Is this the part I want?

[cfaftw]

Hey guys, I'm new to the forum. I just bought an Infinity SSW-212 subwoofer that had a dead amp (doesn't power on, the fuse is fine). I bought it because the woofers were in god physical condition, it was cheap, and I supposed I might be able to fix it. After visually inspecting the entire circuit board, I only saw one visible problem: the inrush current limiter seems to be blown. I'm rather new to DIY electronics repair, but I found this part on digikey and want to verify that it would work as a replacement: Digi-Key - 570-1032-ND (Manufacturer - SL22 10005)

The amplifier itself has a line-in fuse that is 120V, 5A, 3AG slow blow. I have a feeling that ICL on digikey will work, since it's the same part number as what is printed on the blown ICL and it is rated for 5A.

Please let me know if that's correct! I'm really hoping replacing this piece will fix it because it's cheap and a really easy solder job.

Below are pictures of the blown ICL from my amp, including the part number stamped on it.

View of the ICL... you can see that it doesn't look quite right


A closer view of the ICL, showing the damage.


The other side of the ICL showing the part # (it reads "USS 10005").


Thanks in advance guys,
- Brett

[lanchile]

yeah, that thermistor looks broken! and the one from Digi Key looks just fine to replace it. But it is not to often to see a thermistor broken like that. it must be a big "inrush" to break one like that.

[mightydub]

You might want to go up a step on the current rating for the replacement part. Also see if you can figure out why it blew - does the bypass relay work? Does the timer circuit that triggers the relay work? Is there a short circuit downstream from the transformer? Etc.

It could be that the part was under spec'd and hit end of life, or that it had some manufacturing defect that made it die an early death. But it would be prudent to check the rest of the supply and amp to see if there was some other cause and the thermistor was the weakest link and therefore first to go. Don't make any assumptions based on the fuse being good, when the amp died that was probably the first thing that the person you bought it from tried.

[cfaftw]

Thanks for the replies! Indeed, the original owner saw the fuse was blown and replaced it (which obviously did not fix the problem). I would assume the fuse and the thermistor went at the same time (possibly a power surge).

I agree it would be prudent to perform more testing. I don't know much about electronics, but I don't think there is a timer circuit (this sub has no auto-on/off function). How would I do a more thorough test for shorts? Again, visibly there is no indication of a short anywhere on the board. The only visible damage is with the thermistor.

If I were to step up the current rating, how high should I go, and what would be the consequence of going too high? If the amplifier is rated for 5A max, wouldn't I want to limit the inrush current to 5A?

Thanks again,
- Brett

[lanchile]

Originally Posted by cfaftw

Thanks for the replies! Indeed, the original owner saw the fuse was blown and replaced it (which obviously did not fix the problem). I would assume the fuse and the thermistor went at the same time (possibly a power surge).

I agree it would be prudent to perform more testing. I don't know much about electronics, but I don't think there is a timer circuit (this sub has no auto-on/off function). How would I do a more thorough test for shorts? Again, visibly there is no indication of a short anywhere on the board. The only visible damage is with the thermistor.

If I were to step up the current rating, how high should I go, and what would be the consequence of going too high? If the amplifier is rated for 5A max, wouldn't I want to limit the inrush current to 5A?

Thanks again,
- Brett

you can go higher with the thermistor but, never go higher with the specified fuse.if anything goes wrong the fuse will burn.

[cfaftw]

Originally Posted by lanchile

you can go higher with the thermistor but, never go higher with the specified fuse.if anything goes wrong the fuse will burn.

Cool. I ended up ordering the 5A thermistor just to replace what was there. If it ends up blowing later I'll get one rated for a little more amperage. As for the fuse, I could only find 5A 3AG slow-blow rated for 250V (checked mouser and digikey), despite the owner manual specifying 125V. From what I understand, it's ok to have a fuse be rated for a higher voltage... I could be wrong though :| We'll see what happens!

[mightydub]

Typically the thermistor is in parallel with a relay. When the switch is first turned on, AC flows through the thermistor to the transformer and then to the power supply, limiting the inrush current. After a few seconds the relay is turned on shorting across the thermistor, so the AC current flows through the relay to the transformer for all but the first few seconds after power-on. A simplified schematic and spice model is here: AC to DC Power Supply - diyAudio (got to put in a plug for the LTspice wiki.)

So look for the relay, then check that the relay is getting turned on a few seconds after the unit is switched on. You wouldn't want your sub amp to be sucking current through the thermistor full time - that would limit performance and mean an early death for the thermistor.

[cfaftw]

Just to give an update. There is no bypass relay. Current goes straight through the thermistor always. Basically current flows like this: line AC -> fuse -> thermistor -> switch.

Anyway, I got the new thermistor and put it in. I plugged in the amp, flipped the switch, and... the fuse blew. Thermistor is fine. I'm hoping no components were damaged.

I found the schematic for my amp online for $1 (I should have bought it sooner), and it actually shows a thermistor with a resistance of 20 ohms. Would using the 10-ohm thermistor cause the fuse to blow (by letting in too much current)?

On digikey, the only 20-ohm 5A ICL has a tolerance of +-25%. I saw a 30-ohm 5A ICL with +-15%. Would using a 30-ohm thermistor cause problems or should I just get the 20-ohm?

[cfaftw]

Originally Posted by cfaftw

On digikey, the only 20-ohm 5A ICL has a tolerance of +-25%. I saw a 30-ohm 5A ICL with +-15%. Would using a 30-ohm thermistor cause problems or should I just get the 20-ohm?

For some reason I can't edit my post. The 30-ohm thermistor I found has a +-20% tolerance, not 15.

Also, I am aware there might be a short somewhere, but I haven't done a thorough check yet. I might be able to get an electrical engineer friend of mine to come help me find the short.

[mightydub]

It sounds like the thermistor isn't the problem, most likely there is a short circuit somewhere, perhaps a shorted output transistor in the amp.

Get a multimeter, and start by disconnecting the transformer from the rest of the power supply. Replace the fuse, and then make sure you have the proper AC voltage at the output of the transformer. Check the rectifier diodes - typically these fail open, but check anyway. Do a visual inspection of the power supply capacitors, look for any bulging or leaking electrolyte. If all of that looks good, then disconnect the power suppy output from the amplifier, hook the transformer back up, and then check the DC output of the power supply.

If you can switch on the power supply without the amplifier circuit connected and you get proper DC levels, then chances are one or more of the output transistors is blown. Post a pdf of the schematic here and we can have a look.

Be careful messing around inside the amp - AC line voltage at best can give you a very nasty shock, at worst it can be lethal. The bulk capacitors in the power supply can hold a charge for a very long time, and depending on the voltage can be very dangerous. If there are no bleeder resistors across the bulk capacitors, then it is a good idea to short across the caps with a screwdriver to make sure they are discharged before working inside the chassis.