulb mounted on a board or box with a line
cord and plug, an AC outlet and a toggle switch to short the bulb.
Spend the money you saved over the Variac on an ESR meter and you'll
have a much more useful test setup.
73, Garey - K4OAH
Atlanta
Drake B & C-Line Service CDs
<http://www.k4oah.com>
francobevi francobevi wrote:
"francobevi francobevi" <[EMAIL PROTECTED]> made an utterance to
the drakelist gang
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I have some tube equipment to power up safely. I heard about someone
use Variac. I do not have one and I would know what it is (a kind of
transformer to gradually increase AC voltage?), what type to
eventually buy and a procedure to use it correctly to revive old rigs.
Bests,
Frank
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