fusible resistor

[jessenator]

So I'm tinkering more and honing in on some cheap as free™ cosmetic tweaks on my little Monoproice/Harley Benton/Laney 5W guy. The power light jewel has a nice amber color that shifts full blood red with the red LED on the other side. jwin sent me some 10mm clear white LEDs and so that'll go in. I want it to be a bit brighter than stock, so I fiddled with some resistor values on the bench and got it locked in at 1k2

Looking closer at the amp main board, it's got a regular value for the CLR at 4k7. Looks like a 1W resistor, and the silkscreen says 'Fusible' on it. Wondering if that's standard practice on an amp for the power lamp/LED, and what I should get to replace it, or if I could get by with a standard metal film 1W and not risk a fire or some other hazard.

[jwin615]

I'm going to guess that they went with a fusible resistor for temperature protection, or more likely over temp indication. So, if that resistor gets warm enough(via the amp), it opens and your indicator turns off.
Or maybe it is to protect if the jewel is broken and the circuit is shorted?
This makes more sense if a bulb was used vs an led. Though the 22R seems as it may be sacrificial as well.
Ohhh. On second/third thought, maybe it's for temperature stability. Those nichrome resistors are stable at muuuuch higher temps.
Is this portion of the board exposed to high temps when in use? Maybe the same PCB is used in higher wattage amps where this is more of a concern. If it's not getting heated, I don't think there's much downside in using a standard film resistor. Maybe use a wire wound to safeguard against higher temps if that's a concern.

[jessenator]

I'm going to guess that they went with a fusible resistor for temperature protection, or more likely over temp indication. So, if that resistor gets warm enough(via the amp), it opens and your indicator turns off.
Or maybe it is to protect if the jewel is broken and the circuit is shorted?
This makes more sense if a bulb was used vs an led. Though the 22R seems as it may be sacrificial as well.
Ohhh. On second/third thought, maybe it's for temperature stability. Those nichrome resistors are stable at muuuuch higher temps.
Is this portion of the board exposed to high temps when in use? Maybe the same PCB is used in higher wattage amps where this is more of a concern. If it's not getting heated, I don't think there's much downside in using a standard film resistor. Maybe use a wire wound to safeguard against higher temps if that's a concern.

Thanks.

Interesting thing, I was able to trace it on Saturday, but now I can't at least not reliably. I'm not even measuring across a component. I'll go from that via to the leg/pad of R2. I'll get a continuity beep and then it stops and I can't get anything until I touch something else, then R2, then it's cut off again. No idea. Happens with my Ryobi and my Fluke. Weird.

But at least I know that's the one.

A parallel resistor would do it without having to clip or desolder the one that's in there. There's a 7k5 wire wound on a compact mac analogue board I could salvage, but that would only take me to 2k9—not much brighter. Would it be a hazard to just use something like a ≥2W regular 3k3 resistor? That would get me close to the 1k2 value I tested on the bench for brightness.

[jwin615]

If all it is powering is the LED, should be fine I would think. The "LED only" 4007 is interesting as well.
Was their an AC powered lamp in there previously? And the 4007 is a crude rectifier for alternate led use?

[jessenator]

Yeah, might be a vestigial part of the circuit.

Shared LED board for the 15W version which has its own pad for an LED...for some reason?