Need help with my LANEY

[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

[lowell]

Did you check the surrounding resistors and caps near the output transistors? can you post schematic?

[bandit200]

Hi,
Yes, I think ts3 and ts4 (s0692`s)have gone down too as well as r15 220ohm.
I think I`ve attached the drawings , i really hope you can help.
Many thanks Ian

[lowell]

make sure you test all the resistors around there. But it really seems like you have a power supply problem. I'm far from an expert just so you know. But try measuring resistance between both supplies and ground. See the connector clip on far left of the schematic? Use those as your test points. Should be very large or OL on your meter. Let us know if either supply is shorted to ground and then check the resistance from +supply to -supply on the same connector. Might also do voltage drop test on D3 and D9.

[Twist]

Check all the diodes and other transistors across the whole power amp. Take care checking components in circuit. They may show a short or test good due to other components elsewhere in the circuit. You may have to remove them from the PC for an accurate reading. Also check all resistors with a value of 2.7K or less. Check your emitter resistors on the outputs. C2 shorting would likely not be caused by another component but by internal failure of the cap itself or over voltage.

[bandit200]

Hi,
Thanks for your advice ,I will get on to it asap. I need to swop out the two transistors first cos i know that they have gone short. I will post my findings as i find them so please keep the good advice comming as i need all the help i can get.

Cheers Ian

[lowell]

Hi Ian,
do you have a current limiter - a light bulb in series w/ the amp? You can make one of these very easily w/ a 2 socket junction box. One of these will save you many fuses. When fuses are popping, after I perform a repair to fix the problem I use the limiter on first power-up. If the light bulb is quite or very bright then I did not fix it and the fuse would be blowing. So I troubleshoot and fix, then use the limiter until the bulb is glowing slighty, THAT'S when you know the amp is not drawing mega current anymore.

[bandit200]

hi,
I replaced the two power tranistors and a couple of surface mount transistors on the back of the main board. This seemed to do the trick after setting up the bias. But the sound seems to have suffered badly. There is a short delay then the sound comes in very crackely and there is a very short sustain. The harder I strike the string, the shorter the initial delay. Is this the setup or have I damaged the driver ? There is sound , just very quiet for a second.
There is vertually no sound if i strike softly.
Cheers Ian

[bandit200]

ooo

[lowell]

the closest situation where i had similar symptoms, I found that the vbe multiplier thermal-sensing transistor (was attached to heatsink) was bad. check the bias circuit for problems with this component and surrounding components.

[bandit200]

Thanks Lowell, i`ll take a look.
Cheers Ian