HP MicroServer PSU repair: weird resistor

[Deckert]

Hi,

I'm repairing an 150W SMPS, but came across a rather odd resistor. It looks like it powers the bootstrap circuit (via a zener), but the value seems off.



The resistor is clearly blown (and measures open), but there's enough to see the colour codes.
Yellow, Violet, Gold, Gold and Black.

The second two Gold bands don't make sense for a five-band resistor - should I be treating it as a four-band and ignore the black ring? I've found zero information on five-band resistors where the 4rd band is gold.

When ignoring black, this comes out as a 4.7 ohm resistor, yes?

--deckert

[wraper]

should I be treating it as a four-band and ignore the black ring?

Yes, fifth band is such cases usually is manufacturer specific and is used to mark type of resistor.

[coromonadalix]

correct me:  but i think the first color is green, doesnt seem yellow to me  loll

it would be an 5.6 ohms at 5% precision

[WattsThat]

If it is in a mains connected circuit, chances are good it is a fusible device and it has blown as evidenced by the hole in the body.

[Deckert]

If it is in a mains connected circuit, chances are good it is a fusible device and it has blown as evidenced by the hole in the body.

Yes, that was what I was thinking as well - fusible resistor.

Indeed, it is a mains-connected device. The resistor feeds a part of the bootstrap circuit from the high-voltage DC side (320VDC). Now to figure out what other component is faulty that caused the fusible resistor to blow in the first place.

--deckert

[madires]

correct me:  but i think the first color is green, doesnt seem yellow to me  loll

it would be an 5.6 ohms at 5% precision

With the second band being quite violety?

[madires]

If it is in a mains connected circuit, chances are good it is a fusible device and it has blown as evidenced by the hole in the body.

Yes, that was what I was thinking as well - fusible resistor.

Several manufacturers use an additional black color band to indicate a fusible resistor. And a blown resistor doesn't imply it's meant to be a fusible one.

[SeanB]

5R6 0.5W fusible resistor, and very likely the faulty component on the other side is a power switch device, and it's associated controller chip, probably because either the power supply capacitors for it went high ESR, or the current sense resistor for the chip went open as well, or the secondary side capacitors went open circuit.  Less likely is the snubber network for the switch device being faulty, either the diode shorted or open, or the ceramic capacitor open.

[Deckert]

5R6 0.5W fusible resistor,

Still think it's yellow and violet, but that apart, definitely seems to be fusible, specifically looking at this resistor supplier:
https://supertech.manufacturer.globalsources.com/si/6008800039268/pdtl/Fusible-resistor/1130423460/Fusible-Resistor.htm

and very likely the faulty component on the other side is a power switch device, and it's associated controller chip, probably because either the power supply capacitors for it went high ESR

You hit the nail on the head - main reservoir cap vented downward and "chewed" off one of its own legs :-)

There's a TNY268PN (AC-DC Converter) chip after that resistor with the MOSFET stage in series with the resistor that blew. The TNY268PN itself is also blown now (but I was able to recover the top of the device to get the device ID). Currently waiting for a replacement part, but the lockdown in ZA will delay that part's delivery.

or the current sense resistor for the chip went open as well, or the secondary side capacitors went open circuit.  Less likely is the snubber network for the switch device being faulty, either the diode shorted or open, or the ceramic capacitor open.

Will definitely chase this path further as well - thanks!

--deckert