Marshall MG30 DFX HELP PLEASE!!!

[Rikshand00]

Hey everyone. Can anyone help me? My marshall MG30 DFX hasn't been working for some time. At first it would cut out and go really quiet when I was playing on it. Now the sound doesn't work at all. The strange thing is that If I plug headphones in to the output socket I can hear the sound coming out fine.
Im not an electrical engineer so if anyone replies, please simplify for me. :duh Thanks   :dbtu:

[phatt]

Hi Rik, and welcome.

IF your amplifier has a send return setup (usually on the back) then get a spare guitar cord and join that loop via the cord.
Does that bring it back to life?
Phil.

[Rikshand00]

hi phil and thanks. No I cant see one  . Does it look like a normal Jack socket? IF so there is nothing on the back

[phatt]

Yes they are standard 6.5mm sockets just like the front input. Sometimes they are on the front.
But ok if they don't exist then you have to try other ideas.
All is not lost because if the headphones work then I doubt much is wrong.

Wait a while and other minds here will likely know more, meantime check the speaker terminals have not fallen off or something like that.
Maybe Even use enzo's hi teck fault finding tool. :lmao:
Give it a good whack on the top of the case and see if something happens.
Often over time solder joins crack on the PCB and cause problems and a good belt reconnects things. It won't stay working for long but it gives clues as to what maybe wrong and it may need to be opened up and checked.
Phil.

[DrGonz78]

I read some people on the Marshall forum saying that the Power supply is no good on these amps. Then they double the capacitance of the main filter caps and installed some kind of higher spec PSU. All that and they never posted one thing about voltages on the amp. So, poo poo on that idea!!

Personally, without looking at the schematic I would assume to start by whacking it a bit... Also, clean that headphone jack with some contact cleaner and see if that helps.

[Rikshand00]

iv tried all these but its still not working  . but thanx 4 helping  :dbtu:

[DrGonz78]

So, personally I would inspect the inside of the amp to see if anything is detached or shorted out. Thumping it might not always be effective but sometimes can help determine for loose connections. I would be looking for solder joints that are cold or something loose on the output to the speaker area.

Also, being able to test voltages on the amp will help make sure the power is good through out the amp too. Schematic is attached for all to help if you continue with troubleshooting.

[phatt]

OK I just had a look at the schemo (Thanks Dr Gonz :tu:) Now you mentioned the Headphones worked fine.
Well the schematic shows the the signal that drives the HP's comes from the output of the main power chip (IC6).
In which case I'd be looking for broken wires/tracks AFTER that point.

First and easiest test is to disconnect the speaker wires (at the speaker) then check for continuity of the speaker itself.  Should read low OHMS.
If the speaker is 8 OHMS then look for a reading below 8 Ohms. (maybe around 6)

If you don't have a DMM then momentarily tap the lugs on the speaker with a 9 volt battery. The speaker should pop/thump and move in one direction and then return upon removal of the battery. Don't leave the speaker connected to battery for long time as speaker coils don't like raw DC volts.
That will at least tell you the speaker is alive or dead.

Now if it is dead (No pop or movement) then look for broken speaker braids,, these are the woven leads that go from the tags to the actual Voice Coil.
Sometimes these break as they cop a lot of mechanical movement.
Often they break right next to the solder on the tags and a quick resolder can bring them back to life.

Don't worry I'd say it's got a good chance of being fixed.
Cheers, Phil.

[Rikshand00]

Thanks people I have used your advise. I am slowly making progress. My speaker is Kind of working, Its just really, really quiet. You can barely hear it unless you are close to the amp. Any more suggestions? Thanks again!

[Roly]

There isn't much after the amp output, the headphone socket with a set of switch contacts to turn the speaker off when headphones are being used (and which are normally the number one suspect in a case like this), then the speaker itself.  There just isn't much else.

A fresh 9V battery flashed across the speaker terminals (disconnect it from the amp first!) should make quite a loud Ker-Splat!

While you have it disconnected, try running the amp output to a different known good speaker (being very careful not to short the amp output).  If it comes good it's your speaker, but if it's the same then it is either dirt in the headphone switching contacts, or (if the headphone socket is mounted directly on a printed circuit board), a wrenched solder joint under the socket.

You've really got your fault backed into a corner where it can only be a few things.

HTH

[Enzo]

Get out the ohm meter.  measure resistance to ground from the speaker terminals.  The cold side of the speaker shou.ld measure 1/10 of an ohm, so essentially grounded.  If you get an open, it is almost surely the phones jack cutout contact.  And if you picked the other speaker terminal, it would add the very few ohms of the speaker voice coil to that zero.   So either way, if you get a high resistance, like about 2k, then the jack is the first place to check.

[Rikshand00]

I put the 9v battery to the speakers and its making a noise. I guess it is working? So Il try and see what the problem is on the circuit board. I dont have a ohms meter by the way. I appreciate all the help  :dbtu: 

[J M Fahey]

You made the Roly test, which tells you the speaker works, but not the Enzo test , which checks the headphone contacts.

[Roly]

Working on electronics without a meter really is flying blind.  Even the cheapest is better than guesswork.  You can pick one up for under $10 these days, and have a look at;

http://www.ssguitar.com/index.php?topic=2555.0

Google "How to use a multimeter" and read/view a few.