Marshall MG100fx hissing like crazy

[diydidi]

Hi guys.
There something fishy going on with these amps. I have two from different customers, both with the same problem- excessive hissing.
Also weird to me is that on the back it states model as MG102fx. Either way a schematic is proving impossible to find.
Where is the hiss coming from? More importantly, how do I fix it?
Regards.

[DrGonz78]

You could first connect the preamp out to a known good amp to see if the hiss is coming from the preamp. If it is coming from the preamp then I would at least take voltage measurements on the opamps (all the pins) for anything out of the ordinary. Got to first find out though which side of the amp is producing the hiss.

[diydidi]

The send output has no hiss, and sounds clean. There is a digital board in this amp, but it is after the return socket to the amp. So it could be coming from there...

[Enzo]

And it could be coming from just about any semiconductor on the amp.

The preamp is clean, but do ANY of the controls affect the noise in ANY way?

This is not likely a voltage problem.


How did you determine the DFX board was after the FX loop? If that were true, then you could plug the guitar into the FX return and still have the FX. On the other hand, if the signal on the send jack includes the digital FX, then the card is before the jacks. In either case, does turning the reverb and FX controls to zero stop the hiss? If not, then the DFX is innocent.

[J M Fahey]

The regular MG100 schematic is available on the Net.
If you can´t find it, I have it somewhere and can repost it.
Don´t remember if it´s the DFX version or not, but should be basically the same, at least to mentally divide it in blocks for testing, the only difference being one has a standard Op Amp driven reverb tank and the other shows a connector sending and receiving from a DFX board whose schematic is not shown, 99,999% possible it´s an externally made bought part.
Anyway you can test its return line , power rail and ground to see whether they are dirty/noisy or not.
In fact, you can now unplug that connector, no need for a schematic, simply to check whether hiss dissappears or not.

EDIT: there I found it .
You can unplug the little DFX board (with power OFF) or simply set the EFX pot on 0 , do the same with Reverb just in case.


EDIT2:
this one is pertinent to this post, it shows the small digital effects daughterboard, but I also think will be interesting for alan0354: Marshall, who is a *heavyweight* in the Musical scene (does anybody doubt that?) STILL uses a single sided board (and Phenolic to boot, not Epoxy glass) to build a massively produced amplifier, NO through holes , instead of laying tracks on top they make a liberal use of jumpers, and ...... I VERY VERY much doubt the chinese PCB supplier

"buys double sided boards and etches the top layer off"

As I often state and to keep feet firmly on the ground, beyond noob imagination on how things are or should be , fact is that real world producers are first of all solid businessmen, not dreamers, produce good to very good stuff on a budget (anything else is suicidal) and DO fulfill needs of Millions of customers.

[diydidi]

The DSP is definitely after the return socket. I have added a pic of the front panel. If I have the FX turned off and all controls associated with it down, the only controls that really push the hiss up, are the volume (close to center), and the Master volume (far right). Turning the reverb up, and also fiddling with the fx parameters, make no difference. However switching to the overdrive channel, obviously increases the hiss substantially.
I can't find schematic for this one. It seems to differ from the HFX and DFX schematics.

[Enzo]

I am not sure how the photos tell me the electronic order of things. But it sounds like the DFX is not involved.

I am unsure of your report. You say the SEND is clean and hiss free, yet you also say the volume, master, and channel turn the hiss up. Those controls are all in the preamp. And do the tone controls also affect the tone of the hiss?

Use a signal tracer or scope to see what is on the output pins of the various op amps. At this point it sounds to me like a noisy one in the first stage. Can you determine which IC the input signal visits first?

[loudthud]

Did anybody notice several of the opamps in the schematic Juan posted in reply #5 are drawn wrong with feedback connected to the positive input?

[gui_tarzan]

Wouldn't that just create a phase inversion? It's been 20+ years since I've messed with op amps.

[Enzo]

Come on, LT, that's because he is in the Southern Hemisphere.

[J M Fahey]

Come on, LT, that's because he is in the Southern Hemisphere.

Sorry, I forgot to tell you that you had to look at the schematic from the bottom side, through the paper as if it were transparent, to see it the way I do

By the way, WHO decided North should be drawn on top?

Many Geographers think otherwise, and to boot correct old map distortions:

[J M Fahey]

Did anybody notice several of the opamps in the schematic Juan posted in reply #5 are drawn wrong with feedback connected to the positive input?

Incredible but true.
So far I´ve found IC1b , IC8a/b

Mind you, *there is* at least a case I know, where switching normal use of inputs is necessary: in some power amps the main (discrete) part is inverting and has an Op Amp front end which is inside the global NFB loop.
In that particular case you need to treat the + input as inverting and the - input as non inverting, to keep phase relationships correct ... but here they receive NFB from their own outputs, so it´s a gross drawing mistake.

Another surprise: the amp diyididi has on his bench is absolutely different, looks like a Line 6 product !!!!
Shares outside aspect , font panel and model number, but it seems to have a very basic analog front end, just to get input signal to the proper level, then it might be digitized and from there on it´s all digital.
Also look at the flat ribbon connecting the front panel controls , the large PCB on the right with a clock crystal, the lack of wiring or tracks connecting different gain stages to the controls, etc.

The only "good" thing is that they ditched the pesky TDA7294 and reverted to a pair of large discrete power transistors.

And surprisingly the PSU is still EI transformer powered, not the SMPS I´d expect.

FWIW this might an outside-Marshall design.

And I´m very sorry but I suspect in this case Diyididi won´t have many things to do.

Disposable digital device.

[diydidi]

This one is turning into a bit of a puzzle for me. I like the comments/ replies however.
Will report back after the weekend. Gonna do some more fiddling around.

[J M Fahey]

Puzzling here too.
Would NEVER have imagined to find that inside a Marshall labelled head.
Not surprised on, say, Line 6 stuff, or even a few "Vox" ones ... because we all know they really are Korg made, the kings of digital keyboards and pedalboards, but ..... a Marshall???
From what I saw in such amps, they have 1 or 2 (at most) Op Amp stages just to provide 1M input impedance as expected by the guitar, enough gain to what the DAC requires, *maybe* a little preemphasis so late deemphasis after all digital processing lowers noise floor ... and little else.
Then signal dissappears inside a 100 pin per side plastic block which does all the work.
What I mean is that if the cause of hiss is not in the minimalistic front end or in the output analog stage, there´s not much you can do

[diydidi]

Ok, so I've given up on these... I admit defeat. Anyone have a contact at Marshall I could email? They aren't returning my emails at all.