Carvin Legacy Help Needed!

[pontiacpete]

Hi All,

I have a carvin legacy combo here that is having a problem with the output signal. With a guitar plugged into the input the amp doesn't make much sound, just a very broken distorted sound but when plugging an open ended jack into either the send or the return the amp sound like it should, I guess. Perhaps it's not as loud as it should be at 100 watts, it may only be getting 50 w. I have to check that.
I thought there was a bad solder joint issue at the base of Q5. So while i was in there I changed Q5 and re-soldered most of the board.
I didn't think the send or return jack would be cause but changed them anyway to rule them out.
Here's schematic, hope someone here can help me, thanks

[Jazz P Bass]

You have to narrow it down a bit more.
So it appears to be a preamp problem.
Did you Send to another amp?
How about some basic voltage checks of the preamp plates & cathodes.
(going in to the loop jack will produce 1/2 of the volume. The schematic indicates a required 240mv signal.)
Do both channels act up?

[pontiacpete]

Thanks Jazz P I think the connector cable is causing the problem. Perhaps I should hard wire it. Or try and drop some solder at the connector

[tedmich]

I'd suggest reflowing the board pins and carefully re-tensioning the socket with a tiny screwdriver, if its a typical molex

[Chuck H]

Yes, or replace it with a new one. Some of the ugliest and most layout damaging things I've seen have involved hard wiring previous ribbon cable assemblies.

[pontiacpete]

Is it best to just buy a new one from carvin ?

[Jazz P Bass]

Depends.
Can the customer wait for the new part?
Do you want the repair to look professional or hacked.
(remember, you never know who is going to be looking at your work)

[Chuck H]

(remember, you never know who is going to be looking at your work)

Oh crap... I gotta go.

[hasserl]

The fx loop on those amps use switching jacks that are well known to cause issues when not used for long periods of time. Often just inserting a plug into the send/return jacks will break up the corrosion and knock it from the contacts and "fix" the problem. Since your problem seems to go away when plugging into the return jack your problem may be with the send jack. On the Carvin forum I've coached many posters through this over the years, it's a common malady. The other common problem is the ribbon cable that connects the small pcb with the FX send/retun jacks to the main pcb, the problem being with poor connections made between the pins on the pcb and the plastic connector on the cable. You can try to clean those and that might work, but I have had many that just resisted being cleaned and were only corrected by replacing the cable. Carvin does stock those and will send them out to you quickly.

[pontiacpete]

funny that you are posting this now, because I just got the new ribbon connector in today and that hasn't solved the problem. Re-soldered the pre amp board. Had a talk with a carvin tech and he suggested looking into C66, C67 and C2 because they have an effect on both channels. I changed them out and no joy. Changed out the return and send jack also with no results.
This is a very frustrating amp being that there isn't that much to it and yet it has such a difficult problem to solve.
Thanks very much for the post. Do you have any other suggestions.
Voltages are looking good.
I'll post them if needed.

[hasserl]

Yeah, I would trace the signal through the path to see where you lose it. A signal generator and a scope is the best way to do this, but if you don't have them there are ways to do this without them.

[pontiacpete]

That's what I've been trying to do with little success. Signal looks good up to the relay then I lose it, but when plugging an non shorted plug in the send or return some volume comes back. I might be sending this back to carvin. Although the tech there didn't have an idea as to what the problem is.
The weird thing is I put a signal in the input and it comes out the output pretty loud and clear, but when playing a guitar through the input there is no sound unless the volume is all the way up and the guitar is hit hard.

[g1]

Are you saying you put a plug in either FX loop jacks and leave it there? This is how I am understanding it.
If so, either this is the wrong schematic or there is something strange going on with the FX loop jacks. It is shown as a series FX loop with B1 connecting FX send tip to FX return switch contact.
According to the diagram, inserting an open plug into either the FX send or FX return jack should completely kill the signal. Find out why it doesn't.

[pontiacpete]

when inserting and un shorted 1/4 plug into either the return or send brings the signal back to normal, i.e. at a lower volume than the full 100w.
It seems to be something weird with the FX loop. When plugging thru the loop, from return to send it works better also, but the clean channel not so.
I have the board out now, going through the connections.

[Chuck H]

The weird thing is I put a signal in the input and it comes out the output pretty loud and clear, but when playing a guitar through the input there is no sound unless the volume is all the way up and the guitar is hit hard.

Perhaps a problem with the guitar or the instrument cable you use with the guitar?

Are you using the same instrument for the guitar and the signal generator? Does the guitar work correctly plugged into the amps input when a dummy plug is inserted into the effects loop or are you saying that plugging a signal or the guitar into the loop it seems OK??? I'm a little unclear on some of your test descriptions.

Signal looks good up to the relay then I lose it

So you DO have good signal leaving the outputs of P6 and P9???

g-one is spot on. Inserting a jack into only the send or return (not a cable between them, but just a jack. And then not a signal of any kind, just an open ended jack.) should kill the signal completely. Which is one reason I'm in quesion about your test method descriptions. What exactly are you doing and what exactly are the results?

When the guy at Carvin says "check X, Y and Z" that means you need to check the signal through those parts including any signal path to and from those parts. If you just arbitrarily replace them that doesn't rule out all the possibilities for problems associated with the circuits those parts exist in. Like broken board traces or lifted pads on the PCB. If fact, just arbitrarily soldering joints, and now removing the board, is actually counter productive. You could well end up with other problems like poor connections that will complicate finding the original problem/s by appearing as a new symptom. The initial problem/s would be most easily diagnosed in a working circuit. The circuit isn't working with the board removed. This problem requires diagnosis before it can be solved.