Glass fuses / amp-fix (how to be happy with a free lunch)

[clintrubber]

We all have seen advertisements of people selling broken gear: "it's likely just a fuse". (Sure! )

So you can imagine some mixed feelings here when that all that this 'broken' bass-combo (old Rickenbacker TR120B) seemed to need was indeed some new fuses
(2A slo-blo, they're before the power-TX and are the only protection in the whole amp).

So it does happen - but for how long?
All fine here then, but those older Rickenbacker combo amp seem to have a habit of needing new power devices, so that's what I was expecting - this one not yet then.

Long story short: how likely is it that glass fuses themselves are simply the ones that cause the problem of an amp going silent?
Amps get hauled around, so can imagine some mechanical impact that makes a fuse to go open, and the next evening the amp remains silent.

Curious to your eventual amp-repair-by-just-a-fuse experiences, thanks.

 

[ccaudle]

Usually something causes the fuse to blow. Measure the current through the fuse holder. There should be a significant margin between normal operating current and the fuse rating, if the current is close to the rating then the circuit is not operating properly and the fuse will eventually blow again.

 

[Tubetec]

Ive seen a handful of fuses over the years that had suffered some kind of thermal fatigue , it usually just a matter of time before they go out ,

Glass fuses can also fail badly if the magnitude of the short circuit is great enough to evaporate the metal onto the inside of the glass tube in such a way it still ends up conducting , sand filled ceramic fuses are much better in this respect .

If a fuse of the correct type and rating blows theres something wrong somewhere .


`

 

[Brian Roth]

More than a few times I've had fuses quit for no good reason. The largest one I recall was a 100 Amp cartridge fuse on one of the "hot" legs of a 3-phase power entrance to a FM radio station's transmitter shack. It had been in that outdoor metal box for years.

Bri

 

[Tubetec]

Perhaps if the power happens to be switched at just the moment the AC waveform is at max the inrush can vastly exceed expectations , anyway just because we dont have an explanation doesnt mean a rational one doesnt exist .

 

[pucho812]

I suppose like any component fuses are not impervious to failure from age.

 

[clintrubber]

Thanks all for the replies, appreciated.

Good to hear that fuse fatigue (without actual amp-trouble) is not that rare.

FWIW, here's a pre-ampcleaning pic of the old glass fuses (2A slo blo).
Don't know if that spring inside is related to the time-lag, or actually puts it also under mechanical tension.
If the latter, could well imagine some mechanical bumps motivate the fuse to open after some years.

(The sand-filled fuse for amp-debugging, didn't have new 2A at hand, so for now took smaller value of 1.6A)

 

[Disco Volante]

I've seen a number of glass fuses blown on otherwise "healthy" equipment. They can (sometimes) protect amps from damage by shortcircuit of the speaker terminals or power surges. Of course, if it blows again after a short while, it's hood off time...

 

[JohnRoberts]

Fuses experience multiple thermal cycles so can fatigue and fail. It is not super common and it depends on how much turn on surge current stress they experience.

The fact that it is using slow blow fuses, suggests they are dealing with some turn on stress.

JR

 

[clintrubber]

Of course, if it blows again after a short while, it's hood off time...

Thanks, so far so good. This is a 120W amp though, haven't had the opportunity yet to really awake that 15" of this combo, but no strange things so far.

Fuses experience multiple thermal cycles so can fatigue and fail. It is not super common and it depends on how much turn on surge current stress they experience.

The fact that it is using slow blow fuses, suggests they are dealing with some turn on stress.

JR

This Rickenbacker TR120B is mainly meant as bass-combo and with 12x 4700uF in the power supply*, they didn't skimp on the milliFarads. So as you alluded to, some significant turn on inrush current, would most likely be taking out fast fuses indeed.

(*: after switching off, amp happily keeps playing on for a while)


So OK, so far this 'amp-repair' seems to be limited to just a few new fuses, nice, and again good to hear this is not too abnormal.

(Next to bring out a dummy-load and see if things remain OK at higher sweat-levels...)

 

[Disco Volante]

Crank it up!

(I listened to some of your stuff, love it;-)
Cheers, V!

 

[CJ]

The smaller glass fuses are crap. They go out all the time. Maybe vibration, I don't know but they are not as tough as the standard AGC types.

I usually just short them with a piece wire. Not the main pwr fuse, of course not.
But the b+ or heater fuses that were not there when people knew his to build amps.

Probably fixed 20 amps that had this same problem.
Sometimes I will simply up the rating if it is a real expensive boutique. Engineers cut the plate current margin too thin on the B+ fuses, since they are not solo blo, they can't handle the leak excursions very well.

 

[JohnRoberts]

The smaller glass fuses are crap. They go out all the time. Maybe vibration, I don't know but they are not as tough as the standard AGC types.

I usually just short them with a piece wire. Not the main pwr fuse, of course not.
But the b+ or heater fuses that were not there when people knew his to build amps.

Probably fixed 20 amps that had this same problem.
Sometimes I will simply up the rating if it is a real expensive boutique. Engineers cut the plate current margin too thin on the B+ fuses, since they are not solo blo, they can't handle the leak excursions very well.

UL will require internal fuses for circuits that can misbehave and get hot enough to start fires, be careful about second guessing UL.

JR

 

[clintrubber]

Crank it up!

I will at the earliest opportunity!

(I listened to some of your stuff, love it;-)
Cheers, V!

Thanks for having a listen! I like your username, I figure you either like Mr.Bungle, or the sci-fi movie (where Mr.Bungle on their turn may have sourced their album-name from )

 

[clintrubber]

The smaller glass fuses are crap. They go out all the time. Maybe vibration, I don't know but they are not as tough as the standard AGC types.

I usually just short them with a piece wire. Not the main pwr fuse, of course not.
But the b+ or heater fuses that were not there when people knew his to build amps.

Probably fixed 20 amps that had this same problem.
Sometimes I will simply up the rating if it is a real expensive boutique. Engineers cut the plate current margin too thin on the B+ fuses, since they are not solo blo, they can't handle the leak excursions very well.

Thanks, 'good' to hear about this 'just failing' happening more often. It's lame, but the easiest fix.

As it happens I have another 'broken' yet-uninvestigated Rickenbacker combo-amp that I expected to have to check for blown power devices (as actually does happen, according to repair reports), but who knows it's also there just the fuses... (I may be pushing my luck here)

Note these Rick-amps here are all solid-state, no further fuses after the power-TX. Nor anything else like overcurrent-protection. Then at least a speaker fuse would have been welcomed...

 

[JohnRoberts]

Thanks, 'good' to hear about this 'just failing' happening more often. It's lame, but the easiest fix.

As it happens I have another 'broken' yet-uninvestigated Rickenbacker combo-amp that I expected to have to check for blown power devices (as actually does happen, according to repair reports), but who knows it's also there just the fuses... (I may be pushing my luck here)

Note these Rick-amps here are all solid-state, no further fuses after the power-TX. Nor anything else like overcurrent-protection. Then at least a speaker fuse would have been welcomed...

When UL adds extra fuses to prevent product fires these are not considered user serviceable parts, like rear panel fuses. UL makes the manufacturers include internal fuse stickers declaring the amperage of the fuses to prevent bypassing the protection with larger fuses.

JR

 

[clintrubber]

UL will require internal fuses for circuits that can misbehave and get hot enough to start fires, be careful about second guessing UL.

'Internal' as in post power-TX fuses, so in DC-supply lines and towards speaker-connections?

When UL adds extra fuses to prevent product fires these are not considered user serviceable parts, like rear panel fuses. UL makes the manufacturers include internal fuse stickers declaring the amperage of the fuses to prevent bypassing the protection with larger fuses.

I recall the UL-stickers on old SWR bass-amps I used. Those amps had indeed way more protection-fuses than these Rickenbacker amps.

No idea how far back those UL regulations go? These Rickenbacker amps will be from around ~1981 (no UL), but I seem to remember those UL stickers on earlier Fender silverface amps as well...

Also thought to have understood that at a certain point Rickenbacker stopped making amps, due to something like liability/insurance becoming too costly, but I may be joining unrelated things into a wrong story here...

 

[CJ]

those powdered fuses they use for three phase 480 use to fail all the time when i was building watt meters,

some would be open right out of the box, i think they need current to keep them working, we were using 1/2 amp fuses for about 5 milli amps which might or might not have been the reason,

 

[clintrubber]

FWIW, another fuse-discussion: https://www.edn.com/fuse-failures/

Actually not too sure yet what to think of it; the actual article itself is short and nothing new, but let's see where the comments-part will go.

 

[evil grill]

Some years ago we had new units coming in to the workshop where they had the fuses blown or showing resistance. Some of the faulty fuses were sent to the manufacturer. The product manger said it was impossible and therefore could not fall under warranty. After a while the problem disappeared.