SWR Silverado Cooks!!!

[themusic]

Working on a SWR Silverado. I'm working off SWR2000 power amp module schematic.
Q12 the sc3264 was fried. I replaced and immediately fried again.
Disconnected incoming J3 , checked getting -75vdc , also +75vdc at J2 .
Started checking component's ,board, loose connections , everything seemed tight and within tolerance.

Popped in another 3264 and BOOM fried again. I can't really meter anything with out voltage, and at $6.80
a pop it's getting kinda pricey ...Thanks again for your input

[Enzo]

First advice - stop throwing parts at it. One transistor might fail on its own, but as soon as you burn a second, then the transistor is not the problem. Until we find something additional to fix, don;t feed it more output transistors.

second advice - if you are putting a load on the amp, stop. No speaker or load until the amp is stable.

third advice - look up "light bulb limiter" and make one of these very simple things and USE IT.


With the dead transistor removed, what does the circuit do? The schematic shows paired outputs, so without Q12, Q11 would be able to function alone. No, not at full power, but all we care about at the moment is stop blowing fuses, we are not up to getting power out of the amp.

ANy time you have a shorted or blown transistor, always check the resistors associated with it. The 0.1 ohm ballasts of course, but also the 100 ohm resistors with the drivers. and so on. Open circuits are just as dangerous as shorts to the circuitry.

Does your 2000 schematic exactly match your amp? That one and the original Silverado are very similar, down to the number of Q12, for example, the 1999 schematic has 2.7 ohm base resistors for Q11,12, but the 2000 does not. other resistor numbers are different.

[Jazz P Bass]

Schematic link: http://music-electronics-forum.com/a...amp-module.pdf

[themusic]

Found my light bulb limiter ... pulled dead transistor...still shorted , disconected -75vdc still shows short with bulb limiter.
Checked resistors...ok , thanks

[The Dude]

Understand that when the bulb glows brightly it means something is drawing a lot of current. It DOES NOT necessarily mean something is shorted. It's easy to get going in the wrong direction if you don't understand this. An open resistor or anything causing an output imbalance can cause excessive current draw. You need to check and re-check all associated parts. It only takes one small part in a direct coupled amp to upset the apple cart. Be thorough. It's best not to fire up the amp with parts pulled. It also helps to follow the schematic. You know of a transistor that is shorted. WHY does it continue to short? What components are attached to it? What is the current path? Usually if you study the circuit, you can figure out where the excessive current went when the transistor shorted and that will lead you to other failed components.

[Jazz P Bass]

Just to add, I do not think 'pulling a rail' is going to tell you much.

In a solid state output section it's all about balance.

The top half pulls a slight bias current, as does the bottom half.

And when everything is proper, they balance out.

The key word here is 'slight'.

I would carefully go through & check for any low resistance connections on the output & driver transistors.

[themusic]

Thanks , your right I didn't mean direct short.. just got bright when first turned on then went to a low dim.
I'll get down to it tonight ...thanks

[Jazz P Bass]

Low Dim is good.

The initial brightness was the capacitors pulling current & 'charging up'.