No sound from Marshall MG50dfx

[upl8tr]

Hi all,
Noob here, hoping for some (a lot of) help with a Marshall amp.
Long story, friends amp used for gigs = no problem.
Suddenly it stops working, lights come on and fan operates but no sound whatsoever.
Friend approaches me to ask if i can fix it? WTF?
After listening to symptoms i suggest it sounds like simple problem and advise him to open the case up,
look for blown fuse/s loose speaker connections and/or anything obviously dislodged or broken.
A few months later i meet him again and he says do you want that amp before i throw it in the trash?, Hell Yeah?
Sooo, after following my own advice and checking the amp i found nothing obvious.
In fact, the inside of the amp is very clean and tidy.
I looked at each resistor and capacitor closely for bulges, leaks, burns, loose connections etc, but they all appear sound.
I smelled the power input section and closely smelled the coil for burning smell, nothing so that’s also all good, as far as I can tell.
I checked the speaker with my CD line out and it is working, np.
There is no sound out through the head phones from the Marshall.
I’d welcome any suggestions where to begin fault finding and hopefully get this amp working again, please.

[52 Bill]

I checked the speaker with my CD line out and it is working, np.

Welcome to the place.

When you used your CD to check the speaker, did you connect it to the CD input on the amp or directly to the speaker terminals?

Also did you try and plug in a patch cord from the FX loop out to the FX loop return jack?

[upl8tr]

Hi there, thanks for the reply.

When I tested the speaker I used a patch cord (male to female) from the head phone socket on my CD player straight onto the jack plug from the Marshall's Speaker, no amp involvement at all.

Since your reply, I connected the FX loop out to the FX loop return jack, and get some sound from the amp; it is patchy fading in and out and cuts out if the master vol is raised to about 3 or 4.

[upl8tr]

Hello again,
I disconnected the FX loop out to the FX loop return jack, and plugged the guitar in to see if by any chance I still got some sound from the amp, I didn’t it’s still the same, but I noticed this time that the two internal red and green diodes attached to the channel selector switch light up when the guitar is strummed, don’t know if that means anything but thought it worth mentioning.

[52 Bill]

Try plugging in your guitar into the FX return jack. Can you hear the guitar through the speaker?

How about if you send the output from your CD player into the CD input jack on the amp, any sound?

As a warning, for these tests make sure that the speaker is plugged in before you turn on the amp, and if you should need to unplug the speaker, make sure the amp is off and has been allowed to sit for a few seconds before doing so.

[upl8tr]

Yes to both, the guitar through the xf sounds bad, but the CD is crystal clear.

[52 Bill]

Yes to both, the guitar through the xf sounds bad,

By bad do you mean weak and thin, but undistorted?
Or distorted?

This test limits your problem to the pre-amp section, as the CD inputs go pretty much directly to the power amp chip.

[upl8tr]

Yes, weak and thin, but undistorted.

Also, like before, master volume set at 4 but sounds just louder than normal TV listening volume.

Massive hiss, fades, disappears totally at 5 and above.

[52 Bill]

Yes, weak and thin, but undistorted.

Also, like before, master volume set at 4 but sounds just louder than normal TV listening volume.

Massive hiss, fades, disappears totally at 5 and above.

Try cleaning the master volume pot and the return jack switching contact with a high quality contact cleaner spray like DeOxit.

[upl8tr]

Okay, will do, I have some contact cleaner, not sure about quality but should give an indication if it makes a difference.

If so, I’ll buy some good quality stuff tomorrow.

[upl8tr]

LMAO, I think I owe you a few beers!!!

Sprayed all the pots and jack inputs and voila!

Seems to be working as it should, but it’s too late to test it out, so ill have to update you tomorrow, okay.

Once again thank so much, I can’t believe how quickly you sussed the problem out!

[upl8tr]

Success

Hi 52 Bill,

Had a quick blast through the amp today and it appears to be fine, both channels working and effects doing what they should, so all in all, a very successful venture!

(Just need to give it a good clean and screw it all back together now).

[52 Bill]

Glad to hear you got it working.

I think you owe a few beers to your friend that gave you the amp.

[upl8tr]

Well funnily enough, I called him today and told him I had got it working.
I offered back to him, he said no, but I’ll have a few beers from you!!!(lol).

Again, thank you very much for your time, and keep up the good work.

[beanoir]

I'm having a similar problem with the same amp, only I don't get any sound from the main speaker when plugging the guitar or cd player into the loop in.

I've never ventured into taking an amp apart, i've got the back off and the speaker is connected, and works too if I connect it directly to my cd player. There is also perfect sounds coming out of the headphones socket.

Any help for a serious amateur would be greatly appreciated!

Thanks

Nick