Generic answer to "blew a fuse" questions

[R.G.]

This question happens again and again and again and again and again and again and again here, so it makes sense to post a general answer.

If an amp (or anything else) blows fuses when plugged in or turned on, there are only two things the beginner troubleshooter can hope to do. These are:
(1) Look for obviously burned parts. These parts may not be the only problem there - they may be victims, not perpetrators - but they will need replacement in any case.
(2) Set up some means of limiting the AC line current so you can let the item run with a limited power and give you time to poke around inside with a meter and find what's really busted. The best of these is the Light Bulb Limiter, which limits AC line current but lets a trickle through to run things while you debug. Some people use Variacs, with a current meter to let them see the AC current and manually adjust the voltage down to a level that lets them debug. The light bulb limiter is vastly superior because it limits the current all on its own.

The usual warnings apply: do not try either of these techniques if you don't already know how to work safely inside a plugged-in and powered-on amplifier. You can get seriously dead if you touch the wrong thing. The internet is NOT where you learn how to work inside a live amp safely.

[rjb]

The light bulb limiter is vastly superior because it limits the current all on its own.

Just a note on using the limiter, that I found here: Powering Your Radio Safely With a Dim-bulb Tester

If the bulb's wattage is too low, it will light brightly even if your radio has no problems, and your radio won't play at all.
Generally speaking, your radio should play normally without fully lighting a bulb that is 1.5 to 2 times the radio's stated wattage.

If you haven't already, better stock up on a variety of incandescent bulbs before they become unobtainium.

-rb

[big_teee]

Fuses are expensive these days.
It makes sense to make a cheap dim bulb tester.
a one hundred watt bulb is usually sufficient.
https://www.google.com/search?q=dim+...w=1366&bih=596

[Tom Phillips]

Personally I mostly use a variable transformer with a built in voltage meter and a current meter. However, I'm not suggesting that is the best, or the only, approach. A big advantage of the light bulb limiter is that it provides automatic current limiting. That feature is especially useful if the equipment that you are troubleshooting has an intermittent problem that causes a large current draw or a problem that happens suddenly only after the line voltage reaches a certain level. In that case the bulb limiter will kick in faster than you could turn down the variac. A problem could also happen when the equipment is left running unattended on the bench. The light bulb limiter is a great tool in those situations.

[R.G.]

I agree, Tom. I have and use a variac for situations where I can afford to watch the current meter while I'm turning the knob. Generally that's on new bringups.

When I am trying to repair something that has blown fuses, especially where the fuse blew so fast that the inside of the glass cartridge has a shiny metal surface composed of the ex-fuse link vaporized onto the glass with extreme prejudice, I reach for the light bulb. They're complementary approaches, not competitors.

I suspect that a trip to home depot for light bulb limiter parts is easier for most people to do that find a variac.

Fuses are expensive these days. It makes sense to make a cheap dim bulb tester. a one hundred watt bulb is usually sufficient.

Just as your government has decided that you can't be trusted to flush the toilet responsibly, they have decided that you can't be trusted to turn on lights responsibly and have mandated that all incandescent lighting be removed from the market by making manufacture and import illegal. DOE sez:

The EISA 2007 act specifically limits the import or manufacture of inefficient bulbs. Stores will be able to sell remaining inventory.

"Inefficient bulbs" means "incandescent". While they are also forbidden, there are "exceptions" from the regulation for heavy duty bulb, appliance bulbs, colored bulbs and three way bulbs, at least for the moment. So at least for now, you can get old stock (are we going to have NOS light bulbs, too?) bulbs or one of the excepted bulbs.

[nickb]

Can I suggest rule #(0)?
(0) If the fuse is blown, don't replace it until you have a current-limited supply available. Failing to do this could cause more damage to your equipment.

For sure, 4/5 of folk that come in with a fuse blown scenario, the first thing they did was replace it and so try to break something else.

[Mick Bailey]

Mostly when I get a fuse-blower the owner has already replaced the fuse (several times) with anything other than the correct value.

Plus, there's often a 13A fuse fitted to the mains plug. So with that, a T8A in place of an F100mA heater fuse, along with a shorted output tube means lots of damage.

[Richard]

I think halogen bulbs will still be available, just the traditional one are being phased out. What annoys me is that I have 4 light bulbs in my studio apartment, I should be able to use any damn bulb I want. That and they only seem to have a 40 watt "equivalent" that uses only 13 watts. How about make a 300 watt equivalent that uses 100 watts?

[rjb]

I think halogen bulbs will still be available, just the traditional one are being phased out.

I still say if you use a bulb limiter, stock up on spares while you can.

How about make a 300 watt equivalent that uses 100 watts?

They may be pricey, but not rare. ~300W equivalent CFLs are generally 65W.
https://www.1000bulbs.com/product/16...wVtBoCABTw_wcB
http://www.target.com/p/feit-300-wat...B&gclsrc=aw.ds

[olddawg]

Honestly... I spent 25 years on the bench and never used a Lightbulb Limiter. I used a variac with a voltage and current meter. The only techs I saw using a lightbulb limiter (and the only ones allowed too) in my shop(s) were old TV techs. The tech reps from certain manufacturers would pull your authorization if they saw even saw one in the shop sometimes. That said... Working at home now I use a lightbulb limiter at times simply because I didn't want to invest in a variac, lol. More than one way to skin a cat of course, but an LBL has its own set of idiosyncrasies.

[rjb]

...but an LBL has its own set of idiosyncrasies.

Are you referring to this?
https://en.wikipedia.org/wiki/Incand...and_resistance

Current and resistance

The actual resistance of the filament is temperature dependent.
The cold resistance of tungsten-filament lamps is about 1/15 the hot-filament resistance when the lamp is operating.
For example, a 100-watt, 120-volt lamp has a resistance of 144 ohms when lit, but the cold resistance is much lower (about 9.5 ohms)....
Electrical contacts may carry a "T" rating symbol indicating that they are designed to control circuits with the high inrush current characteristic of tungsten lamps.
For a 100-watt, 120-volt general-service lamp, the current stabilizes in about 0.10 seconds, and the lamp reaches 90% of its full brightness after about 0.13 seconds.

[Enzo]

I don't use an LBL either, but I have a certain level of bench experience, and I like to think requisite discipline. I do recommend them to learning techs. I don;t speak for olddawg, but I THINK what he means is there are side effects from bulb limiters. How many times have we heard from someone who plugs and amp into it, the bulb stays dim, but all his voltages are way off, and he spends a lot of time trying to "fix" the voltage issues. The whole problem was the bulb limiter causing the voltage drops.

My variac and ammeter are transparent to the unit under test. If I can turn it all the way up to 120v and no undue current, then I can continue to operate it that way and take meaningful measurements. On the bulb limiter, if I power up and it shows no over current trouble, I cannot continue running it that way and take meaningful readings.

To quote one Detective Callahan, "A man's got to know his limitations." And by extension, those of his equipment.

[Steve A.]

Any way to wire 60W bulbs in series and/or parallel to allow the LBL to handle higher powered amps?

Steve Ahola

[pdf64]

Any way to wire 60W bulbs in series and/or parallel to allow the LBL to handle higher powered amps?

It could be done the same as a humbucker series/parallel switch.

[Steve A.]

Just as your government has decided that you can't be trusted to flush the toilet responsibly, they have decided that you can't be trusted to turn on lights responsibly and have mandated that all incandescent lighting be removed from the market by making manufacture and import illegal.

Although we often tend to disagree politically I have to agree with you on this, R.G. What is ironic about the older low-flow toilets is that you usually had to flush them twice. (BTW you can still buy parts to repair older toilets, thank God.)

Steve