Gently winding up Variacs

Hi all,

If a piece of faulty equipment hasn't been fired up for like many

*years*, what rate would it be advisable to slowly wind up the supply voltage to it so as not to blow anything? I'd guess 24hrs for a 240VAC DUT would do it but it's just a guess. I blew something in a scope up one day after being too impatient; very annoying. Whichever component it was that went phut filled the room with acrid smoke. I still have yet to track it down. And I'm also guessing one could get away with starting the process at 50VAC? What does the Panel think?
Reply to
Cursitor Doom
Loading thread data ...

Its not a panacea, just gives better odds against an electro cap failure, a totally dry one will not reform. I don't know if its on the WWW but the Colossus at Bletchley Park is wound up daily via a geared motor drive , over 40 minites IIRC, and also down again in the afternoon. As a result they just loose about 1 thyratron a year, replacing caps would be no problem

Reply to
N_Cook

Ever hear of a dim bulb tester ? This is an incandescent lamp wired in seri es with the mains. If you have a blown AC fuse you can just jump it out bec ause the DBT will limit the current. In the US, a 100 watt lamp will limit the current to under an amp. On like a solid state audio amp or alot of oth er things it may light up in the beginning but will dim out as the main fil ters charge up. If it never dims you start disconnecting things until it do es and that is your short.

Some technologies do not like this, here and there, some depend on the stee p rise in main B+ to start, sometimes through a cap to a lower source but t hat is used less often because it precludes certain methods of line isolati on.Some TVs that had scan (LOPT) derived sources had problems with it, some SMPSes have a problem with it and most things that are microprocessor cont rolled for the power on have a problem when the standby is up, then the inr ush of the main power goes on and kills the AC. Steps must be taken to rect ify this problem if you want to use the DBT on such units.

Tube units also need to be handled differently. If it has selenium or coppe r rectifiers, they are hellishly inefficient and you might need a higher wa ttage bulb. In fact I have been considering building one that has multiple bulbs that can be switched in and out for different equipment.

If you have a tube rectifier, like a 5U4 or whatever, you can probably just fire it up as is, because that B+ will not be applied all at once. It only comes after the filament of the rectifier tube warms up. Usually, when an ALL tube (valve) unit blows the AC fuse immediately it either needs a power transformer or a small noise suppression cap across one of the windings. B ut you do have to be a little more careful if it has solid state rectifiers . In that case, usually ohmmeter reading will tell you if you have a dead s hort.

Any caps that emit smoke at power up need to be replaced. A shorted power t ransformer stays shorted. Main thing is the smoke and smell really. Anythin g that smokes is bad, so mainly the DBT and variac protect DC coupled ampli fier circuits and some regulator problems.

If you use the DBT in conjunction with the variac, you should be able to in crease to full voltage in less than a minute unless the thing has massive f ilters in it. This IS an issues with some high fidelity amps that have like a half a Farad of main filtering. Some really do have that much, though 10

0,000 uF is more common. In any case you take the variac up a bit and the b ulb lights up a bit and then dims down. When it dims down you give it a lit tle more and a little more. When it stops dimming down (and this is with no signal and no load) THEN is when you look for the problem.
Reply to
jurb6006

This is fine most equipment with a linear power supplies. Switchmode power supplies may be unhappy at significantly below minimum spec'd input voltage and let out their smoke regardless of whether the caps are good.

--
    sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/  
 Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/  
 Click to see the full signature
Reply to
Samuel M. Goldwasser

** That would probably be very safe, but in most cases it can be done much faster. Filter electros are the troublemakers as they can require gradual reforming after a long period of idleness.

Had one like that the other day, a valve amp from the 1960s that had not been run in the last 10 years. With all valves removed, the AC supply current rose quickly with each voltage increases and then slowly fell back.

** That is what overheated electros do.

.... Phil

Reply to
Phil Allison

Not much more, but all depends, of course. Then you got tube rectifiers. I like using dim bulb AND variac. I don't know if it helps, but sometimes I let sit at zero, after slow increases.

Greg

Reply to
gregz

Tantalum caps failing, can go off with quite a bang, leaving an "electrical" smell

Reply to
N_Cook

Probably not a tant, then. I don't recall any bang and certainly no electrical smell. Just choking, acrid smoke. Oh, and when I looked inside, there was a light film of fine, sooty, black carbon powder over much of the innards. I suspect whatever had failed was concealed or screened as I couldn't see any evidence of charred components merely from splitting apart the external cases.

Reply to
Cursitor Doom

T

Mpffff.... a variac without proper metering is worse than useless. And a di m-bulb tester, while infinitely better than an unmetered variac is still a blunt instrument.

A variac needs an ammeter that is accurate within a single % of the scale i t is measuring. Such that one may watch even tiny variations in how the cur rent comes up as the voltage is raised. A few things:

a) Anything with a tube (valve) rectifier will not pass B+ until the applie d voltage is somewhere between 70% and 80% of the nominal voltage. So, refo rming caps by way of slow uptake on a variac for such a devices is a comple te myth. Those caps will see, all at once, no less than 70% of the operatin g voltage - not hardly a soft start.

b) If any device is using as little as 5% more current than it should-if-op timal, that current will be expressed somewhere as heat. If in the winding of a power or output transformer, that will, eventually mean *POOF*. If in an IF transformer for a tuner or receiver, also, eventually, *POOF*.

c) Electronics, old and new, do not typically 'cure' themselves of faults, even if cajoled, massaged and seduced very slowly. Sure, caps _can_ reform, but that reformation is unlikely to be either reliable or permanent. Given the low cost of caps these days, even the thought of reforming vintage cap s should give one the cold shivers at the very least.

d) One can, with the proper instrumentation, figure out what is going on wi th any given piece of equipment typically within several hours anyway. 99-4

4/100% of them, within several minutes. Sure, individual component thermal failures or intermittents can be a huge diagnostic problem, but as to gener al function, that should take little time at all.

So, cutting to the chase, a metered variac with a precise meter that is cap able of reading *accurately* in very small increments is a very fine diagno stic tool, especially at the triage level. A simple, unadorned variac is us eful only for dimming the lights - its original purpose. IF that is all one has, put it aside and start with the dim-bulb tester and a number of diffe rent 'bulbs' to get a very-ballpark S.W.A.G. at how much current any given devices is actually using.

For the record, if it takes me more than 5 minutes to conclude how well beh aved any given device might be under full power, that would be a long time. HOWEVER!! Once any new-to-me device is at full operating voltage and runni ng, I will watch it like a hawk for several hours at least before deeming i t fit for polite society. And very few very vintage (pre-1960) devices get even that far without at least the power-supply caps being redone. It is si mply not worth the risk.

Peter Wieck Melrose Park, PA

Reply to
pfjw

been run in the last 10 years. With all valves removed, the AC supply curre nt rose quickly with each voltage increases and then slowly fell back.

How would that happen unless the unit had a solid-state rectifier? Typicall y that quick-rise, slow-fall is due to the "empty" caps accepting inrush, t hen as they charge becoming more 'resistant-for-lack-of-a-better-word'. And the only way the caps would be in the circuit is if the rectifier was in p lace - hence the question.

Put another way, it is a bad idea to allow a tube amp to make B+ without a load. Many marginally designed amps ran their filer caps at very near their ratings - and both vintage and marginally capable caps running unloaded mi ght just go *POOF*.

Peter Wieck Melrose Park, PA

Reply to
pfjw

** It wouldn't.

Soon as silicon diodes became available in the 1960s, designers used them t o replace the inefficient and troublesome valve rectifiers in TVs and amps of all kinds. A further efficiency was obtained by using four diode bridges or two diode voltage doublers and eliminating the centre-tapped secondary needed with valve "full wave" rectifiers.

** Inrush surges as you increment a variac are normally very brief, but in this case they were sustained, taking many minutes each time for the curren t to fall back to the residual being drawn by the primary of the transforme r. I well knew this meant dodgy filter electros.

On my bench, the variac's output is followed by a 3.5 digit current meter w ith 1mA resolution and a (safety isolated) waveform output to view on a sco pe. 100Hz current pulses being drawn by filter electros appear very distinc t from the steady 50Hz current draw of the power transformer.

With good electros and no valves installed (or the standby off), current pu lses should be absent.

** It's something to be aware of and one should check electro voltage rati ngs to make sure they are not being exceeded in this condition.

BTW: I have come across a few modern valve amps where electros further down the chain from the main filters were of such lower voltage that removing s ome of the preamp valves caused their rating to be exceeded.

.... Phil

Reply to
Phil Allison

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.