Fuse rating question ?

baldy

I have a Laney RB9, 450 watt bass head.
It was lent out & when returned I was told that it had been plugged in, switched on & didn"t work.
I have taken the chassis out & both fuses are blown. 
On the chassis back panel it says fuse rating & supply voltage
T6.3A L - 115v
T3.15A L - 230v

Both fuses in the amp, which are both blown, are stamped T6.3A, 250v.
Should I put the same fuses back in or should it have the 15A fuses as our supply is 240v (230v) ?

HHwarner

For 220-240v you need 3.15A L

ICBM

3.15A not 15A!

15A will probably cause a substantial amount of smoke to come out of the amp...

baldy

Thanks both & forgive my stupidity for reading the fuse ratings incorrectly.

ecc83

I have struggled to find an info on the head but it seems its mains power pull is 450W which would give a current pull close to 2A. Now, that might seem well below 3.15A but it isn't really when you consider "Inrush Current" That can be two or three times the standing or working current. The result can be random, nuisance fuse blowing, fusing is more art than science!

The best thing is to keep a pack of T 3.15s handy (and get them from a reputable dealer, RS or CPC) and change it as and when. I would never advise upping  fuse rating UNLESS the mnfctr agrees. What have you got in the mains plug? If 13A you could swap that for 5A as that might pop first and be an easier swap.

Dave.

baldy

Thanks. 
I read the ratings as 
T6  3A L - 115v
T3  15A L - 230v
And not 6.3A & 3.15A, doh.
I did wonder what point a 15A fuse in the unit was when the most the plug could be was 13A lol.

ICBM

If both 6.3A fuses are blown it doesn't bode well though... that sounds like someone has already been messing about with it. For it to blow a 6.3A fuse at 230V (not 115V) means there's something fairly serious wrong inside. The only other possible explanation for it if not is a shorted speaker cable or cab.

baldy

The head was working fine & was lent out to a band that took it to tour Germany (they used their own cab).
All I know is I was told it was plugged in, switched on & didn"t work.
I will have to see what happens when I replace the 2 blown fuses.

baldy

Oh & it was a squat in Germany, so god knows what the power supply was like.

baldy

Well changed both fuses for new 3.15A & it is dead.
Are Laney RB9"s any good, would it be worth spending any money on fixing it (I can"t find them on the Laney site so it must be old & discontinued) ?
If it is worth fixing I have a DMM any suggestions of what to check & what sort of readings I should be looking for ?

ICBM

baldy said:

Well changed both fuses for new 3.15A & it is dead.
Are Laney RB9"s any good, would it be worth spending any money on fixing it (I can"t find them on the Laney site so it must be old & discontinued) ?
If it is worth fixing I have a DMM any suggestions of what to check & what sort of readings I should be looking for ?

It's probably worth fixing, even though it's not worth a lot.

Open it up and disconnect the power transformer completely from the main circuit board, making a note of where the wires go if they're individual connectors (I can't remember). Replace the fuse and power up again - if it now blows, the transformer is fried. A replacement will cost a fair bit, but is easy to fit so the labour charge should be low.

If it doesn't blow, it's very likely one or more power transistors have shorted. An outside possibility is a rectifier diode or two, but this is less common. If it's a power transistor it's also possible that it's done other damage to surrounding components, although often not. Finding which one(s) is fairly easy and still not a huge job for a tech familiar with these.

If it does prove to be the transformer, PM me - I have a couple of spares of about that size, although I can't remember if either of them are exactly that spec, I'll need to check.

baldy

"Replace the fuse and power up again - if it now blows, the transformer is fried"
It didn"t blow the fuses when I switched it on, just dead ?

ICBM

baldy said:

"Replace the fuse and power up again - if it now blows, the transformer is fried"
It didn"t blow the fuses when I switched it on, just dead ?

Ah - you mean without disconnecting the transformer but with new fuses, still dead?

There are some internal fuses on the main PCB I think, have a look at those. If they're blown then it's not the transformer since the fuses are downstream of it.

baldy

Yes, fitted new fuses, plugged in switched on & nothing but the new fuses are okay, they didn"t blow.
I"ll open it up again tomorrow & see if I can locate any more fuses although I din"t notice any yesterday when I had it apart.
Also none of the transistors or diodes looked burned.

ICBM

Transistors don't look burned when they're shorted, diodes sometimes do although not always.

If it doesn't have fuses on the board there's a possibility that there's an internal thermal fuse in the transformer which has gone. To check that, meter from a ground point to the wires that go from the transformer to the PCB (AC voltage).

baldy

Great, thanks for all the advice ICBM.
I shall check what I can tomorrow following your suggestions.