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Old 16th January 2012, 07:17 PM   #1
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Default connector on schematic

Hello

I am doing some studying and came across this schematic. I do understand most of it, but i am getting confused with some of the notation they use on the drawing.
On the drawing there is a connection from JK2 (input plug) to a point after the clipping stage. What am i supposed to understand when the plug is inserted on this? Is the point after R6 going to ground? If not where is it going to connect with the plug inserted?

http://www.amparchives.com/Amp%20Arc...%20MG30DFX.pdf
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Old 17th January 2012, 12:57 AM   #2
Enzo is offline Enzo  United States
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Your link didn't open for me, but it appears you have a Marshall MG30DFX, yes?

They will probably move this to the instrument amp section.

JK2 is the input jack. There are four connections to it on the drawing. Top to bottom we have:
The tip contact for your guitar cord plug.
The tip cutout contact which is grounded.
The ring contact, which I believe to be in error
The sleeve contact - the shaft of the plug and is grounded.

The tip cutout keeps the tip grounded until a plug is inserted. Plugging in lifts the tip away from the cutout, thus removing the ground. The purpose is to keep the amp silent when nothing is inserted.

As drawn, the ring contact would be grounded by the sleeve (shaft) of the plug, and so it would ground out the signal path at R6. Not very useful.

I think what they intended was the ring contact would have been grounded, and that line to R6 would go to the ring cutout contact. That way R6 stays grounded until a plug is inserted, thus reducing his and "gain noise" from the first stages. When a pl;ug is inserted, then that contact would lift and just like the tip, open the ground connection, thus freeing the signa path to do its job.

The MG50DFX schematic is drawn correctly in this regard. The MG10CD drawing is even more clear. A number of MG series models are mis-drawn in this fashion. I suspect cut and paste.

That later point in the circuit is ONLY grounded when NOTHING is in the input pack. The drawing is wrong.
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Old 17th January 2012, 03:01 PM   #3
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thank you sir...that makes a lot of sense...i actually suspected that, but figure i would ask anyways.
On that same regard, would you know how can i get the specs for some of the capacitors they refer to on that schematic? Like C13, C14, C58 just as an exmaple...

Thanks again
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Old 18th January 2012, 12:26 AM   #4
Enzo is offline Enzo  United States
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I don't know off the top of my head, I usually determine that by looking at the part. The schematic has a legend, but not apparently on the same page. I can only guess that caps with a C are ceramic, the ones with F are film, and maybe the M one is mica.

I may bring down the wrath of the purists, but I don;t think in an amp like this the type cap matters much.
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