variac/dc power supply repair

The fuse is easily enough tested using a simple ohm meter, or by substitution, but to be honest, if you do not have sufficient knowledge to be able to troubleshoot a simple power supply of this vintage, I really would not recommend that you start poking around inside, as there are dangerous voltages present in any mains power supply. Sorry to be so negative about this, but I'm only thinking of your safety ...

Arfa

If it is similar to this model it shouldn't be too difficult to find out what is wrong with it. Looks pretty basic but I haven't actually seen one. BTW, if you can't determine whether a fuse is blown or not then you shouldn't be playing around inside it.

Agreed with the other postings, especially on the SAFETY issues.

Most likely problme is a blown fuse (there might be nother one inside), next most likely would be a bad connection or short somewhere, next most likely might be a bad carbon brush on the Variac. There really isn't much else to go wrong, though a shorted rectifier might be the cause of the blown fuse.

--- sam | Sci.Electronics.Repair FAQ:

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Yeah, that's the same style of fuse. I just don't know how to test that type of fuse. I am only familiar with repairing PC power supplies and the glass/filament type fuses that they use.

What do you mean by "that type of fuse". What type is it?

In any case, you test that fuse like any other, with a multimeter on ohms or continuity. It should be read as close to 0 ohms.

Respectfully, if you don't even know how to test a fuse, what are you going to use this power supply for, and do it safely?

--- sam | Sci.Electronics.Repair FAQ:

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I am fully aware of how to test a fuse, given that I can find the terminals of the fuse, which is not usually an unreasonable assumption. If you look at the picture, it is a type of fuse that screws in. It is not like the "edison" type screw-in fuses. There is kind of a "rivet" in the end of it that, when removed, retains a metal cylinder that hangs loosely over the rivet. I can't figure out how the fuse is supposed to work, because when I've removed it, it physically becomes one big short because of the loose cylindrical part.

I am fully aware of the hazards, but from the replies, you'd think I was prodding around in it with a kitchen knife while plugged in. All I am trying to do is test this fuse, and if disassembly is required, I have no plans to do that with it plugged into the wall or with an energized filter cap.

It sounds to me like the fuse is missing.

The fuse that goes in there is probably 1-1/4" long by 1/4" in diameter.

When someone asks how to test a fuse, it is no wonder that people are questioning your experience.

--- sam | Sci.Electronics.Repair FAQ:

Important: Anything sent to the email address in the message header above is ignored unless my full name AND either lasers or electronics is included in the subject line. Or, you can contact me via the Feedback Form in the FAQs.

It may simply be that the fuse holder is broken...

If the fuse is a screw cap then you simply unscrew the cap anti-clockwise. The fuse will most likely be a 3AG (1-1/4" x 1/4" tubular glass). Usually the screw cap has a flat spring retainer to hold the new fuse when inserting it but these sometimes break or go missing butthis doesn't usually affect the function of the fuse holder. At the far end of the fuse holder the rear terminal is spring loaded so that pressure is applied to both ends of the fuse when the cap is screwed home. You can test to see if the spring exerts pressure by pushing a screwdriver blade down inside the fuse holder.

I have found these fuseholders sometimes go open circuit usually through a terminal or phenolic body breakage internally. The only way to test the fuse holder is to disconnect (unsolder) one of the wires connecting to it (the far end one is easiest) and with a known good fuse inserted measure across both terminals of the fuse holder with your DMM. Naturally, you must ensure that no power is applied while doing any of this.

Are you sure there's even a fuse in there? Almost sounds to me like an empty holder.

Sam, save the flames for someone more deserving, please. I said that I have never seen THIS TYPE of fuse before, because I mistook what is apparently a broken fuse holder for the fuse itself. That is not equivalent to asking "how do I test a fuse?". As for "questioning my experience", go right ahead, I'm not going to get into a pissing match, but it is usually less effort to simply answer the question as posed.

I openly admit that I rarely get to deal with nice test equipment. And I don't see where that means that I should simply accept that it is broken and not attempt to learn about it - just as I learned about microwaves, televisions, monitors, amplifiers or any other piece of electronics that could be dangerous if one does not employ common sense.

Yes, it appears to be an empty, broken fuse holder. When I first unscrewed it, I was confused and thought it was similar to the mains fuses that screw into the panel. A step in the right direction, thanks!

It doesn't even sound broken to me, often the fuse is all that holds the bottom contact down away from the cap, with no fuse at all installed all bets are off.

So, are you now sure that the problem was caused by a broken fuse holder in the first place?

Well there's no sense in arguing about it or belittling one another, I would however strongly recommend doing some research on the subject before mucking around with it too much. I do agree that a person should be able to clearly identify just about any type of fuse or fuse holder they come across before poking around in anything mains powered.

I got a service diagram from Lab-Volt after much consternation. Looks like the holder is supposed to be like that, when the holder screws in it both tightens up the "link" to the fuse as well as completes the connection. So the holder may not be broken, I'll just see if it blows another fuse before proceeding. Thanks for the helpful replies.

You are a worry... this is the first time you have even mentioned that the unit is "blowing fuses". Your previous posts inferred that the problem was with continuity related to this fuse, but this new information puts a whole new complexion on things. The very fact that the fuse is blowing infers there is nothing wrong with the fuse holder or the fuse. Assuming that you are using a properly rated delay fuse (slo-blo, or time delay fuse usually with a 'T' marking), and it is still 'blowing', then the fuse and holder are fine. The most likely problem is that there is some component following the fuse which is overloading it and causing it to do its intended job.

Have you even opened the case to see if there is any obvious sign of a short circuit anywhere? Perhaps I shouldn't ask that if your attempt to repair the unit up till now is an indication of your knowledge.

It's of more concern that if he doesn't know a fuse from a fuse holder, he may not know the difference between a live wire and ground and may end up dead.

--- sam | Sci.Electronics.Repair FAQ:

Important: Anything sent to the email address in the message header above is ignored unless my full name AND either lasers or electronics is included in the subject line. Or, you can contact me via the Feedback Form in the FAQs.

I know the difference between a fuse and a fuse holder in terms of their function. However, I have not seen every fuse holder nor every fuse. I jumped to a conclusion at the very beginning when I thought I was looking at some strange, obsolete type of *fuse* when I was looking at a *holder* of type which is apparently common and well known in test equipment. How should I know? That's why I asked. (You know, the holder says "Slo Blo" on it too.)

To reply to the GP, with this knowledge of what is a fuse and what is a holder straightened out several posts ago, I concluded that it was given to me without a fuse. Isn't it a reasonable presumption then that it previously blew a fuse? What other reason would one have for removing a fuse? No, there are no obvious short circuits in that there are no loose wires. Since the presumption was that a fuse blew, that was the first thing to check. But I can't tell whether it will blow another fuse without putting another fuse in and powering it up, can I?

I don't see what has been so difficult to understand about what I have posted. The largest part of the problem is that respondents would rather jump to conclusions about my mettle than ask for clarification if what I have posted is ambiguous or seems wrong. I try to anticipate replies when I post, but it is inescapable that descriptions will be lacking at times. Furthermore, I have visited this project approximately three times; since it is at home and I work all day my opportunities are limited.

My experience is limited to consumer electronics component-swapping (including fuses). That's why I haven't seen this type of holder before - fuse holders in consumer equipment are almost invariably the flex metal type. I can take the appropriate precautions vs becoming a crispy critter when working on a monitor, a PC power supply, or a microwave, so why do I feel like I need a disclaimer at the end of every post I make about this power supply?

I realize you get what you pay for in terms of soliciting advice, but I recall this group being a lot less hostile 5 years ago.

You came across something which was obviously a fuse or fuseholder which you hadn't seen before. You knew the item you were asking about on the unit was a fuse because it was marked so, and even had the value and slo-blo written alongside it. The thing has a screw cap which you removed so if this didn't give the clue that "duh, some type of fuse must go in here". then no wonder other people are frustrated at your posts.

So, before getting on the NG and posting did it occur to you to do some research on the web, even if it was only to protect yourself from ridicule because of your ignorance on the matter? By typing two words "fuse fuseholder" (without the quotes) into Google would reveal in the second picture a 3AG fuseholder similar to the one you had in front of you.

Clicking More Info on this item takes you to

Browse by Category menu on the right would have revealed there were separate categories for "Fuses" and "Fuseholders", so have a look at Fuses.

Here we find a sub-category "MDL (Slow-Blow)" so that takes us to

Item 5 is the closest you will get at 3A so now you know all you needed to before getting on the NG and posting.

Look first, and when you have done your research ask questions from a basis of at least having done something yourself.

Now hang on a minute... You didn't tell us before now that you were supplied the unit "without the fuse", so how was anybody supposed to guess this fact. So, NO, it is not a reasonable assumption for us to "guess" that it had "previously blown a fuse". Only yourself would have been able to make that assumption. We aren't psychic you know...

Given the information you provided we had to assume (wrongly it now seems) that you knew what a fuseholder was and the type of fuse to use, and that you had powered it up but it didn't work. We assumed you would have ensured there was a fuse in it since you had all the info written on the unit. And since you didn't say that fuse blew, we assumed that your problem related to continuity within the fuse. These things are so obvious to us that we didn't think to ask "have you tried a new fuse in the fuseholder?" because we thought you would have done this already.

You are dead right there.... so why didn't you do it?

All right, we NOW know that you weren't aware that this style of fuse holder used a tubular fuse but we couldn't guess that could we. After all tubular fuses have been around since about 1930 and panel mount fuseholders have been around for at least 50 years and anybody who is even vaguely familiar with electronic equipment will have come across them. The fact is you knew the object was a fuse because it was there in white letters. It is obvious you must have unscrewed the cap and found nothing there so pray tell me why this didn't prompt you to ask yourself "now how can this fuse thing work when I have this completely disconnected screw cap in my hand and there is nothing at all in it?" Bloody hell, if it wasn't immediately obvious there was a bit missing then your logic is non existent.

Try to imagine that you are asking for help "by remote control". We can't see what you have in front of you so you have to describe it in detail. Heck, if I hadn't dug up that pic of the power supply by Googling nobody would have had much of an idea of what you were talking about.

You seemed a candidate for a disclaimer because when it became obvious you had never come across a panel mount fuseholder in your life. Then it seemed possible you didn't have any electronics knowledge and you might put yourself in danger by following suggestions. Anybody who has dabbled in electronics in the last 50 years or more will have come across this common type of fuseholder so it would have been highly improbable that you hadn't seen one before.

What is a "flex metal" type fuse? I've never heard of it. What type of "consumer electronics" are you talking about? Are you talking whitegoods such as microwave ovens, washing machines etc? If so, you are right, they don't have panel mount fuseholders. They don't want ignorant people replacing the fuses when they don't know what they are doing.

When whitegoods have a fuse it will usually be concealed inside but in most cases that I have come across they are still tubular fuses either ceramic or glass and generally size 3AG and mounted in an open style fuse clip with a plastic insulating cover of some sort. A microwave oven is a good example.

If you had given all the info to start with, and done some research, you would have received the correct advice. Given the paucity of info then you can't blame people for getting up your arse. The "hostility", as you put it, came about because of frustration due to you not providing all the info you had, or should have had, before posting.