Marshall MG15HFX amp - no sound on clean but crunch works

[Alkay]

Hi everyone I hope this is the correct section to post this.

I gave a Marshall MG15HFX amp to a friend but there is no sound on the clean setting on the amp.

Crunch works with the same toggle button and also both overdrive modes work on the other button.

Does anybody have any idea how I could fix this please?

Cheers,
Alkay.

[Simon Barden]

I haven’t found a schematic for that exact amp, but if it is anything like the MG15DFX, then almost all the components in the signal chain are similar, with the clean volume ‘channel’ really only having it’s own volume pot as a likely suspect for failure. Either that or the channel selector switch. The selector switch has two changeover contacts; one just associated with isolating the input to the dirty channel gain circuit, so won’t be an issue, whilst the other selects between the output of the clean circuit and the drive circuit. It could be that the clean circuit contact isn’t making. This then feeds a common EQ and effects and power amp circuit, so the possible fault are is very small.

The clean volume pot would be my first check. I had a Mesa Nomad with really poor quality pots where the back was held on with folded metal ears, and a slight knock to the knob would push the shaft back against the rear of the pot and bend the metal ears open so the back came loose. As the carbon track was attached to the back, contact was lost between the pot input and output.

You’ll need to open the amp up and check the volume pot. A bit of work to get the knobs off and the various nuts and screws removed so you can access the bottom of the PCB. You should be able to then measure the resistance between the pot input and output, and it should vary as the pot is turned between full resistance and none. Full resistance is likely to be about 5K. You should be able to see if the pot back looks loose. If it is, you can normally just push it back on, bend the metal ears to keep it in place and I’d add a drop of CA glue to each tab to help it stay there. You should also be able to work out the channel switch contacts (two input pins, one output pin x 2) and check for continuity. If one input pin doesn’t connect direct to an output pin with a short-circuit reading on the meter when the switch is operated, then it’s the switch.

Also check for any obvious dry or broken solder joints around the volume pot and selector switch on the PCB. Any doubts, then remake the joints using lead-free solder.

If you can find a link to the actual amp schematic that would help confirm the above, but I doubt the MG series amps have changed that much over the years with maybe a few component value changes, surface mount rather than discrete components and maybe a new DSP chip for effects.

If it’s not any of the above faults, then it’s going to require circuit checking using a signal generator and a scope to track down a faulty component. Which means a pro repair (otherwise you wouldn’t be asking the question in the first place), and that will probably put it beyond economic repair as it’s likely a used working model would cost less.

[Alkay]

Thanks for the advice Simon. I'll have to try to find instructions for taking out the PCB. It looks a bit difficult.
Is there a possibility I can get electrocuted even when everything is unplugged? I read somewhere that some amps still hold voltage even when off.

The last post on this website says a similar model: https://music-electronics-forum.com/...arshll-mg15hfx

What do you think?

Cheers,
Alkay

[Simon Barden]

Now I know what the amp is (I should have looked up the amp first rather than going straight for the schematic), I still can’t find a circuit diagram for it, but as it has a common gain and master volume for the channels, it’s going to be far more complicated than I thought in my first post.

There aren’t any dangerous voltages stored in the capacitors in a transistor amp (probably the highest voltage is 24v as opposed to 400v for a valve amp), though you are best to leave it off for an hour first before opening one up to allow voltages to dissipate.

Without a circuit diagram, it’s going to be almost impossible to diagnose a fault. The channel storage almost certainly means digital control of the various amp sections, making it very complicated electronically; far more so than the amp I thought it was based on. So much that could be wrong that it’s definitely not a self-repair job. It’s probably a dead transistor, but you’d need the diagram and a scope to follow the circuit through to find out where.

Which puts it in pro repair territory, so normally a base charge plus hourly rate costs. Very easy to exceed its value or replacement cost. It takes just as long to take apart, diagnose, repair and reassemble a $200 amp as a $1000 amp. And whilst it may make sense to spend $250 repairing the $1000 amp, it doesn’t for the $200 one.

[Alkay]

Ok thanks Simon.
Yes I understand what you are saying it not worth fixing. He's better off saving his pocket money and getting a new one.
Thank you for your help anyway. I appreciate it.
Cheers,
Alkay.

[McCreed]

Have you tried the "technical tap" or "controlled drop" methods of fixing???

Just kidding!!!

At least it's producing sound, so your friend could set the crunch channel as "clean" as possible then use an OD/Distortion pedal to get dirtier tones. That could get him by until he can afford another amp.

[Simon Barden]

I presume it has the same issue with the foot pedal doing the selecting as with that disconnected and using the front panel buttons? If the front panel buttons work on their own, then it is very likely just a foot pedal issue, which is much easier to fix.

[Alkay]

Hi McCreed,
That was my thoughts but I've spoken to him and the sound issues are intermittent.

[Alkay]

Hi Simon,
He doesn't have the footswitch unfortunately. Best to get another amp and not attempt fixing it given the cost as you said.

[Simon Barden]

Intermittent sounds like either a bad switch or a dry/cracked joint. So it may be worth taking it apart, removing the PCB and looking for bad solder connections. Most of the components will be surface mount components on the top side of the board, but switches and pots will be through-mount components with solder connections on the bottom of the board, and so fairly obvious.

I've just noticed that the digital FX are on a piggyback board, so it may be worth making sure that the connectors are making properly. Remove it and push it beck in full, just in case. Unlikely, but you never know.

And as there is no footswitch being used, it maybe that the footswitch jack contacts themselves are getting corroded through not being used. As the channel selection signals from the front panel switches are probably passing through the jack socket, I'd spray some contact cleaner in the footswitch jack and push a jack in and out (any type of jack) several times, to get the contacts making and breaking. This is often a problem with series FX loop jacks that don't get used on an amp. Ideally done with the PCB exposed so you can positively spray claner onto the switching contacts, but spraying through the hole with the board in situ is better than nothing.