Marshall mg50dfx

[Ren]

Greetings everyone,
I ws wondering if someone could please help me please ! my 50dfx playin up the sound cuts of completely and after constant fiddling with the internal speaker Cable lead into the internal speaker socket it works for few hours and then goes off completely again . Sometimes the fiddling don't help as well it's just NO SOUND at all sometimes .everything else seems to be working fine the fan and everything .
Thanks in advance

[Jazz P Bass]

Two things come to mind that may be at fault.

One: the jack needs replacement.
Two: the cable itself has an internal break.

Oh. Welcome to the Forum.

marshall_mg50dfx_sch.pdf

[Chuck H]

The schematic seems to be a dead link. I looked around for a better link, but no luck. I did read that those amps seem to have a lot of trouble with cold solder joints. Try another speaker cable like Jazz said. If the problem remains you could open it up and touch up the solder joints with some "flux" or "rosin" core 40/60 solder if you can get it. It seems to fix a lot of these amps. Just use a small amount of fresh solder at each spot. Melt the joint and then touch the new solder to that melted joint. You want the flux to re wet the contacts and it won't do that well if you melt the new solder onto the iron first. Work quickly to avoid overheating the contacts and components. Just touch a hot iron to a contact and the moment it melts touch the end of the new solder to it. It should instantly be smooth and shiny. Remove the iron at once and go to the next contact. It won't take that long once you get rolling.

[Ren]

Thank you mate !very kind of you to welcome me I will replace the cables but before I do could something be wrong with the actual socket ??and cld I replace it at home ? I'm sorry I'm very new to amps and stuff so if I end up asking silly questions please be patient with me .....

[Ren]

Thank you chuck will try and work my way around . I will experiment .I will learn it the hard way

[g1]

Here's that schematic:

[Jazz P Bass]

The Speaker Jack is on it's own little board.

Pull the chassis & you will see it.

MG50DFX Speaker Jack.pdf

[Ren]

Hey there should I get a mono jack or a stereo jack ? As they have two options for me thanks

[km6xz]

The jack is a TRS, Tip Ring Sleeve, or Stereo but the ring contacts are not used so you can use either that fits the pc board traces.
I would caution you about resoldering the pc boards just for the heck of it. The original solder connections will be better than yours and will not run the risk of creating a real problem with solder bridges. The most common recommendation on web forums is "cold solder" but ask a tech who works on these every day and they will say it is rare. I don't see them every day, make 1 time every 10 days but they are very popular here so there must be a thousand of them in this one city. Overall, that amp is rather reliable and if there is a real problem most often it is the power amplifier IC that is low cost and easy to replace with the right desoldering station.

That jack pc board might have simply cracked trace or broken solder pad/solder however right at the jack where the jack pins connect to the jack board. That can be caused by someone tripping over the external speaker cable plugged in or the short internal speaker cable catching on something when moved. It would be obviously cracked or intermittent upon inspection. Remove the jack board and lool closely on the solder side of the board. Plug a 1/4 in plug in to the jack and wiggle it, if there is a broken pad or cracked joint you will see one of the jack connection pins move with the wiggling. Only replace things that you prove are bad or else with each addition of new parts the risk of damaging the board or making a wiring error increases. Find THE problem, not just replace until it appears to work. You will never know for sure what the problem was and whether it is actually repaired if you do.
The main reason that and its bigger brother are popular is that they are one of the Marshall solid state amps that has very similar voicing as the much more expensive tube models. The sound pretty darn good despite using a $4 power amp chip instead of $200 in tubes and transformer.

[R.G.]

I would caution you about resoldering the pc boards just for the heck of it. The original solder connections will be better than yours and will not run the risk of creating a real problem with solder bridges. The most common recommendation on web forums is "cold solder" but ask a tech who works on these every day and they will say it is rare.

It is rare in amps made before or a year or so after the changeover from 60-40 tin-lead solder to lead-free solder. I have personal experience with the problems in a manufacturing line trying to make that change. The entire electronics industry had problems with this until lead free solders got better known and the best formulations and wave solder setups were discovered, typically one factory at a time.

I did personally repair many, many articles from the changeover period just by resoldering all of the joints with tin-lead and liquid flux. It is not clear when this amp was manufactured, but the exposure is there for items manufactured during the changeover period. That is probably the reason that "resolder it all" has become common internet wisdom.

As with everything, re-doing something done by a factory means you have to be more skilled than the factory to do it well.

Find THE problem, not just replace until it appears to work. You will never know for sure what the problem was and whether it is actually repaired if you do.

I would add to that to never think you have found THE problem; it is common enough that there is more than one problem, perhaps related (... TWO cold solder joints in a lead-free unit! Wait... three... four...nineteen...) that when you find A problem, you do indeed verify that the problem you fixed is really fixed, then go on to test whether there's another problem or not.

[Mick Bailey]

I always have an uneasy feeling about re-soldering a lead-free joint using lead solder. It seems to work and I've never had any subsequent re-work to do, but it just seems wrong. The material left is a random alloy with uncertain long-term properties and it usually seems to transition from liquid to solid very quickly, often with a dull, gritty surface. Often I'll remove as much of the original solder as possible and then resolder.

One of the main problems with cheap/modern boards is they're only capable of bearing the heat of original solder process and incapable of withstanding the heat of a rework. Especially so due to the alloying process of soldering to copper raising the subsequent melting point of the original solder. It's useful that a lead alloy will 'wet' such a joint and it can be persuaded to flow - especially the SMD rework solder with 5% silver. But it still leaves the risk of lifting tracks or pads. Some boards are so bad I think the only reason they're not made of cardboard is they found a cheaper material.

There's also the problem of a joint that's cracked through long-term thermal cycling (especially the early lead-free formulations) becoming oxidised within the crack. Especially worsened if it's a high-current component such as a regulator or power resistor. Soldering over these can give a nice crowned joint, but it can still be cracked underneath and only the end of the component is secured and has limited durability.

[Jazz P Bass]

Quote:"Some boards are so bad I think the only reason they're not made of cardboard is they found a cheaper material."

Good one, Mick.

[Ren]

Hello people ,
Thanks for all your advise also yesterday I've just discovered something ,the amp was workin for sometime luckily (after fiddling with the cable and the jack ) I also found out that I had to twist the master volume knob quiet few times quickly for the sound to come out .

[Chuck H]

Hello people ,
Thanks for all your advise also yesterday I've just discovered something ,the amp was workin for sometime luckily (after fiddling with the cable and the jack ) I also found out that I had to twist the master volume knob quiet few times quickly for the sound to come out .

Like manual tremolo

If you read Stan's posts, he talked about stressed areas. Pots and jacks are usually board mounted and often also act as structural support. Using the component and it's solder connection inappropriately to hold the amp together. This leads to connection failures. Inspect the pots connections for lifted pads or cracked traces. If you don't see any you can try cleaning the pot. If that doesn't help, re flow the connections with fresh solder. If that doesn't help, replace the pot.

[Ren]

Hey !! You guys are amazing thanks for your suggestions .And JAZZ thanks mate !! just changed the jack ,it just sound like how it did when I first bought it ,God I even forgot it could sound this loud. I had my doubts tho whilst I was changing the jack but I was so surprised it worked !!! Thanks ))