solidstate Marshall, bad hum on second channel

[Modded D+]

There seems to be a terrible hum on the lead channel of this G30R CD. Not a ground hum, because I can get that on top of this one too. But this one doesn't present itself in the clean channel. On the lead channel it is there even with the volume and gain all the way down. At a certain point when raising the volume, the normal noise associated with high gain will more or less overtake this noise, but I got this off a friend for cheap just as a bedroom practice amp.

I should mention that the input jack had cracked solder joints on the PCB. I put a new jack in and use a short wire for the connection. I would have reused the old jack but the plastic threads were stripped. But the wires are not crossing any components on their short journey to the board. I guess I should have played this prior to ripping into it, but I knew what the problem was from a while back so I didn't bother with it yesterday.

Anyone familiar with this type of noise?

Here is the mod I did, just for the sake of full disclosure. I have since moved the yellow wire off of that jumper and the blue away from the channel switch. Anyone know why there are two ground connections?

[Enzo]

The top photo shows the cracked solder on two pins of the jack. That is a simple resolder, except your threads were stripped.


The pin of the jack nearest the camera is the ground, the left rear pin is the tip contact - the hot lead of the input. The right rear pin is the cutout contact for that tip. It is the extra contact that is used to ground the tip contact when nothing is plugged in.

THAT is why it is grounded.

Now look at your new jack. It has no cutout contact, only the tip and ground. That means with nothing plugged in the inoput is still live. Bring a finger up next to the jack and hum shoud increase.

What is the other jack for? If it is not used much, I might have swapped it in place of the inut jack and wired your new jack there, If it was for some function like a foot switch in particular.

Is the new jack a metal bushing jack? If so, it will ground the circuit to the chassis at the front panel. The old plastic jack was unsulated from the chassis. This can introduce all sorts of hum into a system, it upsets the gounding design.


Now, it could be that none of this is involved in your noise. If the noise is new after the jack swap, it is possible you broke a wire connection from the board, or cracked a control, or missed a cracked solder connection elsewhwere.

[Modded D+]

The top photo shows the cracked solder on two pins of the jack. That is a simple resolder, except your threads were stripped.


The pin of the jack nearest the camera is the ground, the left rear pin is the tip contact - the hot lead of the input. The right rear pin is the cutout contact for that tip. It is the extra contact that is used to ground the tip contact when nothing is plugged in.

THAT is why it is grounded.

Now look at your new jack. It has no cutout contact, only the tip and ground. That means with nothing plugged in the inoput is still live. Bring a finger up next to the jack and hum shoud increase.

What is the other jack for? If it is not used much, I might have swapped it in place of the inut jack and wired your new jack there, If it was for some function like a foot switch in particular.

Is the new jack a metal bushing jack? If so, it will ground the circuit to the chassis at the front panel. The old plastic jack was unsulated from the chassis. This can introduce all sorts of hum into a system, it upsets the gounding design.


Now, it could be that none of this is involved in your noise. If the noise is new after the jack swap, it is possible you broke a wire connection from the board, or cracked a control, or missed a cracked solder connection elsewhwere.

Thanks for explaining the extra ground, but I am not sure that should have anything to do with my problem. The two ground contacts should only be an issue when nothing is plugged in, otherwise they are on the same trace on the PCB, so they should be grounded fine when the guitar is plugged in. The noise is only on the lead channel, and it is there even with the volume turned all the way down.

Is the new jack a metal bushing jack?

I believe so. I will have to really take a look at it, but I know it has metal where it mounts. As to the metal being connected to the sleeve I don't know, but I wouldn't doubt it. If anything, in order for the jack to fit the metal threads are up against the top of the hole in the chassis. But again, this sound is only in the 2nd channel, and it is a noticeable sound.

[Enzo]

The metal threaded bushing is connected to the grounding, so if the thing is metal it is as I described. Your noise in only apparent on the high gain channel, but that doesn't rule out the jack. The higher gain may be what is making it audible. Demount the jack and see if it makes a difference.

Ther is still the other stuff I briefly mentioned in my closing pargraph above.

[Modded D+]

I ordered a new Marshall replacement jack for the MG series, the picture looks like it is the same thing. I will look it over when I pick it up tomorrow at AES. But I opened it up and pulled the jack out away from the chassis and the problem seems to be gone.

I will post when I get the new jack it. Thanks.

[Enzo]

So just what is the second jack in the view? Is it a footswitch jack or something? Would swapping it into the other spot get the amp back in service while waiting for parts?

[Modded D+]

Yes, the other jack is a footswitch. But swapping it over is not necessary at the moment because Antique Electronic Supply is just down the road a ways. I am picking up the part tomorrow afternoon.

[Enzo]

Oh, I pick the one guy who lives next door to the source...

[Modded D+]

Thanks a lot. It works great now.