What have I gotten myself into...?

[SlickNick]

So long story short, I bought a Marshall MG50DFX for $75 because I want to use it as a speaker cab for an Orange Tiny Terror.  I WOULD however, like to use the Marshall amp head and this particular amp had an input problem but I figured I could fix it so I offered the dude $75 down from $100 blah blah blah.  Upon removing the pcb mounted input jack, I managed to melt of two "caps?"  I'm going to work around this by making jumper wires that go down line to the next point in the "trace."  However, I've been racking my brain as to the 4th point on a TS input jack and I thought it was completely unused...but then I noticed some down line "stuff" attached to this mysterious contact.  2 of the others are shield related and the 3rd is the tip or hot.  Running a continuity test between the mystery contact and the shield of the cable I had plugged in yielded nothing.  Then with the cable unplugged, I noticed I could get continuity between the mystery and the would-be shield contacts meaning that there was some sort of contact within the pcb input jack that the plugged in cable would disengage (maybe some sort of hum prevention circuitry?).  What can I do to get around this?  I really don't want to pull the Schematics and order the original part because I dont want it to fail again in the future.   Thanks in advance.


Edit:  I've attached the schematics in a PDF format.

[mah62]

Wow! you are a braver man than me if you are dabbling with mains and are unsure what you are doing! You did discharge the caps didn't you?

"Let's be careful out there"!

Mark

[mah62]

Wow! you are a braver man than me if you are dabbling with mains and are unsure what you are doing! You did discharge the caps didn't you?

"Let's be careful out there"!

Mark

[Govmnt_Lacky]

Im gonna try to give this a shot......

In the schematic, if you look at JS1 (Input jack) you will notice that BOTH the tip and ring connection have contacts that short to them when a jack is NOT installed.

Contact 1 is the Tip. Contact 4 is the Ring.

Contact 2 is the shunt to the Tip. Contact 3 is the shunt to the Ring.

When the guitar cable is NOT installed, Contacts 1 & 2 are shorted together. Also, Contacts 3 & 4 are shorted together.

When a guitar cable IS installed, BOTH of these shunts DO NOT make a connection with their respective contacts.

Based on what you wrote quoted below, it seems like the Input jack is functioning properly.

Running a continuity test between the mystery contact and the shield of the cable I had plugged in yielded nothing.  Then with the cable unplugged, I noticed I could get continuity between the mystery and the would-be shield contacts meaning that there was some sort of contact within the pcb input jack that the plugged in cable would disengage (maybe some sort of hum prevention circuitry?).

[SlickNick]

When the guitar cable is NOT installed, Contacts 1 & 2 are shorted together. Also, Contacts 3 & 4 are shorted together.

When a guitar cable IS installed, BOTH of these shunts DO NOT make a connection with their respective contacts.

Based on what you wrote quoted below, it seems like the Input jack is functioning properly.

Ok I see what you're saying but the input doesn't work properly.  You have to pull the guitar cord to the left or right to get signal to pass and it's definitely not the cable.  I just recently soldered my cable perfectly and the seller's cable had the same problem.

By looking at the schematic, can you tell me what would happen if I just didn't hook up those pins that shorting out when nothing is plugged in?

[Govmnt_Lacky]

From what it looks like to me it appears that you are just shunting the input to Ground.

If you are having problems passing signal then the jack is most likely busted (from the explanation you have given)

Replace the jack and move on 

[SlickNick]

If you are having problems passing signal then the jack is most likely busted (from the explanation you have given)

Replace the jack and move on 

I'm Trying...lol.  I want to put a metal, non pcb version in its place.

[blearyeyes]

If that is a solid state Marshall, it might also be the mute circuit running to the main power amp chips. I know my Valvestate has that.