Marshall MG50DFX No sound from main input

[beanoir]

Right here goes, i'm posting a new thread rather than hi-jacking the previous chaps thread with a similar but not the same problem.

The problem I have is that no sound is coming from the speaker on either channel when I plug my guitar into the main input, unless I turn it up to max on the volume at which point I can just about hear some sound leakage from the speaker.

If I plug my heaphones in, it sounds fine.

The speaker works, as i've connected that directly to another source. And the CD-in works too.

I've had the head out and sprayed contact cleaner on everything, and still no improvement.

From here, I'm now stuck as to what I can do. I'm a complete novice when it comes to amplifiers and electrical equipment in general if i'm honest, but I can normally put my mind to such things if I know where to start. I wont be beaten by this!!!

Appreciate any help that is offered.

Cheers

Nick

[Phostenix]

Schematic is here:
http://www.ampix.org/albums/userpics...ll_MG50dfx.pdf

Do you have any test tools? Do you know how to solder?

[Phostenix]

If it's OK at the headphones out, it looks to me like the signal is good all the way to the power amp chip input. So, my guess is that you need to replace the TD7293, but I'd wait for confirmation from one of the more experienced techs here.

[Tom Phillips]

Hi Nick,
Try connecting the FX Send on the rear panel to the FX to the FX Return Left (Mono) with a guitar patch cord. Does that bring it to life when you play a guitar into the front panel input?
Tom

[beanoir]

Originally Posted by Phostenix

Schematic is here:
http://www.ampix.org/albums/userpics...ll_MG50dfx.pdf

Do you have any test tools? Do you know how to solder?

Thanks for the schematic, i'll have a study of that. I dont have any test tools, but I can solder...

[beanoir]

Originally Posted by Tom Phillips

Hi Nick,
Try connecting the FX Send on the rear panel to the FX to the FX Return Left (Mono) with a guitar patch cord. Does that bring it to life when you play a guitar into the front panel input?
Tom

I just gave it a go, and bridged the FX Send to the FX Return and if I turn up the volume to the max I can hear just about hear it through the amp's speaker then. What does this tell me?

[Tom Phillips]

That rules out a problem with the switching contacts in the FX jack.
That is a common problem. The test is easy so it was worth a try. I also thought there was a good chance this would work because you already found that the CD In works. That tells me that you power amp is working.

[beanoir]

Originally Posted by Tom Phillips

That rules out a problem with the switching contacts in the FX jack.
That is a common problem. The test is easy so it was worth a try. I also thought there was a good chance this would work because you already found that the CD In works. That tells me that you power amp is working.

But the sound out of the speaker when I tried that was awful, I assume thats to be expected then?

So if the main amp is working, but it still works out of the headhone jack, then that must narrow it down considerably i'm guessing?

[Tom Phillips]

Originally Posted by beanoir

But the sound out of the speaker when I tried that was awful, I assume thats to be expected then?

So if the main amp is working, but it still works out of the headhone jack, then that must narrow it down considerably i'm guessing?

Yes, that does narrow it down.
Maybe the CD input was being overdriven. Did you try lowering the CD input signal?

[beanoir]

Originally Posted by Tom Phillips

Yes, that does narrow it down.
Maybe the CD input was being overdriven. Did you try lowering the CD input signal?

No sorry, I meant when it sounded awful (or perhaps more accurately I should have said faint sounding) I had the guitar plugged in to the main input, and bridged the FX jacks at the rear. I haven't tried the CD input today, but it worked the other day.

So if I refer now to the schematic, where should I be looking as a first starter do you recommend?

[Phostenix]

Let's clarify the CD input. If you plug a CD player in, does it have normal volume to the speaker or is it faint & distorted too?

[beanoir]

Originally Posted by Phostenix

Let's clarify the CD input. If you plug a CD player in, does it have normal volume to the speaker or is it faint & distorted too?

The CD input also sounds very faint in the same way as the main input, I think I may have confused with my first post post actually, apologies.

[beanoir]

Just a quick note to thank you chaps (Phostenix and Tom) for your help, I appreciate I popped on here and launched straight into "please help me".

Anyway, I've decided that in this instance, the cost of the parts required to do the proper testing, new solder iron and parts will be more expensive than taking it in for repair, so I have thrown in the glove. I hate to have to admit it too!

Thanks again!

Nick

[Phostenix]

Sorry it wasn't an easy fix.

[Tom Phillips]

Originally Posted by beanoir

... I've decided that in this instance, the cost of the parts required to do the proper testing, new solder iron and parts will be more expensive than taking it in for repair, so I have thrown in the glove. I hate to have to admit it too!

Thanks again!

Nick

Your welcome. Sometimes it's best to let someone else do the repair. At least you accomplished some troubleshooting. It's hard to go further without some basic test equipment.
Cheers,
Tom