Variac protocol?

pfcs49

Phil
Subscriber
I need to test some solid state gear that's been out of service 20 years. I want to do no harm.
What's the consensus on bringing it up?
Starting voltage and time to 100% voltage?
 
This is a tricky one. The variac is of most value with tube gear, where the high voltage capacitors require reforming. It doesn't offer any protection against shorts though, which are what will do the most damage in solid state equipment. I am curious what others have to say, but I've never used my variac when I repair solid state equipment, I just hit it with line voltage and watch the current carefully.
 
Dim-bulb tester is better suited for solid-state stuff imo. But, if you have a Variac that shows current draw, you can ramp the voltage up slowly and shut it down if current draw accelerates. Also, some gear doesn't like being current limited (V-Fets for example) , so asking here is a good idea.
 
I've had some SS gear get pretty stupid at reduced voltages. Usually I go the dim bulb route when I have to work on this stuff, using a large enough bulb that it won't actually limit anything at idle. If its class A, you'll need a decent size bulb but AB stuff tends to idle at a pretty low current.
 
Don't variac any of the Sony V-Fet units.

Dim bulb tester at low wattage to start reforming the caps then up the wattage a couple steps every hour then if all is good, go line level.
Steps would be 25w, 60w, 75w, 100 or 150w depending. Each level limits amperage and higher wattage lets voltage go up. Or you could use the DBT with a big bulb and the variac at low medium high and line levels to do the same thing.

In each case the DBT limits the current which the variac will not do and it is the current that will help release any smoke.
 
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For what it's worth, with most old SS gear, I start at 50~60V on the line, wait an hour or more, then ramp up 10V, wait an hour or more, etc., until I reach 120V or trouble ensues. Always bring up the variac through the first few volts very slowly while watching the ammeter. If there's a short, this is when you will learn about it in most cases. As mentioned above, there are a few cases where the equipment normally exhibits bad behavior at low line voltage.
 
I like the variac because I can set it to 10-20% mains voltage
and take my time and do a full health check of the psu,,,bias etc...
If things are in order then increase another 10-20% and repeat checks.
Certainly a DBT would be helpful, must build one.

I've had some SS gear get pretty stupid at reduced voltages.
Spent too many hours trying to work out what was wrong with a Luxman L-480, using the above procedure, answer=nothing wrong.

Seen some posts where some of the yamaha (VFET??) power amps shouldn't be variac'd.
 
SS gear, use a dim bulb. It’s best for detecting shorts.

Variac for reforming was more of a thing way back on tube gear with wax paper caps.

Either is useful for unknown gear that has been out of use a long time but are not going to “save” anything if it truely is in need of repair.

I’ve had old tube gear come up to play around 100v and then caps let go. That’s fine, tells me the gear is working enough that a recap is a worthwhile effort. All old tube gear I see needs recap anyway.

I’ve had SS gear dim on the DBT but still show some glow, and let go on line voltage. Or didn’t dim enough to know condition but run fine on line voltage. Big units are tough to dim bulb even with 150w bulbs.

On SS gear a dim bulb with a 60W bulb will give the system an easy initial charging. A minute is plenty. Then you could jump to 100W and do a minute. If you have dimming, it’s line voltage time.

As mentioned. V-FET and any FET dont’t do well in limited voltage so Variac on SS probably is not a good plan. Current limit (dim bulb) is probably not good on V-FET. That stuff is tricky to revive and repair.

I get a LOT of unknown SS gear and some probably out of service for many years. A quick DBT to check for shorts and then straight to line voltage. Works well.
 
As above, find out what units should never be brought up on a Variac. Caps that have been sitting can benefit from a slow start, even on SS gear. Most of the problem happens when the line voltage goes over 100 VAC. Caps that aren't complete trash will form quickly at the reduced voltage and be OK. Then, slowly increase the voltage, maybe steps of 5 VAC, to get to 120 VAC over 5-10 minutes. Since the line can go even higher, maybe another slow increase to 125 after it's been running a while.

FWIW, the manual for my big Tek scope says to use a Variac during service and troubleshooting. It has a switching supply. It was working. I ran it up with the Variac. It blew the supply. Will I do that again? No.
 
ConradH, nice directions!
I have dbt and variac.. I don't do tubes so the dbt is for all the ss stuff. I use the 130vac/10A variac to adjust my 2 speed mikita grainder sander to a polisher speed and play with house hold appliances for fun :D, but avail for the bench, too
 
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