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These are two resistors that were in series with a transformer on the mains (120 V AC) input of an audio amplifier power supply. The PCB under them has become discolored with heat after being in service for 22 years. I removed them from the board and measured them. Surprisingly, they are both 2.81 kΩ, but the color bands would suggest 10 kΩ ±10%.

I am surprised that they measure much less than expected. Further, I wouldn't expect the measured resistance to be so similar to each other if indeed they have failed over time (2.815 k and 2.813 k).

Either I have interpreted the color band incorrectly, or they have managed to degrade very similarly to an out-of-spec resistance.

They measure 11 x 5 mm which I think may be rated for 1 W but I am not sure. The color bands are faded.

What value would seem reasonable for these resistors? I don't know what purpose they serve in a linear power supply where they are between fuse and transformer (current-limiting? soft start?).

Here is a partial schematic. There is a TRIAC and opto-isolator after the resistors in question (R1, R2).

schematic

simulate this circuit – Schematic created using CircuitLab

And here is a picture of the resistors:

Power resistors

Edit:

I updated the schematic after one of the answers indicated that I had not sufficiently probed the board to see that the TRIAC is in parallel with the resistors.

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    \$\begingroup\$ That could be orange,black,red. Which would be 3k. \$\endgroup\$
    – HandyHowie
    May 20, 2021 at 18:26
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    \$\begingroup\$ Did you pull the via barrel out? The upper left lead looks suspicious. \$\endgroup\$
    – qrk
    May 20, 2021 at 18:28
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    \$\begingroup\$ Given that there's a fuse in series, I would expect a resistance in the single-digit Ohms range. What's the output voltage of the transformer and what's the rating of the fuse? \$\endgroup\$
    – Jonathan S.
    May 20, 2021 at 18:28
  • \$\begingroup\$ @qrk Yes, I will have to repair the PCB when replacing that one. \$\endgroup\$
    – JYelton
    May 20, 2021 at 18:34
  • \$\begingroup\$ @JonathanS. sorry I omitted those details! The transformer is 59V 150VA with center tap. The fuse is 1.8A (250V). \$\endgroup\$
    – JYelton
    May 20, 2021 at 18:35

2 Answers 2

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They look like they could be Orange, Black, Red, Gold to me - 3k.

This page also backs this up where someone has the exact same problem -

The last comment there was - “I contacted Paradigm and the tech told the resistor values: 3K ohms 2Watt for R90 & R91. Some model versions might be different.”

On this page you can see a photo of the board with the two resistors on -

Here is what I believe the schematic should look like. You can see the 3k resistors on there. Probably used for standby power mode. enter image description here

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  • \$\begingroup\$ Why would you place two 3k resistors in series with a fuse? That doesn't make any sense. \$\endgroup\$
    – Lundin
    May 21, 2021 at 6:47
  • \$\begingroup\$ Also the resistors on your pic appear to be orange, blue, violet, gold (? it's hard to see) It ain't 3k that's for sure. \$\endgroup\$
    – Lundin
    May 21, 2021 at 6:52
  • \$\begingroup\$ @Lundin The OP has reverse engineered the board, maybe they have made a mistake. \$\endgroup\$
    – HandyHowie
    May 21, 2021 at 7:21
  • \$\begingroup\$ @Lundin I have just checked on a few different monitors and they all look like Orange, Black, Red, Gold to me. Admittedly they are not the clearest of photos. \$\endgroup\$
    – HandyHowie
    May 21, 2021 at 7:29
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    \$\begingroup\$ They are probably for a soft-start at power-on. \$\endgroup\$
    – Kevin White
    May 21, 2021 at 18:07
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Those might be 0.1 ohm (brown, black, gold) and have been discolored by heat over time. If they're reading 2.8K then they've definitely failed.

More about color codes here: https://www.petervis.com/electronics/Standard_Resistor_Values/0R1.html

MORE/REVISIT:

Or, they could be 3.0k (orange, black, red). That makes more sense, given that they haven't failed open and read about that value. Given @HandyHowie 's answer, backed up with a photo, that seems to be your huckleberry. So, dear reader, if you voted for this answer, unvote and vote for that one.

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    \$\begingroup\$ What would be the purpose of two 0.1 ohm resistors in series with the primary of a transformer? There is already a fuse in the circuit. \$\endgroup\$
    – Andrew Morton
    May 20, 2021 at 20:56
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    \$\begingroup\$ @AndrewMorton - Two components such as these resistors in series (also sometimes parallel combinations are used) is often done to spread out the overall power dissipation across multiple components. Resistor values adjusted according to design requirements. If a design wants a net 2.2 ohm resistor then two 1.1 ohm resistors can be used in series. \$\endgroup\$
    – Michael Karas
    May 20, 2021 at 22:00
  • \$\begingroup\$ 3.0k? Might be (orange->brown, red-> burnt away). It's a bit like that blue/black or white/gold dress, isn't it? \$\endgroup\$
    – hacktastical
    May 21, 2021 at 8:10

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