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My SVS PB13-Ultra subwoofer stopped working. It happened after a voltage sag, probably followed by some spike.

After inspection, the fuse and a power resistor were blown.

I am sending you pictures attached. This is the PCB of the 1000W power supply of the sub driver. The missing indicated component is a 7 Watt, 5 ohm resistor.

Along with the other same resistor, they are the very first thing (after a usual varistor) that the AC circuit sees. I assume both these resistors are part of the LC filter at the input stage, the resistors probably dampening the filter's response.

These sand blocks are old fashioned resistors. I tried to find a similar one in mouser and digi-key, but no luck at all. 2 questions:

1) Is there ANY difference in replacing it with an ordinary wirewound power resistor, eg:

http://www.mouser.com/ProductDetail/Welwyn-Components-TT-electronics/WHS7-10RJT07/?qs=sGAEpiMZZMtTURnxoZnJAET%2fHU6QB3QMhCsG69Ld%252bHU%3d

or a 10W one:

http://www.mouser.com/ProductDetail/Ohmite/TWW10J10RE/?qs=sGAEpiMZZMsPqMdJzcrNwsOTZqx7B%2f%2fzcWOTb0%252bMDhQ%3d

I didn't find any 5ohms, >= 7W, so I'd have to replace both resistors.

2) What is the story behind these sand blocks? Aren't they wirewound internally? Why did they stop being produced?

Funny thing is that the date signature in back of PCB is 2010. It is not that old! tks!

img1 img2 img3 img4

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Cement resistors are still common as dirt, and they are usually wire-wound. 5ohm cement resistors, however, are not. Either use a 4.7ohm and .33ohm resistor in series, or 2 10ohm resistors in parallel.

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They're really common still- the vertical style less so outside Asia. You can find 5W and 10W 5.1 ohm axial types at a Digikey etc.

They're designed to be fusible, so ordinary WW resistors may not be as safe.

You could try to hack in the 10W part or try to source the original type from Asia.

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  • \$\begingroup\$ If these are fusible then OP should really check the cause why they have 'fused' and they should really be replaced with fusible ones. \$\endgroup\$ – jippie Jul 2 '14 at 5:46
  • \$\begingroup\$ Well, both the fuse and this one resistor fused probably because of a voltage sag. I know it because I was home when it happened, lights got dim and the UPS biped and kicked in for one or 2 secs. The probable spike after the sag is a candidate to blame. So I believe I just have to replace them. \$\endgroup\$ – tfm Jul 2 '14 at 17:30
  • \$\begingroup\$ I didn't know these are fusible. That is a very important info. I wouldn't like to miss that in my sub power supply and amp. But I just can't find any fair replacement! If they are fusible, how come there is no class of resistors with this feature available at mouser, digikey,. etc?! Any idea where I can find those, even if in Asia? \$\endgroup\$ – tfm Jul 2 '14 at 17:34
  • \$\begingroup\$ The ones I pointed to are equivalent (10W) but larger. Or try eBay Alibaba etc. where Asian sellers congregate. \$\endgroup\$ – Spehro Pefhany Jul 2 '14 at 17:44
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If the resistor is really bad, take a hammer to it and find out what's inside. Inductance at audio frequencies due to being wirewound should be negligible. The ceramic construction can get hot without damage. All this may not be very important if the fuse blows as soon as you apply power next time.

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I wouldn't use a wirewound resistor, unless it's a very high quality one, because the inductance could affect the performance of your sub. You can use a higher power rating, the only problem there being that the new part might be too big. Just FYI, the resistors you've shown are probably ceramic. Also, you're right, I checked Mouser and their highest rated ones are 5.5 watts. Anyway, a possible replacement would be this, and the slight resistance difference and modest price would warrant changing the non-damaged one as well.

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  • \$\begingroup\$ I'd like to see you justify that "the inductance could affect the performance of your sub". Do you know which part of the circuit the resistor is in? It's a 5 ohm resistor and may be taking 1A. How much inductance might produce a reactance of 5 ohms at sub frequencies (say 1kHz maximum) - answer something a about 800 uH. Let's say the reactance was one-tenth of the resistance at 80 uH - do you honestly believe the self inductance of this wirewound resistor is going to be anything more than 1 uH? \$\endgroup\$ – Andy aka Jul 2 '14 at 10:47
  • \$\begingroup\$ player3, I tend to be as careful as possible with audio components as well. However, in this case, since this is the dampening (and fusible) resistors of the first input filter, I can't see how it could affect audio. Circuit still goes to diodes bridge, gets filtered again, isolated, switched, filtered at the output... I would kindly like to hear your thoughts on affecting performance in this case. \$\endgroup\$ – tfm Jul 2 '14 at 17:39
  • \$\begingroup\$ You're both right. I'm no expert, and I tend to err on the side of caution especially when I don't know exactly what I'm working with. I only said the inductance could affect performance, if at all- not that it would ruin anything, nor that the author should avoid wirewound resistors like the plague. \$\endgroup\$ – player3 Jul 2 '14 at 20:26

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