Laney rbg500 blow out

[bandit200]

I have a Laney RBG500 and on power up I get a low crackly speaker sound followed by the fuse (fs1) blowing and a few seconds later C2 (10nf 100v) popping. On checking the power transistors (MJ15003 + MJ15004) finding that they have both gone short to ground but the middle two checked ok. After replacing all of the above (thinking the fault lay at the transistors) powering up blew them all again.So replacing all components "again" but being too affraid to try power up (knowing the outcome allready) it sits in my kitchen waiting for someone a bit more technicaly minded. So please if anyone can cast some light on my situation I would be eternally grateful.
Ian

[amptech]

laney blow out!

Hi there,

There's no point powering up back up again and wasting new componets!

do some cold checking to make sure you've not missed any leaky or shorted
componets and use your meter to check for high leakage between the output and ground point. remove the fuse's that feed the dc power to the main pcb and fit 100 ohm 10w resistors to limit to current and also use a r c d breaker to protect you and the amp under test.
make notes of what you have done in case you get called away....

[MarkusBass]

Ian,

Remove the FS1 and FS2 fuses, power up the amp and measure voltages on C14 and C15. They should be +/-50V.
Switch the amp off, make sure there is no voltage on C14 and C15, and check all transistors on the power board, especially drivers: TS5 and TS18, and bias servo TS12.
Make sure that newly installed output transistors are properly insulated.
I assume that at least one of these steps would show some problems.
If not, I would remove the ouptut transistors (in order not to loose any more components), install 100 Ohms/10W resistors instead of FS1 and FS2, power the amp and see whether it works. Of course without any load. This may be a problem if you don't have signal generator and osciloscope but you can check the voltage on TS5 emiter. DC voltage should be close to 0, and AC should be slightly > 0 when you touch the input of the amp with finger.
But I would guess that one of transistors would show up as faulty.
If you find it, assemble output transistors (making sure that there is no short circuit), and power up the amp - still with 100 Ohms resistors. Check whether there is 0V DC on the output. Check the voltage on emiter resistors. Should be just few milivolts (5). Let us know what is the outcome.

Marek

[bandit200]

Hi,
Thanks for the info it is much appreciated, i`ll let you know the outcome asap.

Thanks again Ian