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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

How does the DG212CJ function in the post secondary transformer set up? I've tested and found the 5V fed from the voltage regulator and also found +/-15V on the DG212CJ. I found 15V on pin 8 of the 4558DD Dual Op amps in the schematic above. Since I haven't looked under the board is it safe to say that pin 8 of one of the op amps is feeding the voltage to the DG212CJ?

I'm having a little trouble reading the schematic as I don't see where they would take off the 15 volts to feed the pre-amps section. Where does that usually happen?  Thanks, Hawk (the new guy!)

DrGonz78

Many times on schematics there are voltage supplies given but no explanation to exactly what other circuit components it is derived. In this case, trace the board from the chip (pins 13 & 4) to confirm that it's voltages are fed off the opamp power supply. I guess we just know that the opamp power supply is the same source and the engineers simplify the schematic that way. So, DG212 is connected to pins 4 & 8 of the opamp power supply.
"A person who never made a mistake never tried anything new." -Albert Einstein

Hawk

Thanks DrGonz78! Appreciate the response. I follow what you are saying.

Loudthud

That's a really poorly drawn schematic. I wouldn't be surprised to see "blue wires" that connect the DG212 supply pins to +/-15V.

J M Fahey

Well, that's a usual problem .... but in this particular case, you are being unfair with poor old Marshall (RIP)

In page 2 of the schematic, it's clearly shown that +/- 15V feeds pins 8 and 4 of all dual Op Amps (RC4558 and NE5532) and pins 13 and 4 of DG212.

It is also clearly shown that said DG212 gets +5V on pin 12 and that pin 5 is grounded.

So stop bashing poor old Jim  :trouble

:lmao:

Hawk

Thanks for the last comment regarding voltages fed from. But, silly question I'm sure, where exactly is the -15/+15 volts picked off from? On page 2 of schem. where it shows V+, V-, I measure +34 volts to ground, and -34 volts to ground, both from R10 and R11. Thanks.

J M Fahey

Quote from: Hawk on February 06, 2015, 08:43:51 AM
Thanks for the last comment regarding voltages fed from. But, silly question I'm sure, where exactly is the -15/+15 volts picked off from? On page 2 of schem. where it shows V+, V-, I measure +34 volts to ground, and -34 volts to ground, both from R10 and R11. Thanks.
That's the "main"supply, for the power amp (which runs on +/-34V) but there's also others  :o

Depending on how you count them, you have 3 , 4 , or 5 supplies there, in 3 blocks.

The top one is simple, main windings W5-6-7 feed BR1 and charge C33-34 to +/-034V ... what you measured.

The middle and bottom ones are fed from W8-7-10 , rectified with D10-11-12-13 and, curious enough, because it's a voltage doubler circuit, they get roughly +/-10V and +/- 20V  :o

The middle +/-20V get regulated down to +/-15V with regulators Reg1-2 and thereb you have your +/-15V .

In the lower one, they again rectify with D4-5-6-7 , get roughly 10V (might be as much as 12V raw) and straight feed the fan, which probably is a PC 12V type (cheap and plentiful) and those 10/12V get regulatyed to +5V with Reg3 , for the +5V pin on DG212 .

Hawk

#7
Thanks JM! Off to work now but will study up on your info later today. Thanks for taking the time. (This is a fantastic forum!) :tu:

D11 is charred and reading O ohms both ways. Any comments on how that would effect the circuit/amp? Hoping to replace it tonight or Monday

Loudthud

Quote from: J M Fahey on February 06, 2015, 01:42:53 AM
Well, that's a usual problem .... but in this particular case, you are being unfair with poor old Marshall (RIP)

In page 2 of the schematic, it's clearly shown that +/- 15V feeds pins 8 and 4 of all dual Op Amps (RC4558 and NE5532) and pins 13 and 4 of DG212.

It is also clearly shown that said DG212 gets +5V on pin 12 and that pin 5 is grounded.

So stop bashing poor old Jim  :trouble

:lmao:

The only reason that schematic was drawn is to generate a netlist so that a circuit board could be layed out. The person who does the layout probably never even looked at the schematic. The netlist utility will tell the circuit board program to connect everything that is defined as "+15V", but not to the output of the 7815 because it's not defined as going to the net "+15V". See how there is "+5v" near the output of the 7805? There is also a "+5V" in the upper left corner near R79. Those two points would get connected.

Poor old Jim probably wouldn't even recognize what his company has become. All over that schematic there is text on top of wires, components and other text. This tells me that there is no pride in workmanship. Nobody even put their name on the schematic. Perhaps they don't even speak English.

g1

  Amen to that!  For me that pretty much describes the state of modern Marshall schematics.  Occasionally someone online will make up a drawing showing which connectors go to what.  Otherwise they are a nightmare.  I often wonder if the actual factory techs have secret maps of the connector routing.  If they still have such a thing as factory techs....

Hawk:  D11 is part of the supply feeding the 7815 regulator.  As you mentioned you are getting +15V to the op amps, it is not causing much problem at idle.  But perhaps when passing signal the +15V would be dropping and making trouble.
 

Roly

Quote from: LoudthudAll over that schematic there is text on top of wires, components and other text. This tells me that there is no pride in workmanship. Nobody even put their name on the schematic.

Indeed - "You never find just one roach in a kitchen".  These observations tell us a lot about the culture of the workplace and how management operates - sloppy is good enough.


Quote from: g1I often wonder if the actual factory techs have secret maps of the connector routing.

Having done some production line servicing I'd say that is highly likely, at least to bring newbies up to speed.  {but after that fiasco of the wrong value resistors in the output stage you really have to wonder...}
If you say theory and practice don't agree you haven't applied enough theory.

J M Fahey

#11
QuoteThe only reason that schematic was drawn is to generate a netlist so that a circuit board could be layed out. The person who does the layout probably never even looked at the schematic. The netlist utility will tell the circuit board program to connect everything that is defined as "+15V", but not to the output of the 7815 because it's not defined as going to the net "+15V". See how there is "+5v" near the output of the 7805? There is also a "+5V" in the upper left corner near R79. Those two points would get connected.

Poor old Jim probably wouldn't even recognize what his company has become. All over that schematic there is text on top of wires, components and other text. This tells me that there is no pride in workmanship. Nobody even put their name on the schematic. Perhaps they don't even speak English.

You are right, of course.

A BIG complaint I have about many "modern schematics" is that they are not so, but simply a printout of the .SCH file generated by the PCB layout software.

And parts do not interconnect ... because software would try to draw tracks between different PCBs which of course that's nonsense.

Software accurately draws tracks ending in connectors ... because that's all there is in any particular PCB.

Connector/wire harnesses are something else, of course they are drawn somewhere but they are sent to some OEM wire harness supplier.

At least Marshall in general shows connectors (not always); the worst offender I know is SWR.

Just try to make sense out of this:

not a f*****g pot , jack or switch is drawn!!!!  (because they are mounted in small sub PCBs   :trouble  )

In fact the drawing itself is clearly labelled xxxxxx.sch file.

Although Marshall has some schematics where the TUBES are not shown, but plate and cathode connected parts are ... for the same reason, tube sockets are chassis mounted  :duh

As of who signs the schematics, I'd often see small blank rectangles instead of letters.

Until my Son, who speaks fluent Japanese, installed Windows Asian Fonts Pack in my PC ... and all those little rectangles started showing up as Kanji characters, go figure.



Hawk

Thanks for these posts. I just figured I was missing something with regards to schematics. Good to know I'm not alone and I'm not completely out to lunch! Is it worth contacting Marshall/SWR to ask them why they don't supply proper schematics? Or is it a case of not wanting techs to fix their amps? Would they rather the customer chuck the amp in the garbage and buy another one? Seems that way to me...any thoughts?

J M Fahey

Quote from: Hawk on February 06, 2015, 08:24:10 PM
Is it worth contacting Marshall/SWR to ask them why they don't supply proper schematics?
Heavyweight techs I know have complained to Factory representatives, both Marshall and other brands, face to face (or over a couple beers) at NAMM ... they answer "you are the only one who complains, everybody else is happy" which of course is a bucket of lies .... but then,what to do?
They don't care.
Most Techs curse a little, shrug their shoulders and continue repairing.

Mind you, if you have the actual amp spread eagled on your bench, you actually see where the connectors go, so it's not that bad.

But it sure complicates "distributed repairing over the Internet" such as we are doing now.
QuoteOr is it a case of not wanting techs to fix their amps? Would they rather the customer chuck the amp in the garbage and buy another one? Seems that way to me...any thoughts?
Not that much; I think they are just lazy.

I guess the guy they hired to run the "Schematic design and improvement" department is working extra time:



Enzo

For the novice it must be infuriating.  Some drawings might have a takeoff point for say reverb, and there would be a short line off the signal path with REVIN next to it or something.  On the Marshall drawings there might be a little SEND next to the signal path between two stages, and we must infer what is going on.

I don't like it either, I like old school drawings as much as anyone.  But some things are to be universally understood, like the distribution of +/-15v, and it is common these days to put all the IC power feeds on one page, just like tube amps used t draw all the heaters in a row by the 6v winding.

But once you have seen the screwy Marshall convention used here, it will be familiar the next time we see a Marshall.