Active Monitor blowing transformers.

[aviel]

hey all,
I will appreciate some help.
2 years ago my tapco s8 active monitor started blowing fuses. it was a shorting transformer. i opened a post here and ended buying a new one, over powered, just to be safe.

bought a triad magnetics 100VA with correct secondaries, and it worked fine.

yesterday i went to the studio after not being there for few months, turned on the switch, and bam, same thing. the new transfo is shorted, the 800mA fuse of course blew up.

I have no idea what to do next, is that inrush current? if so, something is malfunctioning and causing it, because other monitor still works fine..(and i took a 2x bigger transfo).

circuit is quits simple, the secondaries go to the rectifier and i end up having 2x31 electrolytics looks fine as well.

what do you think? i dont think its overheating since it happens on turn on

 

[radardoug]

Is it a toroid? Theres some dodgy toroid winders out there.

 

[aviel]

yes. but its triad magnetic

 

[Brian Roth]

Just guessing here.....maybe the rectifier(s) or filter caps are dying.....

Bri

 

[aviel]

Just guessing here.....maybe the rectifier(s) or filter caps are dying.....

Bri

do you think a dying cap can cause very high inrush current?

 

[abbey road d enfer]

do you think a dying cap can cause very high inrush current?

I doubt it very much, but instead of invoking spirits  , you should start with an ohmmeter and measure the windings and the rectifiers.

 

[aviel]

I doubt it very much, but instead of invoking spirits  , you should start with an ohmmeter and measure the windings and the rectifiers.

winding is shorted, thats what happens. question is why..

 

[Khron]

Define "shorted", and which winding?

 

[aviel]

Define "shorted", and which winding?

.

i have 2*115 VAC prinmaries that are connected in series to match the 230VAC. one of them measures 11ohms (which looks normal). the other is 3k Ohm.. so thats a problem..

as soon as i connect main, fuse blows up, while secondaries are disconnected.

 

[JohnRoberts]

Transformers have to get pretty hot for the wire insulation to fail. Consumer power transformers also often use thermal fuses inside for protection.

A shorted reservoir cap, or shorted diode can draw excessive secondary winding current.

JR

 

[abbey road d enfer]

.

i have 2*115 VAC prinmaries that are connected in series to match the 230VAC. one of them measures 11ohms (which looks normal). the other is 3k Ohm..

Technically, it's not shorted. A shorted winding is one where adjacent turns have lost their insulation. Typically, it will register significantly less ohms than normal.
What you have is a fried winding, where turns have burnt, losing continuity. Concomitally, some of the turns are shorted either to another winding, or to the iron core.
The only explanation is overheat, which is probably not attributable to current surges, but rather to continuous overload.
Possible explanations are either undersized xfmr or abnormal current draw from the amplifiers.

as soon as i connect main, fuse blows up, while secondaries are disconnected.
[/quote]

 

[cyrano]

Have you tried searching the interwebs?

Seems to be a pretty common failure on those monitors.
There's even a thread about it on this forum:
https://groupdiy.com/index.php?topic=41502.0

Under-dimensioned toroidal?

Quote from that thread:

Seems like they've built their margin on sh*tty transformers...

 

[aviel]

Technically, it's not shorted. A shorted winding is one where adjacent turns have lost their insulation. Typically, it will register significantly less ohms than normal.
What you have is a fried winding, where turns have burnt, losing continuity. Concomitally, some of the turns are shorted either to another winding, or to the iron core.
The only explanation is overheat, which is probably not attributable to current surges, but rather to continuous overload.
Possible explanations are either undersized xfmr or abnormal current draw from the amplifiers.

as soon as i connect main, fuse blows up, while secondaries are disconnected.

but i am sure that it blew up on startup. last time i used it, it was working. then 2 months later i tried to turn it on and it was dead. thats what confuses me here..

 

[aviel]

Have you tried searching the interwebs?

Seems to be a pretty common failure on those monitors.
There's even a thread about it on this forum:
https://groupdiy.com/index.php?topic=41502.0

Under-dimensioned toroidal?

Quote from that thread:

Thanks!
yes i saw that thread. the xformer i bought is 100VA, its almost 3x of the original one.

 

[aviel]

what if i take the fuse down to 400 mA (instead of 800mA) and use thermsistor to limit the inrush current?

 

[abbey road d enfer]

what if i take the fuse down to 400 mA (instead of 800mA) and use thermsistor to limit the inrush current?

Inrush current blows fuses, not xfmrs. It takes some time to fry a xfmr, unless its faulty from the start.
I suggest you do a PM on the xfmr; if the wires are burnt, it implies the xfmr has been overloaded for a length of time.
If the wire is OK, the open winding is a consequence of dodgy manufacturing.

 

[aviel]

Inrush current blows fuses, not xfmrs. It takes some time to fry a xfmr, unless its faulty from the start.
I suggest you do a PM on the xfmr; if the wires are burnt, it implies the xfmr has been overloaded for a length of time.
If the wire is OK, the open winding is a consequence of dodgy manufacturing.

so here is how the primary windings looks like (attached), i see  very small burnt area, do you think this can be caused from overloading for long period of time? it looks too narrow.
i still dont understand how it could have been overloaded while it was turned off.

 

[abbey road d enfer]

so here is how the primary windings looks like (attached), i see  very small burnt area, do you think this can be caused from overloading for long period of time? it looks too narrow.

Agreed. So it confirms the second hypothesis, that the xfmr had an original defect.

 

[aviel]

so i just had bad luck?
2 years ago xformer burned, same symptoms. bought a new one, and now again. so it might indeed be bad xformer this time, or something which is wrong elsewhere.
anyway, what about putting a smaller fuse (~500mA) and use a inrush limiter?
full power current for 100W should be 100/230 ~ 400ish mA (assuming 100 efficiency).
a 100VA traffo should handle max current.

what surprised me is how they used a 35VA one????

 

[scott2000]

Just curious, what is the Triad transformer model and can you link the specs?