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Marshall MG250DFX How does the DG212CJ function?

Started by Hawk, February 04, 2015, 09:31:22 AM

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Hawk

Advice needed: I'm about to remove the circuit board from the amp but it has plastic poles/standoffs. I've tried a small amount of prying but the board starts to bend. I don't want to crack the circuit board so can anyone suggest a good way to remove the board from these plastic standoffs?  Also, would you unplug all the connectors in advance?  I have to replace a diode, need to discharge caps, and I want to trace out the circuit board for learning purposes. Thanks!

J M Fahey

Post standoff pictures, there are different types.

Maybe they are PC motherboard type, but will wait for your pictures, there are many types available:












g1

  Also on some marshall models you release the standoff from the outside of the chassis.  Sometimes by screw, sometimes by 2-piece black plastic push-pin.

Hawk

Here are the pics inside and out of chassis. What is the best way to remove? Thanks!

DrGonz78

Pics are a bit blurry and can't really see the detail all that great. It seems there is one small part that hooks in and holds that standoff in place to the circuit board. Take a small straight blade screw driver and push (nicely) that piece in as you pull (nicely) up the board to remove. Sometimes they spin around and stuff. Also could use needle nose pliers, but they can tend to rip up the plastic piece. If you do it right each time it gets easier to remove however sometimes you end up wearing the piece out if you do it to often.
"A person who never made a mistake never tried anything new." -Albert Einstein

Hawk


J M Fahey

Easy.

It has an "arrowhead" going through a hole in your PCB .

As`a good arrowhead (or a fishing hook yip), easy to go in, hard to go out.

Although your picture is somewhat fuzzy, there are 2 basic ways for that arrowhead to be stuck (on purpose) holdingbthePCB

1) it's a rhomboid shape, with the tip in a narrow angle and the base a much wider one.

When you push it into the hole it narrows, goes through the tiny hole and then expands to original size.

Going backwards is much harder.

You must squeeze it narrow with long nose pliers while lifting the PCB.

2) it may also have a third center leg which is somewhat bent outside the hole,
When inserting it bends inwards (it's very elastic and tough Nylon) , then snaps outwards again holding the PCB.

You pull the PCB while simultaneously pushing the center pin in with a small screwdriver.

3) a combination of 1)  and 2)  .

In a nutshell, imagine it's a flrexible arrow tip which squeezed thin to get into that hole and then expanded; you must repeat that backwards.

Roly

Quote from: J M FaheyYou must squeeze it narrow with long nose pliers while lifting the PCB.

This is easiest if the pliers are perpendicular to the axis of the standoff, i.e. coming from the point-of-view of <Standoff1.jpg>.  I have several pairs of suture clamps/hemostats which are almost ideal for this in "busy" board environments.

Tip: anyone who does this a lot (e.g. working on computer motherboards) can also make up a specific tool using a small length of metal tubing (with the ends deburred/rounded), pushed down over the arrow.  Old (simple) biro tubes will also work 'tho they tend to break fairly easily.
If you say theory and practice don't agree you haven't applied enough theory.

Hawk

Thanks Gents! Easy is right, as it turns out. Used a pair of needle nose pliers, squeezed arrowhead and pushed up simultaneously and it slid through. It was easy, but glad I asked, better than cracking a pcb! :cheesy:

Hawk

Might as well ask. How does one remove the connectors from the board without causing damage? The top section of each with all the wires: Con 1, Con 2, Con 3, Con 4. thanks! (see attachment)

Hawk

Newb question: I measured 1.3v DC on pin 8 (V+),  and -15.36 DC on pin 4 of NJM4558. I checked others in the circuit  as well, same readings. Now, shouldn't V- and V+ have the same voltages? I thought they would both be -15.36. An equivalent circuit usually shows the same voltage on the rails, neg. and pos. Also this is a dual op amp, so....hmm, what am I missing? ??? Thanks.

g1

#27
  The connectors are just push-in.  Try not to pull by the wires.  At each end there are little lip edges that you may be able to get your fingernails under.  Or a flat head screwdriver to pry each end up a little till it comes loose.

In you first post you had your +15V.  Now it's gone.  Either something has gone bad or there is a connection issue.  Check at the output pin of the 7815 regulator, there should be +15V there.  If it is not, either the regulator is bad or something is loading down the +15V line, like a shorted IC.  Or the regulator is not receiving it's input voltage (probably around +20V or more).
  If there is +15V at the regulator output, then it is probably a connector issue or a bad solder joint or cracked trace somewhere.  In this case, check all the IC's for +15V, if some have it and some don't, it may help localize the problem.

Hawk

Update: found 1.3 V output on Reg. 1, -15.36 V Output on Reg. 2....D11 had been burned out so I replaced it and it is reading normal. The amp was working but maybe when I pulled out the circuit board (I was careful) I loosened a solder joint? Thanks.

Hawk

Checked all the IC's and yes -15V on pin 4 and 1.3 v on pin 8, the analog switch has -15v on pin 4 and 1.3 on pin 13.