Not really a casting project unless you count soldering, but my dad's beloved old bose acoustimas 5 series III stopped working and I tracked the issue to the subwoofer. One of the circuit boards blew a resistor. It says 7.5 K ohm * J on the side of it but it doesn't specify the wattage. I'm pretty confident I can solder in a new one but I don't know for certain which one I need to order. Any idea how to figure out which resistor will work in its place? Also there are no color bars on this resistor so I am a little unsure which direction to solder the new one in. Circuit board model number:
I am not sure but I searched this thread for "r437" and the guy said its 7.5 k ohm 2 watt resistor. Not sure if it is the same mode as mine but at least it's something to go on? https://www.ecoustics.com/electronics/forum/home-audio/545019.html Looking at the resistor itself I almost wonder if it is still functional? It doesn't look damaged or burned out. It looks like it just heated up and melted the solder connection to the board then stopped working. I wonder if I could just resolder it and get it working again?
It didn't just heat up by mistake. It will heat up again, go ahead and blow the thirty-five cents and buy a new one.
No problem buying a new one price wise. Do the different colors or material types of resistors matter? Or just that it is 7.5 k ohms and 2 watts?
Apparently people have a hierarchy for resistors for amplifiers. So I guess I'll use their logic when picking types. They said: So these metal film resistors should work alright for my needs. I'm guessing. https://www.ebay.com/itm/10-PCS-2Wa...hash=item261135b652:m:mINQbLZ0-4YrHyyOJqTuBcg
That looks like some of the "flameproof" ceramic resistors around 3 Watts rating while those Ebay ones have a paint coating that can catch fire probably like epoxy. You may also need to scrape some of the burn marks off the board to prevent the charred carbon conducting too. This looks like a good 5Watt rated metal oxide resistor good for 400 deg F, it's physically bigger, nearly an inch but that works in your favour as bending the leads keeps it (and the heat) away from the charred board. https://www.mouser.com/ProductDetail/KOA-Speer/MOS5C752J?qs=sGAEpiMZZMvmQ%2bOLa8n/M5uRxY2%2b5i%2bejjLmSolVLmA=
How much does the wattage matter? If I oversize it then it may let more power through than intended? This looks like a high temp version at 7.5k Ohm and 2 watts. Similar to the one you linked. https://www.mouser.com/ProductDetai...%2bOLa8n/M1uechRDjrdlsQ8GvFbJ4uDCegwfvOAn4g==
Higher rated resistors run cooler all other things being equal, the same voltage is applied through the same resistance so the same current flows and the same wattage of heat is dissipated. This time round the resistor is physically bigger so the body temperature reached is lower. You can bet Bose picked the smallest, cheapest resistor they thought would do the job. keeping the wire leads longer also helps by keeping the hot resistor away from the board and the longer lead length helps heat dissipation. Some manufacturers even bent the component leads in a pigtail loop to reduce mechanical strain and improve heat loss.
By higher rated you mean 5 watts vs 2 watts? So I should order a 5 watt like the one in your link. So if I go with a 5 watt it should run cooler and do the same job without blowing anything else on the board? Related question: Would going with a much larger one then like 50 watt at 7.5 k ohms help more then? *edit* Since the shipping costs 3x more than the resistors I bought a selection, 2 watts, 3 watts and 5 watts. So if one burns out or doesn't work I can try the others.
In answer to your questions above, yes, yes and yes, there's diminishing returns and by the time you get to a 50W resistor the cost, heatsinking and mounting problems make it untenable. The resistor doesn't look like it's actually failed, it's probably still good but the heat it dissipated has cooked the board and caused the resistor lead solder joints to fail. You need to either use a bigger, higher wattage resistor or some more fiddly fix like resoldering the resistor and attaching some small copper shim heatsinks like tiny flags to each lead to radiate heat and keep the resistor leads cooler.
Gotcha thanks so much for the quick help. I figured the resistor was still good and the solder just melted out. I'll put in the 5 watt when it arrives later this week. Hopefully it will get the hifi working again. I know my dad will be very happy if it works out. That system is his pride and joy. One more question, does it matter which orientation I solder the new resistor in? There doesn't seem to be an orientation on the original. I'm guessing worst thing that could happen is the power doesn't flow through it and I have to resolder it the other way around?
I'm sure there is some audiophile who claims resistors need to be oriented, but for most people either way round is fine. Apparently some film capacitors have measurable differences and good old electrolytic caps can explode if reverse polarised.
Well luckily I got 2 of each resistor! Haha. Will let you know how it pans out. Now I just need to scrounge up some resin core wire and my old soldering iron to solder it.
I'm not sure. It's an old machine. Maybe the solder wasn't correctly applied by the factory and over 20 years the heat from the resistor slowly caused the solder to move off the contacts. At some point it got too thin and heated up hot enough to melt the last bit through. Or a combination of that and a lower capacity resistor than was needed? Maybe there was a power surge? Maybe another component burnt out somewhere I didn't see on another chip? Not sure, what do you think happened?
Yeah that's always the worry with this kind of thing. I had a good look at the board and even transilluminated it to check for other burned areas. Didn't see any. I mean it's possible I missed something or didn't see damage on the other boards that I didn't take out but in the past when I repaired a TV with a bad optocoupler and several laptop/PC power supply couplings on the motherboard the bad component was the only issue and replacing it fixed the issue. Maybe luck but who knows unless I give it a go. Worst comes to worst it's not going to get anymore broken! Haha.
