Take a look at this "current in-rush limiter"

[BrianS]

I got in a Crest VS-900 power amp the other day and the customer must have opened it up because he said that "MOV1 looks like it needs to be replaced".  Sure enough, this part...I believe it is a thermistor...looked like it had been chipped away at by little amp gnome pick axes!

After looking the schematic over, I decided to clip the part out(which upon clipping, the unit crumbled apart) and run a jumper wire across the connections...just to see if that would get me power to the rest of the circuit.  It did.  So I ordered a new part and am waiting to do any further measuring/testing until I get it in...after vacation.

My customer wanted to know if this damage simply happens from constant use, or if there was some catastrophic event that would cause it.  I don't know, so I'm asking you more experienced fellows what you think.

FYI, I have this thing hooked up to my current limiter, and it's not registering any dead shorts that I can tell, i.e, the bulb doesn't burn brightly with the unit on.

[Enzo]

Unless you find some problem later, often as not, we never know just what causes most parts to fail.   No, it is not likely from constant use.

Mains spikes or surges MIGHT be involved.  Mechanical damage from transport or shipping MIGHT be involved.  A failed solder joint that went resistive MIGHT be involved.  The industry recognizes that there are random component failures now and then.   Something that happened 6 months ago may have stressed the part and it is only just now failing.  It usually is not something anyone "did".

[Loudthud]

Toroidal transformers have large inrush currents. There may have been an engineering change that upgraded the inrush limiter. I would contact the factory for advise on a replacement since this is a safety critical component.

[lavejim1]

The part is actually called a metal oxide varistor, hence MOV, and is designed as a kind of voltage dependent resistor.  It functions as a safety device to protect sensitive components from excessive current flow, such as shorts, spikes, etc. 

[J M Fahey]

The part is actually called a metal oxide varistor, hence MOV, and is designed as a kind of voltage dependent resistor.  It functions as a safety device to protect sensitive components from excessive current flow, such as shorts, spikes, etc.

Well, maybe ... maybe not.
All the OP shows is a crumbling disk shaped part , with no direct mention of how it was connected.
If a MOV, it normally goes in parallel with the transformer winding ; if an NTC inrush current limiter, it goes in series.

OP mentions shorting it to restore power, so it looks like it´s the second option, an NTC current limiter and not a MOV.

Even if it were a MOV, it would not

protect sensitive components from excessive current flow

but from high voltage spikes.
Anyway this does not seem to be the case.

[BrianS]

Just thought I'd post a couple notes after having replaced this part:

-I ordered the part using the part number on the schematic, and what I received from Mouser was physically much larger than the original component...probably twice the size or more when considering overall diameter and thickness.  This made me kind of wonder if they changed the specs on this part after having some failures in the field?

-Installed the new part and the amp works fine.

-Judging by the schematic, and Mr. Fahey's explanation in the previous post, I'd say this is an NTC inrush current limiter.

Also, just fwiw, this amp has a "standard" power transformer, not the toroidal type.

Toroidal transformers have large inrush currents.

-I should also mention that I could not completely unsolder the old leads that were left in the mounting holes.  After removing as much of the solder as I could with solder-wick, I had to drill out the holes so I could mount the new component.  The solder pads were intact after drilling, and the new part went in fine, but I couldn't believe my 60 watt iron couldn't completely clear the holes...I've never had that happen to me before.

Thanks so much to everyone here for sharing their knowledge!!

[Enzo]

Pull the old wire leads from the hole.  If the hole will not clear of solder, then FILL the hole with nice fresh solder, then suck THAT out.  That usually works.


If I were in a hurry, I'd poke through old solder with a piece of piano wire before I;d get out a drill.

And I may be in the minority, but I hate solder wick.  I have much better success with a spring loaded suction tool.   And to be honest, I usually use a desoldering station instead of hand tools anyway.

[BrianS]

And I may be in the minority, but I hate solder wick.  I have much better success with a spring loaded suction tool.   And to be honest, I usually use a desoldering station instead of hand tools anyway.

I do use the spring loaded suction tool the most, and chemwick to clean up.  Wish I had enough work to justify the purchase of a de-soldering station!

Thanks for the tips!

[Enzo]

Whatever you use, my suggestion to prime the pump, so to speak, with fresh solder still stands.   I like to think of it like a sponge.  A damp sponge picks up water better than a bone dry one.

[Roly]

It's about re-fluxing the joint.  Desoldering generally requires a bit of fresh solder+flux to make it fluid again.  Solder wick has its place but normally a "snort" is a better proposition.

NTC's in this duty do tend to age all by themselves, then thermally run away to carbon, as above.

If the specified replacement is twice the size then there is a good chance they have been having a generic problem with these and uprated it.