Power Amp Shorting Out

[devious]

I got my hands on a used two channel power amp (samson servo). When I first got it, the fuse was blown, I replaced it, and the unit defaulted into "protect" mode when turned on. Further inspection showed that it would work fine when I unplugged Channel 1 from the power supply, running it with only channel 2 activated inside the unit (see picture).
Measuring the resistance between +VCC and -VCC on Channel 1 showed about .6 ohms, whereas the resistance on channel 2 (working) between those two points was a few hundred ohms, at least.
Obviously there is a nasty short somewhere in channel 1, but I'm in way over my head as it is. I'm lucky I haven't electrocuted myself, considering how many times I've plugged channel 1 into power to see it go *POP* and blow the surge protector's breaker.

Any ideas? I don't know much about power amps so please be gentle.

[lineup]

can you see something looking like burnt
either by inpection from topside (component side)
or from bottom side of the amp board?

could be a transistor or a resistor that is destroyed
a burnt resistor is easy to locate
a transistor may not show it is destroyed, from looking at it
the damage is inside it

[johndiy]

youve checked you have a short
next
check some components ie semiconductors

john

[devious]

There's no visible damage. How can I test these transistors? Will I have to remove them from the circuit before I test? I don't trust my soldering skills too much.

[johndiy]

of course youll have to remove them,then use the multimeter's
semiconductor/diode function to test start with the o/p transistors

john

[jaycee]

Chances are, the 4 big transistors on the top of the heatsink will need replacing. You may also have to replace the driver transistors (which are the two transistors in the middle of the board that are placed underneath).

If you are not familiar with power amplifier circuits then to be honest, you should stop now and get someone who knows what they're doing to repair it. The damage is probably trivial to fix at the moment, but you could easily make it a lot worse.

[devious]

I bought four (2x TIP35C, 2x TIP36C) power transistors for $10 shipped to replace the ones currently in there. If that doesn't fix it, I guess I have a 150W mono amp on my hands.

I hope the transistors arrive quickly.

[Enzo]

TIP35? Is that what was in there? WHich Servo model do you have? These amps should have Asian transistor numbers in them as stock. What power transistors are in the good channel?

[lineup]

Originally posted by devious
I bought four (2x TIP35C, 2x TIP36C) power transistors for $10 shipped to replace the ones currently in there. If that doesn't fix it, I guess I have a 150W mono amp on my hands.

I hope the transistors arrive quickly.

Say it is some transistor that is destroyed.
Now transistors do not often destroy without a cause.
And this cause can have been temporary, like some water, or something else causing a shortcut in amp circuitry.
Or maybe some lightning struck into the mains of a house.

But can also be some other component, that is defect.
By age or by some damage or by being 'shortlived' exemplar from the factory.
Then risk is same transistor will break again when turn on power.
It is very common that the big transistors at heatsink
will be the ones that take the beating,
when something else is the real fault.

But as you have not yet possibilty to test your amplifier yourself,
you may be lucky and your amp will work alright after change output devices.


But best advice so far in this topic
was that you should try to find someone with knowledge in electronics
to help fix this for you.

good luck

[devious]

Originally posted by Enzo
TIP35? Is that what was in there? WHich Servo model do you have? These amps should have Asian transistor numbers in them as stock. What power transistors are in the good channel?

Servo 300, both channels had TIP35C and TIP36C in them.