I don't think any of us here would take some semi-ancient item of electronic equipment of unknown provenance and just plug it in the mains socket and switch on. We know from experience or elsewhere that is not an advisable thing to do. So we typically gently wind up the voltage via a variac over an extended period of time. But is that enough? I read somewhere one should also monitor the current drawn by the equipment as one does this. I admit I've never been quite that fussy, but would be interested in hearing what others think about this rather more cautious approach and if it actually achieves anything worthwhile.
Rant pending as I am traveling. But it is better to dump equipment in salt water than to apply power without a precise means of monitoring it. Dim bulb devices notwithstanding.
Well, maybe if it has an old school 50/60 Hz transformer, rectifier, cap power supply. I don't have a lot of those sorts of things.
Cheers
Phil Hobbs
--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics
160 North State Road #203
Briarcliff Manor NY 10510
hobbs at electrooptical dot net
http://electrooptical.net
Variac yes, meter or lamp for current behaviour yes . One cheap-now extra, monitor with a small simple IR thermometer for any hot spots, not the large ones for car engines but key-fob size that you can really get inside kit with . Note, at the collector has to be metal, shrink some heatshrink over the outside of the collector cone to avoid any short circuits
If not tantalum capacitors, and simple linear power supply, then this is usually a good idea.
Tantalum caps can also be helped by this, but appluing very small voltages for a long time (a day or so) may be better before more power is applied. Replacing them is another way to go.
Slowly ramping up voltage is not generally compatible with switching supplies.
** I've been using a variac plus RMS current meter for 30 years and also see a lot of gear that has been out of use for over a decade, mostly tube amplifiers.
Providing you have a good idea what the AC draw should be under idle and standby conditions, you can apply full supply voltage in a short time if the draw seen rises smoothly towards the expected value.
Of course you need a decent current meter to do this, a 10amp moving iron type is not adequate. I use a dedicated 3.5 digit LED meter with internal RMS converter.
And.... Here is the rant, from 2006. It was written for the vintage radio group - so "radio" is used instead of "about anything mains-powered with capacitors".
RANT WARNING RANT WARNING RANT WARNING Well, I might spare you, IF: you have voltage _and_ current meters for this variac that are capable of accurately representing 0.01A and 2-3V AC. If you have this capacity and use it *every* time you use a variac, then ignore the following.
Otherwise, a variac is worse-than-useless for radio work, fit only for dimming lights without excessive rF noise. I cannot think of a single use for them (without meters) that a dim-bulb tester will not do with equal accuracy, far greater safety and less cost (of course). The typical urban-myth uses for a variac:
a) Apply power to a radio slowly to see how it starts without letting out the 'magic smoke': Yeah, right. How would you know unless you either see sparks (it's too late then), or have a current meter on the thing to see that it is pulling 65 watts when it really only should be
A radio might even work just-fine-and-hum-free as far as ears are concerned, but if it is pulling those extra 5-10 watts, that is all being expressed as heat somewhere, usually in an unobtaium part. Can you say *POOF*?
b) Reforming caps: Unless a radio has a SS rectifier, this is simply silly. A tube rectifier won't pass current until about 75% or-so of rated filament voltage. So, hitting the caps with 75% of B+ hardly is a slow reform.
c) Running radios at the "proper" voltage: Great, until they leave the bench.
d) Dimming lights... Yep, that's what unmetered variacs are best at. Other thanbas ballast and as doorstops.
END RANT
In all seriousness, unmetered variacs are silly, stupid, dangerous items for electronics use, and for any of several reasons. Spend the money where it will do some good, such as an isolation transformer. If you have one of those, then look for a metered variac, or the meters to add to the one you have.
What you said, or reranted is not wrong in some instances. One notable exce ption is audio amps that use VFETS for outputs, which are depletion mode an d WILL fry if you try to run the up slow on a variac, at least in the Sony ones. I don't know of any others but to be tolerant of low AC input it woul d take a quite special design with abnormal conditions in mind. I worked fo r a Sony Signature dealer and ASC back before their product turned to real shit and I can tell you that i n most cases, abnormal conditions were not o n their engineers' minds.
Dim bulbs and variacs come in handy when you have repaired most regular sol id state amplifiers. Once you drop $35 worth of silicon in them for a fried channel it is nice not to have all that short out in the first millisecond .
With most tube stuff, honestly I would just plug it in. Possible exceptions would be if it has selenium rectifiers. Even moreso a high end tube amp wi th a big old 5U4 but using a selenium rectifier for the negative bias to th e outputs.
Bottom line is you can't just make a blanket statement about working on all equipment. And with the advent of the SMPS, there are even less assumption s that can be made. You can usually use a DBT on like a DVD player or somet hing like that and it will keep the smoke in in most cases. But the normal current drain is usually so low all a hundred watt bulb will do is keep you from burning foil off the PS board. Some short on the five volt line is no t protected and if it is going to fry something it is going to succeed. The n, if you use a lower wattage bulb, some SMPSes won't start.
Before I got kicked off AK they were always talking about the DBT. And real ly it does prevent pretty much all board burning. You burn the foil of a ci rcuit board it is not fun to fix, especially with audiophiles who are going to want your repair to follow the same path as the original foil. I have d one that type of rework but really most of the shit today does just fine wi th jumper wires.
Lately I am dealing with a design defect in amps built by Apex for RSQ, KDS and Technical Pro. All a bunch of liars on the power specs, Technical Pro claims like 1,300 watts or something on an amp that actually puts out about 90 watts a channel. The defect is that the assholes thought the bias regul ator would just hold itself to the heatsink. Well they don't, and now ater going through several options we just could not make happen I am simply glu ing them. Got me some JB Weld and made some clamps out of a couple of coat hangars. How is that for factory service ?
Enough rant, back to the subject. The DBT and variac have actually lost som e of their usefulness. They are not useless but it does depend on what you are working on. Old tube stuff I would just plug in. For a few seconds. the n I would remove all the tubes ad plug it in for longer. then I would put i n the rectifier tube and see what happens to the current draw, with all the other tubes out. Scope the caps, if all the other tubes are not in there a nd you got ripple you got a problem. Turn if off before you blow that 5U4, which is no longer available for $3. The shipping is more than $3.
I have had a pretty diverse career and even got a little bit into CNC machi nes but not much. I learned automotive electronics when they started with t hose ECMs and shit. We flipped cars, and the reason we made money was becau se the backyard mechanics could not understand electronic and My Goodwrench forgot there was an actual engine under all that shit. I beat them several times in troubleshooting. Three of them making fifty bucks an hour looking at a computer readout and can't tell you have a burnt valve or a fouled pl ug. And the idiots probably made more money than me.
If you think powering up old electronics is scary, try starting an old car that has been sitting for 20 years or more. That's why some people actually take them apart first.
Well not completely apart but they will do a compression check, change the oil, whatever they can coax out of the crankcase that is and put new oil in it, drain the gas tank, pressure check the cooling system maybe, but that can come later after the thing actually starts.
Everything requires a slightly different approach.
I took my 48 hour rule before replying - hip-shots often ricochet.
ception is audio amps that use VFETS for outputs, which are depletion mode and WILL fry if you try to run the up slow on a variac, at least in the Son y ones. I don't know of any others but to be tolerant of low AC input it wo uld take a quite special design with abnormal conditions in mind. I worked for a Sony Signature dealer and ASC back before their product turned to rea l shit and I can tell you that i n most cases, abnormal conditions were not on their engineers' minds.
Consider that statement - it suggests that a brown-out would fry the equipm ent. Brownouts are quite common in certain regions of the US (sadly) and th ough rare in our particular case, we are on a double-fed distribution line as there are neighbors with mains-powered life-support equipment nearby. Wh at will happen under extreme storm conditions (twice in 8 years) is that on e end of the line may short due to a tree fall or similar, and draw availab le current (and voltage) way down. Hell on motor-based equipment such as re frigerators or freezers and some electronics. But, we have suffered no fail ures as a result. Yes, the critical 'stuff' is on surge protectors, but the point is that consumer equipment should be somewhat resilient and designed for real-world occurrences. I have to ask, have you personally observed su ch a failure due to such a cause?
olid state amplifiers. Once you drop $35 worth of silicon in them for a fri ed channel it is nice not to have all that short out in the first milliseco nd.
ns would be if it has selenium rectifiers. Even moreso a high end tube amp with a big old 5U4 but using a selenium rectifier for the negative bias to the outputs.
And a dead-shorted filter cap would do *WHAT* in ten seconds? Or a shorted rectifier tube? How much current for how long to wipe out an output transfo rmer? Remind me not to send you any of my vintage equipment for repairs...
ll equipment. And with the advent of the SMPS, there are even less assumpti ons that can be made. You can usually use a DBT on like a DVD player or som ething like that and it will keep the smoke in in most cases. But the norma l current drain is usually so low all a hundred watt bulb will do is keep y ou from burning foil off the PS board. Some short on the five volt line is not protected and if it is going to fry something it is going to succeed. T hen, if you use a lower wattage bulb, some SMPSes won't start.
I am not so sure that the OP was concerned about equipment made after the y ear 2000, and/or equipment that has been sitting on a shelf for six months. I believe the thrust was towards equipment that has been sitting in a barn , cellar, shelf or garage for 20 - 60 years. Nor would I spend that much ti me on a consumer commodity, the repairing of which would likely cost more t han a new one - were it even possible.
ally it does prevent pretty much all board burning. You burn the foil of a circuit board it is not fun to fix, especially with audiophiles who are goi ng to want your repair to follow the same path as the original foil. I have done that type of rework but really most of the shit today does just fine with jumper wires.
Getting kicked off AK is not difficult. Probably speaks well of you that yo u were. As to Dim Bulb Testers - they have their (very limited) uses as som ething of a protective device. They will reduce immediate damage (those fir st 15 seconds) but they are NOT suitable as a final indicator.
DS and Technical Pro. All a bunch of liars on the power specs, Technical Pr o claims like 1,300 watts or something on an amp that actually puts out abo ut 90 watts a channel. The defect is that the assholes thought the bias reg ulator would just hold itself to the heatsink. Well they don't, and now ate r going through several options we just could not make happen I am simply g luing them. Got me some JB Weld and made some clamps out of a couple of coa t hangars. How is that for factory service ?
You well know that equipment is built down to the price/purchase point. Whe re it is perfectly possible, even today, to purchase 'good stuff' at a dece nt price, the research required to do so is considerable. This applies to a nything from appliances to shoes or toys.
ome of their usefulness. They are not useless but it does depend on what yo u are working on. Old tube stuff I would just plug in. For a few seconds. t hen I would remove all the tubes ad plug it in for longer. then I would put in the rectifier tube and see what happens to the current draw, with all t he other tubes out. Scope the caps, if all the other tubes are not in there and you got ripple you got a problem. Turn if off before you blow that 5U4 , which is no longer available for $3. The shipping is more than $3.
*NOTHING*, repeat, nothing 'just gets plugged in' on my bench. Even if I am not doing the 'variac' thing for modern solid-state stuff, I will pre-set the voltage at 118V and then turn on the item to watch how much current it draws. Anything with a tube gets the full treatment.
hines but not much. I learned automotive electronics when they started with those ECMs and shit. We flipped cars, and the reason we made money was bec ause the backyard mechanics could not understand electronic and My Goodwren ch forgot there was an actual engine under all that shit. I beat them sever al times in troubleshooting. Three of them making fifty bucks an hour looki ng at a computer readout and can't tell you have a burnt valve or a fouled plug. And the idiots probably made more money than me.
r that has been sitting for 20 years or more. That's why some people actual ly take them apart first.
A 1968 VW beetle in 1992, the car had not run since 1970. a) Drain oil. b) Mix one quart 30W oil with one gallon kerosene. c) Pour into crankcase. d) Remove spark plugs. e) Squirt an ounce of Marvel Mystery Oil into each cylinder. f) Turn driveshaft by hand ten full turns. g) Drain kero mixture. h) Allow to sit overnight. i) Re-squirt MMO. j) Turn ten times again. k) Add correct amount of 30W oil. l) Install new spark-plugs. m) Points-condenser-rotor-cap-wires
Start. Run at idle for 15 full minutes. Gradually run up to 2,000 rpm, hold at 2,000 rpm for 10 minutes. Check for any oil leaks - and there will be s ome! Run for one hour, varying rpm from idle to 3,500.
It also works on water-cooled engines - I did roughly the same thing to a M ercedes 220 back in the day with good results.
e oil, whatever they can coax out of the crankcase that is and put new oil in it, drain the gas tank, pressure check the cooling system maybe, but tha t can come later after the thing actually starts.
I cannot imagine doing a compression check on an engine that has sat idle f or even a year without bad results - scored cylinders at the least. That is the point of the MMO and hand-turning.
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.