Alesis RA-500 troubles

[dnosella]

Hello there,
I'm new to this forum, so please forgive me if I infringe some forum rule.

I'm writing cause I have a malfunction on an Alesis RA-500 amp, when I turn it on the proptection leds stay on, and so no output seems to happen.

It happens with both with and without load connected.

Any idea how can I proceed?

User manual says nothing helpful about it

I have some basic knowledge about electronic, but I have no idea on how I can detect the fault.

Thank you

 

[JonSnell Electronic]

The first thing you need is a service manul.
Service manual download starting shortly
Check the DC level at both L1 and L2. They should be zero volts.

 

[Keruskerfuerst]

Voltmeter or oscilloscope.
Start from input to output.
Download datasheets of transistors.

 

[dnosella]

The first thing you need is a service manul.
Service manual download starting shortly
Check the DC level at both L1 and L2. They should be zero volts.

Thank you

I'll start from there..

 

[dnosella]

Just for the records...
I found the problem, which are 2 defective cascaded transistor on the left output stage of the amp:

-TOSHIBA 2SC5200
-TOSHIBA 2SA1943

First I found the amp didn't enter in protection mode if I disconnected left channel output stage.
So I analyzed the circuit board and found the Emitter of a transistor was desoldered.
So I soldered it, but when I turned on the amp after 1 minute the fuse blewed out.

Then analyzing the circuit board of the left channel everything seems to be connected (shorted)

So I desoldered the transistors and checked one by one (just checked if BCE were shorted) and 1 of 3 pair were shorted.

So I ordered the transistor and new mica (which wasn't in good shape)...

Now I'm waiting for the spare parts, but it seems I found the problem (I hope)