Mackie SR1530 no output

[tigertenfifty]

Hi I have a Mackie SR 1530 in for repair, it powers up ok and I have a signal trace through to output, but extremely low, it appears it is something to do with the mute/protection circuit, can anyone help please.

[J M Fahey]

Inject, say, 100mV 1KHz at some channel Line in and trace it along the path, to see *where* you lose it.
Otherwise possibilities are infinite.

[Jazz P Bass]

Schematic link: MACKIE SR1530 ACTIVE SPEAKER Service Manual free download, schematics, eeprom, repair info for electronics
It is a .rar file type.
You may convert it online here: http://b1.org/online

[tigertenfifty]

I have injected a signal from my signal generator via a mixing desk, signal at input is 160mV, it increases through the 'crossover' daughter board, ( depending on frequency ) and I get a reading of approximately 3V peak to peak at the speaker output, (there are 3 different speaker outputs depending on frequency). The amp is configured as 100 watt output Hi & Mid and 300 watts Bass. There is a protection circuit that has an internal mute switch which activates from the thermal cut out, and I believe this is where the problem most likely is.

[Enzo]

It may be the problem, but why do you think that?

[tigertenfifty]

There are 3 seperate circuits, Hi Mid & Bass, the input is seperated on the daughter board and the op amps here are amplifying the signal at this point, by the time the signal gets to the input of the output transistors it is very low (500mV peak to peak)across all 3 circuits and the output from the same transistors is very low, (3V peak to peak) I can not see anything in the circuit that would affect all 3 output circuits other than the mute system

[Jazz P Bass]

Maybe U1 is bad.
What signal level are you injecting into the input?
What is coming out of U1?
The SM states that a 250mv signal should start the limiter on the low freq circuit.
If you truly suspect the opto limiter, you could try disabling it by jumpering across the LED portion of it.
Or remove it entirely.(for testing purposes)

[tigertenfifty]

I'm injecting a 160mv signal, the red limiter protection led is not coming on.

[tigertenfifty]

With that input signal level, what would you expect to see at U1 output?