First components to go

[Matteran]

Well, I think something in my pre-amp has died. I get noise coming out of the speaker when i turn the power amp up all the way, but nothing from the guitar. It's my 1978 Roland Cube-40, and I think either something got fried, or some old, cheap component simply died. How would I start to fix this?

[vanhansen]

Plug in to the "Main In" and see if you get anything from the speaker.  From the schematic, it looks like this amp has an effects loop with a pre out and main in.  The main in will go straight to the power amp section.  If you get sound, then the problem is in the preamp.  If no sound still, then something in the power amp is gone.

[Matteran]

okay, well the poweramp works fine, so it's obviously the pre-amp. I get nothing out of it. Any suggestions on where to start? Or would it just be easier to give it to the local amp tech?

[dosmun]

Start at the input with an audio probe.

[dave h.]

open it up and look for bad solder joints, signs of overheated components, small mammals, etc.

im no electrician, but the first thing i would check would be anything that moves or gets moved mechanically, ie jacks, switches, pots, and their respective hookup wires.

[niftydog]

then, measure the power supply rails to see if they're correct.

...and after all of that, suspect the electrolytic caps; particularly those associated with the power supply filtering on the pre-amp board.