Crate BX50 noise

[nosaj]

Is the optocoupler supposed to prevent noise like what I’m getting? I shorted the resistor side with alligator clips and still got the sound.
Prolly worth noting that it doesn’t happen as soon as the amp turns on, but anout three seconds afterward

Did you ever mount the PCB back in the chassis with the nuts on the pots and jacks?
From Juan
optocoupler Led is connected to speaker out and both reference voltages through a diode network, so when speaker out peak is "near one rail or the other" LED turns on and LDR limits drive voltage.

nosaj

[Enzo]

The opto is there to prevent the power amp from clipping...period.

[sunshine_bassman]

I screwed everything back together and put it back in the cab, still makes the same sound. Could the opto possibly be bad? Maybe some of the resistors are open?

[nosaj]

I screwed everything back together and put it back in the cab, still makes the same sound. Could the opto possibly be bad? Maybe some of the resistors are open?

Use that Multimeter to check the resistors there really aren't that many on the board. But it is unlikely a resistor opened just by you fixing the jack.
Just a what if, What if you removed the jack you repaired would the noise still be there? Completely desolder it remove it turn it on and see.
nosaj

[Enzo]

Why are we so focused on the opto? For tests, we could just remove the opto, amp still noisy?

[J M Fahey]

I still have no clue what

loud swooping distortion that sounds only right when you turn it on

means and no,

It’s half like a buzzer on a game show, half like a fuzzed-out guitar bending a whole step. It lasts about half a second.

does not explain it at all.
A YT video or it does not exist

[DrGonz78]

Yeah sorry to get anyone thinking it might have been the opto, not my intention. I was more curious about the limiter function. My advice still stands to measure any DC offset voltage at the output while it makes the noise. I would also check for any related DC voltage at pin 2 just to know. My idea involves the amp waking up stupidly notion and instead of a turn on pop there is a different fizzy sound going on. Maybe I am way off but the simple video Juan suggested is a great idea.

[nosaj]

Yeah sorry to get anyone thinking it might have been the opto, not my intention. I was more curious about the limiter function. My advice still stands to measure any DC offset voltage at the output while it makes the noise. I would also check for any related DC voltage at pin 2 just to know. My idea involves the amp waking up stupidly notion and instead of a turn on pop there is a different fizzy sound going on. Maybe I am way off but the simple video Juan suggested is a great idea.

The few minutes I was there the dc was around .005 even with the noise .

nosaj