Suitable substiture for fusible resistor?

Ork

Member
2016-02-29 11:20 pm
Hi Everyone. I have browsed the forums from time to time but finally registered, yah!

I found a couple of fusible resistors on my old Harmon Kardon AVR30 receiver, blown. They are 560ohms 1/2 watts, green color. I'm having a hard time sourcing this value. Neither mouser or digikey has anything. Mouser did have some 560R fusibles at 1/4 watts but that means I'll have to rig up 4 in series/parallel which will not be pretty, and since these 2 blown resistors are side/side, that means stuffing 8 resistors where 2 used to be. I thought about using two 1.2k in series, above and below the board but apparently, 1.2k isn't even a value anyone produces.

I think I read at one time that metal film resistors might work to replace fusibles but if that were the case, then why would metal film fusibles even be offered in a separate line. Anyone know a source for 560R 1/2 watt fusibles or have any thoughts about this? I'm afraid of using non-fusibles since they blew before and the last thing I want is my house to go up in flames.

Thanks in advance.
Norm.
 
Fuseable resistors MUST be replaced with identical components.

At a push, if you know the current through the resistor, you could series mount a wire ended fuse with a resistor.

That's not correct, it depends upon the application.

If the resistor is blown, find out the reason...

FWIW, it helps if you use the correct spelling "fusible"
 

Ork

Member
2016-02-29 11:20 pm
Fuseable resistors MUST be replaced with identical components.
Sure, given that "exact" replacements could be obtained, obviously those would be the ideal replacement solution. But in the case where components are obsolete, we don't then brick the device because we can't find the "exact" resistor now do we?

470R 0.5W fusible should be OK
RS art number 267-1761
If value is critical then 91R 1/4 imn series with the 470R 1/2W fusible will be safe
@davidrsb, thank you. I'm sure the value is critical in this application. Those resistors are part of a voltage divider circuit that I'm pretty sure directly affects biasing. Not sure why I didn't think of a 91ohm replacement in series with a 470R, I would've thought that 91R would be even more difficult to obtain but I see mouser has them in 1/4 watts. 1/2 watt of course is a different story. Speaking of which.... when I look at the board, all the resistors look like standard 1/4 watt resistors (from size). However, the service manual says all resistors are 1/2 watts unless otherwise indicated. I compared the resistors to some 560R 1/4 watt fusibles and they are approximately the same size. I also compared them with 1/2 watt standard metal film resistors and they are much smaller. I know modern manufacturing techniques are more efficient but didn't think that 25~30 year old amp would have these in ultra small size across the board (all the resistors look the same size). See images.

avr30_1.jpg


avr30_2.jpg


avr30_3.jpg
 
IMO, fusible resistor technology was pretty bad and the parts often go high in value or even open,without being overloaded at all. They are rarely used today, so I don't get very worked up about examining the circuit and considering what's apt to fail if the resistor doesn't. More often than not I can live with the consequences of replacing them with metal films (that won't go open to protect anything until everything else has burned up) or possibly carbon films that will open, though not as elegantly as the fusibles. I'd never fuse individual resistors, but sometimes the addition of an extra supply fuse can protect an entire circuit.
 

Ork

Member
2016-02-29 11:20 pm
IMO, fusible resistor technology was pretty bad and the parts often go high in value or even open,without being overloaded at all.

This one is fixed.

It's been my experience as well that fusible resistors do seem to go bad and change value or go OC without being overloaded, even on portable boomboxes. The ones in question were in one (out of 5) channels with one OC and the other changed to very high. I did replace them as davidsrsb suggested using [email protected]/4w in series with [email protected]/2w, but decided to check the resistors in the other channel as well while everything was opened up. Found 3 more resistors (all fusible) with varying levels of changes in their values. Parts arrived today, swapped them all in, swapped out the bias adjustment pots to sealed multi-turn ones, and set the bias current in several stages over 1-hour. So far, everything has held up without any problems and amp sounds terrific now, distortion and DC balance issues are corrected. Been driving it for several hours now and all is good. Luckily, problems were in the small signal stage and not in the outputs stage.

Problem with fusibles is that certain values are very hard to find, or not at all anymore. Interestingly enough, only the fusibles changed values. All of the non-fusibles were spot on. Even the emitter resistors tested good with very little deviance from marked values of .27ohms tested with the low ohms function on fluke 289.