Marshall MG250DFX How does the DG212CJ function?

[g1]

Shows both having the speaker outputs at W12,13,14,15.

...In my mind these speaker outs should come off pin 7 and 8 as they are the neg, pos signal.

No on both counts.  We do not know where any W connections are going except where shown. W13&15 are shown going to outputs, the others not.  Suggest ignoring them.
  The output IC shown on page 3 is the power amp.  The other circuit with W12 thru W14 is not.
  Pins 7&8 are +/- supplies, not signal.
  The output IC data sheet may be helpful.

[Hawk]

Great stuff thanks. When you pointed out W13 and 15 as outputs the penny dropped. :tu:
The only thing that doesn't makes sense is when you say that  7 and 8 are +/- supplies. According to the TDA7293 Data Sheet pin 13 and 15 are the supplies and 7 and 8 are +/- signal which, on the schematic on page 3, go from 9 on Con1 to pin 14(out) of the TDA7293. To me that would make more sense to have, but I may be missing something. Thanks again!

[Roly]

Dunno what datasheet you are looking at but the one I have from;

http://www.digchip.com/datasheets/parts/datasheet/456/TDA7293-pdf.php

...shows pins 2 and 3 as the inverting and non-inverting inputs and pins 7 and 8 as the supplies for the "signal" or front end of the power amp, and pins 13 and 15 as the supply pins for the power end of the amp.

[Hawk]

Thanks Roly. I have the same data sheet and what you say makes total sense. I think what I'm finding confusing is how the TDA relates back to CON1 and all its 15 pins. How is it that pin 10 of the TDA goes to the  +power rail (12,14,15 of Con 1)? When I look at the TDA block diagram pin 10 is considered to be part of the "mute" and I find it hard to determine if that should be part of the -rail or + rail. Thanks.

[g1]

  Sorry I wasn't more clear about pins 7&8, using the word "signal" on the data sheet was a bad choice for them, as Roly mentioned, pins 2 & 3 are the inputs.
  The chip has both standby and mute options.  In the Marshall they utilize the standby (there is a delay circuit which acts to mute the chip during power up at sector A4 of schem.pg2).  Because they use the standby, they tie the chip mute to the +supply so chip mute is never activated.

[Hawk]

No problem thanks for these details g1. But where exactly is sector A4? I'm on page 2 of the schematic and  can't see it...thanks.

[g1]

  On the borders of the page are numbers (top and bottom) and letters (sides).  It's just a way of describing areas of the schematic.  Sort of like latitude and longitude of  a map.  So find A on the side and follow across to 4 on the bottom and you will have the general area.

[Hawk]

Never noticed those numbers and letters before but I found the are you are talking about. Great!

[Hawk]

Anyone suggest some good ways to put test leads on hard to access components? My multimeter leads often get wedged in between other components and I can't get the metal testing tip down against the component lead. Sometimes I've attached a solid core wire and that has worked. Can you purchase a test lead with a long shaft? I'm thinking of my SSW220 and it's output transistors. I want to check voltages on it's pins but can't get me test leads to reach as they're just isn't room.

[J M Fahey]

Spring loaded "hook" test leads

[Hawk]

Nice, thanks.

[Roly]

I find these small single hook clips very irritating because they don't get a good grip on the lead under test and keep slipping off.

These double (and sometimes triple) "gripper" probes are far superior;



Much longer, flexible, and a much more secure grip.

[Hawk]

Thanks for that! Where did you buy them?

[Roly]

Where I would buy them;

http://www.altronics.com.au/p/p0409-parrot-style-test-hook-probes/


It would seem that "parrot" is a bit of a keyword for these (and if you have ever been bitten by a parrot you will understand why).

I have both types kicking around because, like all tools, each has its strengths and weaknesses.

[DrGonz78]

Both those test leads definitely have their strengths and weaknesses. I have had numerous sets of the hook type ones and they always fell apart on me after months of use. I like how they could fit in tight spaces but they were sometimes not long enough. Then I got the longer double gripper probes and they were way better. I have retired those too as they now measure about 4-7ohms of resistance for some reason. At first I thought it was my extender leads that connect the DMM to the probes, but those were fine. I then measured across each gripper probe and both of them have the exact same resistance problem. Perhaps some quality control problem in China while making those probes? Oh well now I have picked up a nice pair of Fluke AC280 Sure Grip probes and I they are way more durable/reliable. However, I am still looking for some smaller probes that won't fall apart or become a resistor.
http://www.google.com/url?sa=i&rct=j&q=&esrc=s&source=images&cd=&cad=rja&uact=8&ved=0CAcQjRw&url=http%3A%2F%2Fwww.amazon.com%2FFluke-AC280-SureGrip-Hook-Clips%2Fdp%2FB001WADEAK&ei=btLpVKD6MYGdgwSav4LoBw&bvm=bv.86475890,d.eXY&psig=AFQjCNH4uOHfdVEE1sYsfuN6m_lSo_PiVQ&ust=1424696281247259