Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems.

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Default A guide to fuse replacement

https://imgur.com/a/aMsBA
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Default A guide to fuse replacement

On Wednesday, February 28, 2018 at 11:27:10 AM UTC-5, bitrex wrote:
https://imgur.com/a/aMsBA


Many moons ago I had a guy bring in a stereo receiver for repair. The cover was off so he could show me where the fuses went.. He also had a 5 pack of Radio Shack fuses and just one remained. He told me he changed the fuse four times and it blew immediately each time.

I asked him why he stopped at 4..
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Default A guide to fuse replacement

On 1/03/2018 4:33 AM, John-Del wrote:
On Wednesday, February 28, 2018 at 11:27:10 AM UTC-5, bitrex wrote:
https://imgur.com/a/aMsBA


Many moons ago I had a guy bring in a stereo receiver for repair. The cover was off so he could show me where the fuses went.. He also had a 5 pack of Radio Shack fuses and just one remained. He told me he changed the fuse four times and it blew immediately each time.

I asked him why he stopped at 4..


**I love it when they keep putting larger fuses in, so they don't blow.

--
Trevor Wilson
www.rageaudio.com.au
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Default A guide to fuse replacement

On Wednesday, February 28, 2018 at 2:46:14 PM UTC-5, Trevor Wilson wrote:
On 1/03/2018 4:33 AM, John-Del wrote:
On Wednesday, February 28, 2018 at 11:27:10 AM UTC-5, bitrex wrote:
https://imgur.com/a/aMsBA


Many moons ago I had a guy bring in a stereo receiver for repair. The cover was off so he could show me where the fuses went.. He also had a 5 pack of Radio Shack fuses and just one remained. He told me he changed the fuse four times and it blew immediately each time.

I asked him why he stopped at 4..


**I love it when they keep putting larger fuses in, so they don't blow.

--
Trevor Wilson
www.rageaudio.com.au



Yep.

I had a guy bring in a Thomson TX82 13" TV for a dead complaint. In it's fuse holder was a piece of copper tubing cut to fit a GMA fuse holder perfectly. Missing from the TV was in the mains input reactor filter coil. There were four burn marks where the posts used to be mounted to the board. I would love to know how big a bang this made when they plugged it in.



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Default A guide to fuse replacement

On 28/02/18 21:02, Gareth Magennis wrote:

I recently keep coming across brand new unsold equipment which has been
supplied with a fast blow mains fuse that has blown at turn on.

This is from more than one manufacturer.

I have no idea what that is all about.


Second customer returns after a returned "repair" done with the wrong fuse?

If that's true, you would do well to investigate who is actually doing
this. What class of equipment?

--
Adrian C
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Default A guide to fuse replacement

Gareth Magennis wrote:

---------------------------

I recently keep coming across brand new unsold equipment which has been
supplied with a fast blow mains fuse that has blown at turn on.

This is from more than one manufacturer.


** A friend who did Fender warranty work was getting a lot "failed on the showroom floor" amps. Always the same, an F1A or F1.6A supply fuse popped. Fitted a T fuse instead and no more problem. Of course, the fuses concerned were buried inside on the main PCB !!

T fuses were not always easy to buy, so it was common to use an oversize F fuse instead - labelling often encouraged you to do so.

Whatever the combination of dumb ideas, it is not hard to do a number of on-off cycles while keeping a close eye on an F fuse. If it bends severely at switch on it's gonna fail soon.



..... Phil
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Default A guide to fuse replacement

Adrian Caspersz wrote:

----------------------



I recently keep coming across brand new unsold equipment which has been
supplied with a fast blow mains fuse that has blown at turn on.

This is from more than one manufacturer.

I have no idea what that is all about.


Second customer returns after a returned "repair" done with the wrong fuse?

If that's true, you would do well to investigate who is actually doing
this.



** Factory techs working in places like Mexico and China, sometimes elsewhere.


What class of equipment?



** Anything that has a significant inrush surge.

Toroidal transformer based PSUs are major culprits.




.... Phil
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Default A guide to fuse replacement

On Sat, 3 Mar 2018 17:54:19 -0500, Ralph Mowery
wrote:

I had a solid state amplifier for my ham gear that used a 20 or 25 amp
3AG type fuse. While the fuse was rated fine for the current ( about 5
amps more than the current drain), after about a month it would do what
I call a 'sag out'. The element would just be bent to the bottom of
the glass and quit making connection.

I guess that the contacts for the plug in with a twist lock cap was not
making contact well enough to keep the heat down.

The current would cycle from almost no current to full current about 20
or more times a day on the average.


I'm not sure if this is about power line fuses or 12Volt, or something
in the circuit. If this is a for a 120VAC line, you are possibly already
overloading the house fuse or breaker.

As far as that sag in the fuses, I'd suggest using a SLOW-BLOW fuse.
They are made to handle high starting current. Or switch to a breaker.
Of course there could be a defect in the amplifier too, such as a
failing power supply capacitor.

Knowing more about the application for the fuse would make it easier to
suggest what could be wrong.



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Default A guide to fuse replacement

Oh yes please, we're all waiting on pins and needles for your suggestions.

Knowing more about the application for the fuse would make it easier to
suggest what could be wrong.


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Default A guide to fuse replacement

In article ,
says...

On Sat, 3 Mar 2018 17:54:19 -0500, Ralph Mowery
wrote:

I had a solid state amplifier for my ham gear that used a 20 or 25 amp
3AG type fuse. While the fuse was rated fine for the current ( about 5
amps more than the current drain), after about a month it would do what
I call a 'sag out'. The element would just be bent to the bottom of
the glass and quit making connection.

I guess that the contacts for the plug in with a twist lock cap was not
making contact well enough to keep the heat down.

The current would cycle from almost no current to full current about 20
or more times a day on the average.


I'm not sure if this is about power line fuses or 12Volt, or something
in the circuit. If this is a for a 120VAC line, you are possibly already
overloading the house fuse or breaker.

As far as that sag in the fuses, I'd suggest using a SLOW-BLOW fuse.
They are made to handle high starting current. Or switch to a breaker.
Of course there could be a defect in the amplifier too, such as a
failing power supply capacitor.

Knowing more about the application for the fuse would make it easier to
suggest what could be wrong.




The application was for an amplifier for a 2 meter repeater. The amp
is a solid state and operates at a nominal 12 volts DC. I think it had
a 25 amp fuse and operated at 20 amps. The holder was a panel mounted
with the cap on it that you give about a half turn to remove.

The amp draws almost no current when in standby and the full 20 amps
when activated . The amp; did not have any devective capacitors and the
ones in it were only a few MFD at the most and they would only have
about 12 volts on them.
The amp was fed with a seperate 12 volt supply that was rated for over
30 amps.


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Default A guide to fuse replacement

Coming very late into this....

Fuses are very simple beasts made complicated by a fundamental lack of understanding.

a) The nature of the load. A purely resistance load is entirely different from a motor load. Both of which are different from a filament load.
b) How inrush current works. A purely resistance load has a massive inrush until the resistance element heats up - whereupon the load drops in proportion to the temperature. An AA5, for instance, is essentially a dead-short at turn-on. As are most tube loads.
c) The differences between motor loads, filament loads and electronic loads..
d) Rated Operating Voltage. Fuses rated below 240V are to be taken with much salt.
e) The differences between a standard fuse, a fast fuse, a slow-blow fused and a dual-element fuse. READ THE OPERATION TABLES!

And, unless you have no investment in the connected load or its fate whatsoever, eliminate all slow-blow AGC fuses from consideration - full stop (the one with the wire element wound around a ceramic rod)!!

From here, do your own research. For the power-supply in question, I would suggest starting with a 20A DE Fuse rated at 250V. NOTE: Many AGC fuses are rated at only 32V. Avoid them. If this blows consistently, only then would I suggest moving to a 25A DE Fuse.

Peter Wieck
Melrose Park, PA


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Default A guide to fuse replacement

On Sunday, 4 March 2018 19:09:25 UTC, wrote:
Coming very late into this....

Fuses are very simple beasts made complicated by a fundamental lack of understanding.


indeed

a) The nature of the load. A purely resistance load is entirely different from a motor load. Both of which are different from a filament load.


filaments are practically speaking purely resistive - as long as you're not driving them at rf, when their tiny L can become relevant.

b) How inrush current works. A purely resistance load has a massive inrush until the resistance element heats up - whereupon the load drops in proportion to the temperature.


some are like that, many aren't at all

An AA5, for instance, is essentially a dead-short at turn-on. As are most tube loads.


not so

c) The differences between motor loads, filament loads and electronic loads.
d) Rated Operating Voltage. Fuses rated below 240V are to be taken with much salt.


I wouldn't recommend salt really. 32v fuses are fine on 12v.

e) The differences between a standard fuse, a fast fuse, a slow-blow fused and a dual-element fuse. READ THE OPERATION TABLES!


and other types.

Then there's breaking capacity, a rather important thing especially with mains fuses.

And, unless you have no investment in the connected load or its fate whatsoever, eliminate all slow-blow AGC fuses from consideration - full stop (the one with the wire element wound around a ceramic rod)!!


sometimes very slow fuses are just the job. That's why they're made.


NT
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wrote:

---------------------


Fuses are very simple beasts made complicated by a fundamental
lack of understanding.


** Fuses only seem simple ......

Many blown fuses look perfectly OK, needing a continuity test to prove the point. Ceramic case fuses for one.

Blown fuses deserve a careful inspection, to make sure the right amp rating and type ( F or T) was installed. Don't ya hate folk who toss them away, sometimes along with the fuse cap.



..... Phil
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