no sound out of Crate G120C amp

[merk]

I'm trying to fix a friend's crate g120c solid state amp. The amp worked fine then quit suddenly with no intermitent problems. It has 2 channels. When the A channel is switched on, there is no sound at all with a guitar plugged in. With the B channel on you can barely hear the guitar. This is with the vol and gain all the way up. If you plug another amp into the Line In on the Crate the sound is normal and can be adjusted by the other amp while the Crate pots don't work.
There aren't any burn marks or signs of component meltdown on the 2 boards. (One is labeled power amp and the other preamp.) Apparently no cracks or bad solder joints. All the jacks and pots look good with the meter. The transformer is putting out +21 v and -21 v on the secondary leads to the boards (I think that was the voltage if I'm remembering right). The speakers are both connected and show continuity. The schematic I've located is blurry and not much use so I may have buy one.
I'm hoping some components need replacing but don't know where to go from here. I've built some kits and am involved in ham radio but haven't really done any amplifier troubleshooting. My bandmate is going to junk the amp so I said I'd mess with it. Thanks for any advice.   

[phatt]

Link preamp output and power amp input with a guitar cord.
Does that help?
Phil.

[merk]

The amp has a line out/line in for each channel so I guess it's a stereo amp. I tried looping a cord between the line in and the line out with no change in volume. (I did it for each pair of jacks separately.) I assume the line in jack goes to the power amp and the line out jack comes from the preamp? I also tried a cord between the 2 fx in and fx out jacks and got some healthy feedback. I posted wrong before-each channel has a super low volume output with the guitar at full volume.
I'm thinking the power amp is fine and the preamp has a break in the signal somewhere?? Also the 4 vol and gain pots must be OK since the 2 channels act the same? Thanks.

[dndemattia]

I did a quick Google search for this amp and it looks like it has an input for a CD player. Have you tried connecting something to those inputs to verify that that the amplifier section works?

[joecool85]

Apparently no cracks or bad solder joints.

How did you come to that conclusion?  Frequently they all look good, sometimes even under a magnifying glass, but when you reflow the joints they work.  I've seen this on a lot of modern amps, particularly from Fender, but I imagine Crate uses similar wave soldered joints.

Apparently no cracks or bad solder joints.My bandmate is going to junk the amp so I said I'd mess with it.

I'll take it off his hands, I'll even pay shipping (seriously if you want it gone I will).

[merk]

Don't know about the CD input. The power amp portion works cause plugging into the Line In works-just no sound using the regular guitar jacks. I thought the solder joints were apparently good based on a visual under a magnifying glass but I guess I'll reflow the joints starting at the input jacks and see if that helps. I did see where somebody replaced a capacitor and lifted part of the copper trace off the board. I'll give that some attention. I wanted a project amp and now that I've got one I'm going to keep it and get it going. I'm also working on the reverb on my '69 SF Twin Reverb and the Crate looks a lot more involved. Once I get a schematic I'll start testing voltages. And I'll keep on hitting the books to try to learn something...