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Author Topic: Variac  (Read 560 times)
K1QQQ
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Posts: 272




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« on: April 05, 2019, 02:38:41 PM »

 Huh Huh Huh Huh Huh


hmmm...right forum ??

I believe if one had old tubes or tubes that had been unused for a long time they would wish to have voltage applied 'small' at first.  i.e. no sudden rush of full voltage/current ?


Now..if one had old transceivers all solid state that have been sitting unused would a Variac be helpful or not ? New transceivers that never had any electricity applied (after purchase) would a Variac be a good thought ? I am thinking things like 30-40 years.

I have several goodies (nothing fantastic but good transceivers in their time) bought way back in time and put in storage and never used.


Besides capacitor issues ? Who knows. If it were designed for 120 volts all through the circuitry and various things in the circuits were designed to run at different voltages I am not sure what reduced voltages would mean.


Just cross your fingers..I guess....
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KX4QP
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Posts: 320




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« Reply #1 on: April 06, 2019, 04:00:47 AM »

As I understand it (and I might well be wrong -- I'm not one of these folks with sixty years of electronic experience), the point of starting up a set at reduced voltage is to look for failures such as arc-over or leakage in capacitors that might be detectable at voltages that won't cause other damage.  If you just turn on full voltage, a leaky capacitor might explode; if you sneak it up (at least in unregulated circuits) you might get a chance to notice that same leaky cap getting hot before it blows -- or before the leakage current destroys something else.

Given that even computers less than thirty years old often require cap replacement to function (yes, there are collectors who do this with computers), this seems like a reasonable precaution to take when powering up any sort of electronics (new old stock or used) that haven't been operated in decades.
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KAPT4560
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Posts: 550




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« Reply #2 on: April 06, 2019, 05:07:21 AM »

  I think that a Variac is used as a precaution when powering up unknown equipment. I would want to do a visual inspection, ohmmeter check of B+ to ground and possibly do a preemptive replacement of old electrolytics before power up.

 Be aware that a Variac or autotransformer is NOT an isolation transformer. It won't protect you from shock with AC/DC equipment.

 If a tube has gone to air (gassy), a slow power up won't save it. It will likely need replacement. Tube "rejuvenation" is possible for certain tubes:
 https://www.w8ji.com/vacuum_tubes_and_vaccum_tube_failures.htm
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NE1U
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Posts: 109




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« Reply #3 on: April 06, 2019, 06:10:32 AM »

I used a variac to initially power up 2 Yaesu xcvrs that had not been powered on in over 12 years stored in basements & commercial starage. It was all exposed to ample summer humidity in other words. I did this in spite of my scepticism that the only system being tested is the power supply. Power supplies have voltage regualtors that either do nothing until the supply voltage reaches a certain level and/or are only a ripple source (can that be good)?!?!?

Nevertheless, I did this anyway sans VOM. I was just getting back into ham radio & therefore electronics and had *no* test equipment. The variac, dummy load, & cables were all loaned by a newly found ham club for the warm time. After a few days of monitoring for noise (nasty hum from the low voltage times) or noxious odors, I finally said f-it & just plugged into a power strip equipped with a breaker. It all worked fine.

Sold the whole lot as is and without incident.

Would I do this again? Not sure. This is kind of like morals. You don't know what you will do until you are in the situation. Huh
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KD0REQ
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Posts: 2364




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« Reply #4 on: April 11, 2019, 02:38:56 PM »

power tubes, yeah, either sneak then up or fiddle bias to heat the plates and activate the getter at lower voltage (W8JI discusses several methods on his site.)

with small-signal tubes, put them in a tester. if they look OK but register gas, leave them heating for a while and see if they getter. retest in a day or two, and keep the good ones.

the other issue in old stored radios, especially double and cubed, is capacitors. wax jobs are often leaky or shorted at voltage. old electrolytics may reform under a day long Variac cycle, or not. in HV stacks, best to just shotgun them all out. also check the bleeders.
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