Bestec ATX0300F5WA - fast,or slow blow, fuse?

[socketa]

I bought a dead motherboard, and connected it to this PSU,
Was checking the standby voltages on the motherboard, and, whilst doing so, the PSU made a loud bang with a flash of light.
So now i truly got a bang for my bucks, and something else to try and fix.
The fuse was blowen, and the top was blowen off of a TNY276PN chip
The glass fuse didn't have any marking to say if it was fast or slow blow
So i checked the marking on the PCB thinking that that would tell me,
but it 's got a T/F8A printed on the board
So should i get a fast blow fuse, or should i get a slow blow fuse?

[stj]

Re: Bestec ATX0300F5WA - fast,or slow blow, fuse?

nothing etched on the metal fuse ends?

[CapLeaker]

Re: Bestec ATX0300F5WA - fast,or slow blow, fuse?

Fast blow.

[socketa]

Re: Bestec ATX0300F5WA - fast,or slow blow, fuse?

Thanks
I thought that the other end was just trademarks etc,
but on the outer ends it had "CONQ FUSE", which i looked up to be Conquer Electronics
so, on that other end of it, nearly half hidden, was "GFE", which is "Fast-Acting"

[socketa]

Re: Bestec ATX0300F5WA - fast,or slow blow, fuse?

This is working OK enough now, but there is a squealing sound that appears to be coming from the smaller standby(?) transformer, and it gets louder, and sounds like bagpipes, when i turn it on.
i checked all of the electrolytic capacitors that are around it - they test OK
Standby is 5.14V, so that's a bit high
Any ideas as to what could be causing this squealing?

[stj]

Re: Bestec ATX0300F5WA - fast,or slow blow, fuse?

with or without a load?

[petehall347]

Re: Bestec ATX0300F5WA - fast,or slow blow, fuse?

T is for time delay slow blow

[socketa]

Re: Bestec ATX0300F5WA - fast,or slow blow, fuse?

with or without a load?

It's the same in both instances.
Output caps test good also.
When i listen, through a hose, i can hear that the noise at the top of the transformer is an octave higher than the sound that is coming from the base of it.

[momaka]

Re: Bestec ATX0300F5WA - fast,or slow blow, fuse?

This is working OK enough now, but there is a squealing sound that appears to be coming from the smaller standby(?) transformer, and it gets louder, and sounds like bagpipes, when i turn it on.
i checked all of the electrolytic capacitors that are around it - they test OK
Standby is 5.14V, so that's a bit high
Any ideas as to what could be causing this squealing?

What about the output caps? I'm sure you checked those, but had to ask anyways. If they were replaced already, did you use very low ESR caps (as in, motherboard-grade low-ESR, such as Panasonic FR/FM/FS or better)? In some cases, I have seen very low ESR caps cause this. In fact, since I'm kinda cheap and I do still have a stash of Nichicon HN 16V 1500 uF caps (or similar grade Rubycon and Panny caps), I sometimes use these for the 5VSB output of various PSUs. From what I've noticed, the one that use PWM-FET tend to develop a faint squeal after doing this. But I've tested these recapped unit with and without any load, and they've been fine for a good number of years now.

So the squeal noise may be normal... unless it's really loud, of course, and the circuit started doing it all of a sudden without having any components changed.

5.14V is not too bad. Under a proper 2 Amp load, it'll probably drop to slightly under 5V. So not uncommon to see manufacturers do this slightly higher voltage on the 5VSB, so that it stays closer to spec if under full load. Also, don't forget there will be a V-drop in the 5VSB wire with a higher load. So that's another reason 5VSB is always set at slightly over 5V.

[socketa]

Re: Bestec ATX0300F5WA - fast,or slow blow, fuse?

I only needed to replace the TNY276, chip and a TVS diode (which is just behind the blue resistor which is also a replacement, because, although the original resistor tested good, it has some of it's insulation missing.
Using the recommended testing capacitors, i can't get a clear signal on the 5VSB rail, and it gets worse when i power it up (see attached photos)
When it's powered up with a hard drive there is peak to peak ripple of 70mV, which is higher than the allowed 50mV
And i can't trigger a well-enough defined signal on the other rails either (but can on another PSU)
Adding a 1A load to the 5VSB doesn't clear up the signals
It looks messy
But maybe that's OK for these PSU's?

[Gentian]

Re: Bestec ATX0300F5WA - fast,or slow blow, fuse?

I think you have to check in spite of the capacitors on the secondary side, also the filter coils. These signals showing in the oscilloscope look not very clean. There may be something wrong with the PSU.

I have seen situations when the noise from the transformer is related to the maximum allowed duty cycle coming from the PWM IC. It happens when we want to change the output wanted set point i.e. from 3.3V designed to work the transfromer to 5V or when the input voltage is too low regarding to rated voltage i.e. 150V instead of 230V.

Hope it helps

[socketa]

Re: Bestec ATX0300F5WA - fast,or slow blow, fuse?

Thanks for your thoughts
There is 162V across each primary capacitor, so that's OK,
And i don't think that the following is applicable in this situation, because all of the rail voltages are in spec:

It happens when we want to change the output wanted set point i.e. from 3.3V designed to work the transfromer to 5V

It has to be something that is common to the standby and the main rails, because it's affecting all of them.

[stj]

Re: Bestec ATX0300F5WA - fast,or slow blow, fuse?

mabe just recap everything small and see if it helps.

[socketa]

Re: Bestec ATX0300F5WA - fast,or slow blow, fuse?

maybe just recap everything small and see if it helps

unluckily, that didn't help
and i also tested the 5VSB optocoupler - it's working
couldn't find any shorted diodes either