Does anyone know why an otherwise working amplifier, would start intermittantly blowing its correct rating fuse (3.15A slow-blow)?
It has a 0.1uF X2 class supression cap on the 240V mains input (which I have replaced with no different effect) which seems to be the only suppression circuitry before the large torroidal transformer primary.
The unit either caused, OR was affected by, a surge that tripped the household mains breaker.
When I can get the unit to turn on without blowing the fuse it works fine. I'm thinking maybe the in-rush current has increased somehow but it doesn't seem to make sense....
Some general specs:
Alto Powered Mixer - model PBM8.250
Stereo 250W into 4ohm load
Any ideas fellow techo's?
It has a 0.1uF X2 class supression cap on the 240V mains input (which I have replaced with no different effect) which seems to be the only suppression circuitry before the large torroidal transformer primary.
The unit either caused, OR was affected by, a surge that tripped the household mains breaker.
When I can get the unit to turn on without blowing the fuse it works fine. I'm thinking maybe the in-rush current has increased somehow but it doesn't seem to make sense....
Some general specs:
Alto Powered Mixer - model PBM8.250
Stereo 250W into 4ohm load
Any ideas fellow techo's?
Are you sure the fuse is correct ?
Any suppression caps can be removed temporarily to eliminate them.
Toroidal transformers do have huge inrush currents, and if it's a 500va or above transformer a T3.15 may not be enough.
Nothing in the amp could have changed to alter the inrush current and the fact the amp works OK point's to just a "fuse" issue.
Was the original fuse a special type... perhaps a ceramic encased sand filled one ? with a very high surge/time delay rating. I suspect it's nothing more than this.
Any suppression caps can be removed temporarily to eliminate them.
Toroidal transformers do have huge inrush currents, and if it's a 500va or above transformer a T3.15 may not be enough.
Nothing in the amp could have changed to alter the inrush current and the fact the amp works OK point's to just a "fuse" issue.
Was the original fuse a special type... perhaps a ceramic encased sand filled one ? with a very high surge/time delay rating. I suspect it's nothing more than this.
Could be the inrush limiter circuit has failed and is allowing too high a current through on power up.
Thanks a lot for your help guys.
I might just add, the fuse rating on the amp says T3.15A but I never saw the original fuse so it may have been the longer delay 'sand filled' type so I don't know. I must admit I've never seen one in amp before and I've seen a few....
As for the inrush limiter circuit, I'm fairly certain there isn't one which makes me think it may be a bad design. When I turn it on with speakers connected you can hear all sorts of pops and hisses for about three seconds while caps charge, outputs turn on etc until it settles down. By the way there are no relays switching on the outputs, its just turn on and they're on!
Not sure wether to use perhaps a T4.0A fuse or higher, or to add a NTC thermistor before the primary of the transformer as a make-shift inrush limiter.
What you guys think?
I might just add, the fuse rating on the amp says T3.15A but I never saw the original fuse so it may have been the longer delay 'sand filled' type so I don't know. I must admit I've never seen one in amp before and I've seen a few....
As for the inrush limiter circuit, I'm fairly certain there isn't one which makes me think it may be a bad design. When I turn it on with speakers connected you can hear all sorts of pops and hisses for about three seconds while caps charge, outputs turn on etc until it settles down. By the way there are no relays switching on the outputs, its just turn on and they're on!
Not sure wether to use perhaps a T4.0A fuse or higher, or to add a NTC thermistor before the primary of the transformer as a make-shift inrush limiter.
What you guys think?
I checked out the 'sand filled' types of fuse. They are called HRC fuses or High Rupture Capacity. While they are deffinatley time delay but main use is for high voltage spike handling not specifically in-rush current handling.
The amp has a fuse rating of 250V T3.15A and would thus prohibit a higher voltage rating fuse. Or in other words if I put a HRC fuse in its place I could open myself up for litigation if the worst happens and it one day catches on fire and burns a house down!
Looks like I need to look into inrush current limiting circuits. Anyone have some expert tips on retro fitting a thermistor? Do I put in series on active primary side of the toroid transformer?
The amp has a fuse rating of 250V T3.15A and would thus prohibit a higher voltage rating fuse. Or in other words if I put a HRC fuse in its place I could open myself up for litigation if the worst happens and it one day catches on fire and burns a house down!
Looks like I need to look into inrush current limiting circuits. Anyone have some expert tips on retro fitting a thermistor? Do I put in series on active primary side of the toroid transformer?
If it were me, I would try and find why it now behaves as it does raher than trying to mask the issue.
Just re reading your post, you say it might be the amp that caused the breaker to trip ?
And what are these noises it makes on power up... that doesn't sound normal at all... or is it just noisy pots and switches etc causing that.
Does it blow the fuse everytime or most times you power up ? Is the fuse blackened or does it just "part" slowly.
Next step may be to see if you can isolate the secondary of the transformer and see if it then blows fuses... that way you just have the transformer and nothing else.
Just re reading your post, you say it might be the amp that caused the breaker to trip ?
And what are these noises it makes on power up... that doesn't sound normal at all... or is it just noisy pots and switches etc causing that.
Does it blow the fuse everytime or most times you power up ? Is the fuse blackened or does it just "part" slowly.
Next step may be to see if you can isolate the secondary of the transformer and see if it then blows fuses... that way you just have the transformer and nothing else.
Thanks I see your point Mooly, I guess I was getting a little fed up with the thing and wanted a fast solution. A quote a lecturer said to me once comes to mind:
"A Gentleman never hurries"
Going through your questions...
>you say it might be the amp that caused the breaker to trip ?
The amp blew its fuse at the same time the breaker tripped. Which was the cause and which was the effect I don't know.
>And what are these noises it makes on power up
They are not all that noticable really. My thought was that they were noises due to normal power up that would in most other amps be switched out by the output relays (this one doesn't have any).
>Does it blow the fuse everytime or most times you power up ? Is the fuse blackened or >does it just "part" slowly.
A T3.15A slow blow would blow probably 1 out of five times. When it does the fuse is completely blackened suggesting a fairly hefty surge.
>Next step may be to see if you can isolate the secondary of the transformer and see if >it then blows fuses... that way you just have the transformer and nothing else.
I have actually tried this and it was still blowing the fuse from what I recall. I will try it again to make sure. Perhaps a partial short on the primary from a voltage spike is causing this or some other changed chx in the transformer? Also, I thought the inrush current came from the main filter caps?
I will make sure of the situation and post again.
"A Gentleman never hurries"
Going through your questions...
>you say it might be the amp that caused the breaker to trip ?
The amp blew its fuse at the same time the breaker tripped. Which was the cause and which was the effect I don't know.
>And what are these noises it makes on power up
They are not all that noticable really. My thought was that they were noises due to normal power up that would in most other amps be switched out by the output relays (this one doesn't have any).
>Does it blow the fuse everytime or most times you power up ? Is the fuse blackened or >does it just "part" slowly.
A T3.15A slow blow would blow probably 1 out of five times. When it does the fuse is completely blackened suggesting a fairly hefty surge.
>Next step may be to see if you can isolate the secondary of the transformer and see if >it then blows fuses... that way you just have the transformer and nothing else.
I have actually tried this and it was still blowing the fuse from what I recall. I will try it again to make sure. Perhaps a partial short on the primary from a voltage spike is causing this or some other changed chx in the transformer? Also, I thought the inrush current came from the main filter caps?
I will make sure of the situation and post again.
FIlter caps are a majopr surge contributor, but toroids have their own inrush.
Chances are the difference betwen fuse opening or not is one of where on the mains cycle you close the power switch. if it is near a peak, you get a large spike, if it is near zero crossing you get none. SO 1 in 5 sounds like that sort of thing.
COnnect an ammeter in series with the mains. Once it is powered up, how much current is being drawn? if the circuit sits there drawing 2.5 amps or something, then ther isn;t much headroom left for the fuse. On the other hand if idle current is next to nothing, then that scenario is off the list.
A couple shorted primary turns wouldn;t kill the transformer, but it would run hot and draw excess current.
DO these tests with the secondaries unloaded, at least at first.
I never used it with toroids, but I don;t know why it wouldn;t work. GO over to RG Keen's web site: New Page 1
And under Tech Tips - tube amps, there is a very simple shorted turns detector for transformers. You can make one with just a few common parts in a few minutes.
Chances are the difference betwen fuse opening or not is one of where on the mains cycle you close the power switch. if it is near a peak, you get a large spike, if it is near zero crossing you get none. SO 1 in 5 sounds like that sort of thing.
COnnect an ammeter in series with the mains. Once it is powered up, how much current is being drawn? if the circuit sits there drawing 2.5 amps or something, then ther isn;t much headroom left for the fuse. On the other hand if idle current is next to nothing, then that scenario is off the list.
A couple shorted primary turns wouldn;t kill the transformer, but it would run hot and draw excess current.
DO these tests with the secondaries unloaded, at least at first.
I never used it with toroids, but I don;t know why it wouldn;t work. GO over to RG Keen's web site: New Page 1
And under Tech Tips - tube amps, there is a very simple shorted turns detector for transformers. You can make one with just a few common parts in a few minutes.
Contrary to what you might expect, maximum surge occurs if you switch on when the mains is at the zero point. The large inductance of the primary winding prevents a surge if maximum voltage is suddenly applied. That's how inductors behave. And to confuse things even more, when you switch off a transformer, depending on the point in the cycle power is removed, you end up magnetizing the core... not a problem in itself, but that residual magnetism also affects the next switch on as it "modifies" the initial inductance of the core.The magnetising field is 90 degrees out of phase with the voltage.
So back to the problem... if it blows the fuse with the secondaries disconnected then either the fuse is not "right", or the transformer is faulty... that's unlikely I feel, or there is some other issue such as a breakdown in a surge protection cap (but you changed those)
What's the VA rating of the transformer ?
I found on an amp I built years ago a T3.15 amp is no good for a 500va tranny, a 4 amp blew intermitantly and a 6.3 was OK.
I still wonder whether there are "different" specs for time delay fuses... I'm no expert on that... and that this is all the problem really is.
So back to the problem... if it blows the fuse with the secondaries disconnected then either the fuse is not "right", or the transformer is faulty... that's unlikely I feel, or there is some other issue such as a breakdown in a surge protection cap (but you changed those)
What's the VA rating of the transformer ?
I found on an amp I built years ago a T3.15 amp is no good for a 500va tranny, a 4 amp blew intermitantly and a 6.3 was OK.
I still wonder whether there are "different" specs for time delay fuses... I'm no expert on that... and that this is all the problem really is.
Hmmmm after some more searching I have found some fuses by Bussmann that are classified as 'anti-surge' which are ceramic bodied which allow for up to 1.5KA surge at 250V AC. There are also some glass body ones from Littelfuse that allow up to 150A surge for the 3.15A type. I think I might purchase a few of the latter and try them out first, I'm hoping this will solve the matter. It would not surprise me if in leiu of including a decent surge protect cicuitry, Alto have just used a special type of fuse like these ones I've mentioned to save on cost.
BTW Enzo thanx for the transformer tester link, I have a blue ring tester but I'm a little dubious of its accuracy (or perhaps I've zapped it somehow) and would like a way to second guess its results. I will no doubt make it soon.
BTW Enzo thanx for the transformer tester link, I have a blue ring tester but I'm a little dubious of its accuracy (or perhaps I've zapped it somehow) and would like a way to second guess its results. I will no doubt make it soon.
T3.15A 'Anti-surge' 250V fuse with 130A 'Breaking Capacity'.
After successfully switching on and off 30 times I am convinced this is the solution.
So what is the moral to the story...
Don't trust 'off the shelf' slow blow fuses (of the same rating) to replace the ones the manufacturer has put in?
Not sure but I think I'm adopting this policy now.
Thanks for you help all!!
After successfully switching on and off 30 times I am convinced this is the solution.
So what is the moral to the story...
Don't trust 'off the shelf' slow blow fuses (of the same rating) to replace the ones the manufacturer has put in?
Not sure but I think I'm adopting this policy now.
Thanks for you help all!!
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