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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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Looks like R108, the 2.2 K 1W resistor is charred, showing .34 VAC to ground on the circuit side, whereas the other one, R109, shows 11.34 VAC to ground. I measured R108 in circuit and it showed a read of 44K...will have to wait until tuesday to pick up resistor...hopefully this explains why I'm getting such a low voltate output at Reg 1 and therefore all the ICS.


Yahoo! Amp works again! After replacing resistor and diode I found proper voltage on op amps, voltage regs 1 and 2, and the analog switch.

Thanks for everyone's help so far.... :tu:


Next issue: amp still works fine, can play guitar through but when I power on the amp makes this nasty loud honking squealing sound for a good second, almost two then goes quiet and runs fine. When I turn off the amp it makes a very loud clicking sound.


I'm wondering if this problem has to do with the Metallized Polypropolene  100nK 275 Volts cap on the line side...


No.  :)
This is with nothing plugged into the input?  Do volumes have any effect?
Plug something into FX return jack, still same problem on turn on?

J M Fahey

Quote from: Hawk on February 18, 2015, 10:38:08 AM
I'm wondering if this problem has to do with the Metallized Polypropolene  100nK 275 Volts cap on the line side...
NO , DEFINITELY NOT, line transient filter caps can have NO effect on anything soundwise.

Clear enough?


Clear enough. So that nasty sound occurs sometimes but not always. Plugged a jack into the FX Send but couldn't hear the nasty sound when I turned the amp back on. I have the bare pot shafts exposed so I turn them without the knobs. On the Overdrive Channel when I touch the Volume shaft I get a local AM radio station coming through loud and clear!  Wondering if this has anything to do with it... Just turned it on now and that nasty sound came back--noticed that the gain was maybe half way, turned it down and re-tried turning the amp on a few more times but no problem...problem is not consistant...I've tried different volume settings and couldn't get the nasty sound to reproduce.... :-\


When I plug my guitar in and touch the volume pot the radio issue is non-existant.


So I should be embarassed here! I believe my problem was a huge grounding issue!! When I put the board back in I didn't put on my nuts/washers and tighten them because I intended to take out the board and study some more components etc...so now radio issue gone, amp sounds great, no nasty noise at start up :-[ :-[


  Don't feel too bad, modern ground schemes can be fairly complicated and often require complete re-assembly to run properly.
  So it's all done & ready for use now?  ;)


It is thanks! It sounds like it should and no crazy radio signals and horrible sounding start-up noise. It's my mule so I'm sure I'll muck something else up but that's how I've been learning. I was hoping that I could somehow have the board turned upside down so I could read voltages at certain points in the circuit board that I can't access from the component side. But obviously that would screw up the grounding scheme and would be asking for trouble. Guess it's a matter of trying to pinpoint the issue, then turn over the board (without power and of course draining caps first) and measure the components to try and find the fault. I suppose you could try and measure the PT secondary voltages by somehow scooping your test leads under the board but that sounds dicey :)

Second Question: Re: the DSP board on this amp. It's about 2X2" and has three chips on it and SMD components. My guess is that this is for the Digital FX processing. I can't see how this could be repaired as everything is really, really small. Would it be a case of chucking out the old one and ordering one from Marshall? It works, just wondering what others have to say about that and how they've dealt with it. Thanks!


Last question for the night :) At the guitar input there is a line that goes up and all around the page and ends up at Pain R. The line also goes to the Headphone Out and ends in a circle and then on the other side of the Headphone Out the line goes to the circle on Pain L and then Pain Left goes to Pain R. Then on the far side of the page this is either fed to Con 2 or vica versa.  I understand the idea of PreAmp and Power Amp and older schematics make it much clearer. When I look at this it seems backwards to me. Looks like the master volumes are feeding pin 1 and 7 of the outputs of the opamps??? What does Connector One Feed??? Thanks again! ???



Quote from: HawkSecond Question: Re: the DSP board

Well, that depends.  The more "commercial" you are, the more likely you will simply change the module as a part, but if you're curious/crazy/irritated by the throw-away society/equipped to handle SMD you might give it a slash, however unless it is something pretty obvious then it is likely to concern some silicon blob that is custom/unobtainium and be a waste of time and energy.

Quote from: HawkLast question for the night :) At the guitar input there is a line that goes up and all around the page ...

Well two actually.

The line appears to be a fancy framing line to delineate the preamp board.  It is not a conductor.

Quote from: HawkLooks like the master volumes are feeding pin 1 and 7 of the outputs of the opamps??? What does Connector One Feed???

In the lower part of the preamp circuit the signal flow is right-to-left.  It comes out of IC5A and B, through C39 and C44, and onto the top of VR13A and B, picked off by the sliding taps, and out to the main amps at PAIN_L and _R.  Again, "PAIN" stands for "Power Amplifier INput" (implicitly the preamp output).

By convention signal flow is from left-to-right and power supply right-to-left, but not everyone observes conventions.

The inputs of an op-amp are on the vertical stroke, and the output from the point;

We appear to have two CN1's (apparently a drafting error).  One goes to CN2 on the preamp marked "TO REAR JACK SOCKET BOARD", and the other is one of the two that go to the power amp boards and mate with CON3 and CON4.  (note that these connectors are drawn differently to show their gender).
If you say theory and practice don't agree you haven't applied enough theory.


Great, that really opens things up for my understanding! I see what you are talking about.

Still confused about power amp schem. Shows both having the speaker outputs at W12,13,14,15. On Con 3 we have W13 attached to Pin 9 and W12 attached to Pin 1 (similar for Con 4) On the last page of the schematic, pin 9 from Con 3 appears to go to pin 14 of the TDA7293 which is the ouput but Pin 1 goes to the inverting input of the TDA7293. In my mind these speaker outs should come off pin 7 and 8 as they are the neg, pos signal. Is this an AB amplifiers? Shouldn't we have two opposite signals at the same time? Guess I'm showing my ignorance here. How exactly does the signal go to and come out of the TDA7293? Thanks!