KRK 10S Subwoofer keeps blowing fuses

[Diver300]

Your profile says USA where the mains voltage is 120 V

You said you are using 1.6 A fuses

The circuit diagram shows that for 120 V, you should be using a 3.15 A fuse, so I would expect a 1.6 A fuse to blow.

Have I missed something? (Apart from being 2 month late to notice this!)

 

[fingerbib2000]

Your profile says USA where the mains voltage is 120 V

You said you are using 1.6 A fuses

The circuit diagram shows that for 120 V, you should be using a 3.15 A fuse, so I would expect a 1.6 A fuse to blow.

Have I missed something? (Apart from being 2 month late to notice this!)

Hey,
Hmm, I'm in the UK so it's ~230v. Would 1.6 be sufficient that?

 

[Nigel Goodwin]

Hey,
Hmm, I'm in the UK so it's ~230v. Would 1.6 be sufficient that?

Actually it's 240V here - the 230V rating is an average for EQUIPMENT only, so it works in both the UK (240V) and the rest of Europe (220V). But 220, 230, and 240 are essentially the same for most purposes anyway, so no issue with the fuse rating.

The imaginary 'change' was done to prevent problems - previously 220V equipment used in the UK was VERY prone to blowing mains transformers, as they ran at 10% overload permanently. So they invented the imaginary 230V mains - with the proviso equipment has to be designed to operate to above 240V (for the UK) and below 220V (for the rest of Europe).

There has been some suggestion that some small parts of Europe 'may' have changed their mains to 230V, but it's pretty pointless and the UK infrastructure hasn't been changed so remains at 240V.

 

[Diver300]

Hey,
Hmm, I'm in the UK so it's ~230v. Would 1.6 be sufficient that?

Well you've answered my question. I had assumed that you were running it on 120 V because that would have matched the mains voltage for your profile location.

The first thing that I would do is to disconnect J7 and see if it will turn on without blowing a fuse.

If it blows the fuse with J7 disconnected, then either the transformer is faulty, or there is too much inrush for the fuse. I suspect that the 1.6 A fuses might have too low a rating, as the inrush for toroidal transformers can be very large.

Do you know the VA rating of the transformer?

 

[Diver300]

I've just noticed that you said in an earlier post that the fuse doesn't blow with J6 disconnected.

My next move would be to put some resistance in series with the +/- 42 V power lines on J6 (pins 1 and 3) and see what happens.

You will need some high power resistors, maybe some 100 W, 10 Ohm resistors. At low volumes, those should have no effect, but they will reduce the current if the amplifier does effectively short out, and give you time to fault find.

If the amplifier is completely shorted, the resistors will overheat but resistors like that will survive for half a minute or so at 2x overload, giving you time to turn off.

200 W ones are a lot more money.

 

[unclejed613]

what i meant was to make those measurements with the output devices connected.... but now that you have it apart..... measure from the center lead of each transistor on the heat sink, to the heat sink... i'm thinking you have a hole in one of the thermal pads (usually a piece of clear mica between the transistor and the heat sink, sometimes a silicone rubber pad)

 

[arubli]

Hello, same model, different issue, I haven´t been able to locate a schematic, and I wonder if the preamp has a compresor or limiter. The failure this unit presents is not on the output stage. when I turn up the volume the signal appears, and then it drops, not to zero, but very faint, if I turn up the volume again, I have signal, but it drops again. any hint will be appreciated, thanx
Alexander

 

[unclejed613]

theres a mute circuit on page 4 with a jfet (Q5) a problem with this circuit, or anything feeding it could be the cause of it fading out.

 

[arubli]

theres a mute circuit on page 4 with a jfet (Q5) a problem with this circuit, or anything feeding it could be the cause of it fading out.

Thank you for your post, any suggestion where I might get the service manual ?
regards

Alex

 

[fingerbib2000]

Thank you for your post, any suggestion where I might get the service manual ?
regards

Alex

 

[arubli]

Thanks !!!!! will take a look, my guess is that the dark glue gets somehow conductive over time...
will post results

regards

Alex

 

[fingerbib2000]

I suspect that the 1.6 A fuses might have too low a rating, as the inrush for toroidal transformers can be very large.

Do you know the VA rating of the transformer?

I didn't check that before I closed it up. Your suspicion appears to be correct though, I fitted a 3A fuse an it working fine now!

The service manual does state to use 1.6A , as below.

I wonder why it suddenly started blowing 1.6A fuses when it was working OK with them for a good while?

In any case, I have sub bass again, hurrah!

Thanks for all your help.

 

[Diver300]

I have seen high surge currents degrade a fuse, so that it eventually fails.

The replacements could be acting too fast for that application, because there is a large inrush current on toroidal transformers. I can't find a full specification for the ones that you are using.