Re: Help needed identifying failed components

Am 06.07.07 17.40 schrieb Lostgallifreyan:
Wolfi <publicalfa-ng@xxxxxxxx> wrote in
news:iHwji.4419$Oz7.2489@xxxxxxxxxxxx:

Am 05.07.07 23.30 schrieb Eeyore:
Wolfi wrote:

I got one of those small, 1/3W, maybe even 1/2W, fusible metal film
resistors, which burnt through, due a failing diode in a computer
monitor's SMPS.

Now I'm having some trouble to identify the value of this R. There a
5 colour rings on it, distributed symmetrically.
In the center: BlK - Gld - Blk and, a hair thinner, Red and what
appears to be Brn, one on each side on the thicker contact caps.

Now, how is this one to be read, starting with Red or with Brn?
And what is the meaning of the 2 rings following the gold one? Which
one is tolerance and what is the other one for?

The blown diode has the following on it: B119 HER303 (1,3mm wires,
body 9.2mm × 5.2mm diameter).

Could anybody please provide some specs on it and possibly one or
more suitable replacement types?
HER303 is a rectifier.
http://www.google.com/search?&q=her303
Thanks. Yesterday I only had access to a Mail/News client, so I
couldn't use a search engine. I now found a few possible replacement
types.

But I'm still stuck with the colour code puzzle of this fusible
resistor :-\


Probably Red, Black, Gold, Black, Brown. (Assuming the gold really IS gold and not some weird metallic shade caused by burnt pigment). The value likely begins with Red for 2, that would be a very unusual tolerance band.
>
So, maybe 2 ohms with unusual markings, 3 colours for 1,0,/10, unused, and brown for 1% tolerance at the end. The fourth band might have been printed by a machine ready for the 4-colour scheme for value, but set to put the unused black band for the old three colour scheme after the multiplier instead of before it as is usual on metal film types in readiness of an extra significant digit.

If the gold colour is a result of burning (I have seen bands look silver as a result of this, and the centre band is the first to burn), it might just be standard 1% tolerance 5 band 200 ohms, Red, Black, Black, Black, Brown.

Judging from the surrounding circuitry, which I further re-engineered by now, I have to assume, that its value is most likely 1 Ohm at most, perhaps even 0.1 Ohm, (even though 2 other people also looking at it and both came up with gold being the center ring).
From a tap of the SMPS transformer, a HER303 diode, which shorted out is connected via a wire bridge with a ferrite core around it to this little 7mm × 2mm fusible resistor, and from there feeding the CRT cathode heaters with a negative 6.3V voltage. The heaters have a cold resistance of some 5 Ohm and assuming that when hot, the resistance increases by at least 50% to about 8 Ohms, then it still would mean some 0.6W dissipation in the little R (with an assumed value of 1 Ohm).
My guess is, that it isn't rated for more than 0.5 W, so it would be already a bit tight. But on the other hand, it is no DC, it still is a pulsing current, so it might be just fine and the above rule of thumb calculation resulting to high anyway.
But I think it also quite surely eliminates the possibility for a 2 Ohm reading, so it has to be something like Brn-Blk_Gld => 1 0 ×0.1, next Blk = dunno, and finally Red = 2% tolerance OR Blk = 1% tolerance and the final Red = dunno.

Any other thoughts?
.

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