220V to 12V 2Amp power supply modification

[duke37]

Keep a 7.6Ω resistor well away from this circuit. You could pass excessive current or generate much too high a voltage.

Put everything back to what it was and test that you have not damaged anything then put a 8.2kΩ across the 2.2kΩ resistor. If you do not have one, then use higher value say 10k. You will not get the voltage you want but will prove the principle.

 

[73's de Edd]

Sir Nauman Muhammad . . . . . . .

I will also try this one Will 7.6 Ohm resistor work?

Unless you typed in the wrong numbers above NO WAY would as low of a value as 7 decimal 6 ohms be used.

Sir duke37, was interpreting the 2.2K and 3.3 K as actually being the units existing R7 and R6 values, while that was not their values..
Actually, those were two very commonly found values, that one could salvage from surplus electronics.
In reality, the idea was then to use THAT pair to shunt across the EXISTING R7 and R6 pair to see the degree of voltage change.
In your case, you actually pulled out the R7 resistor completely, and installed your 6K combinational pair in place of it..
Which is OK, as we saw that the power supply then produced 7.5 volts instead of the original 12.54.
And YES, you need to pull that resistor pair and reinstall the original R7 10K resistor before moving on to work with the R6 resistor.
AND as I suspected, and had mentioned before, is that the change / altering the value of that R6 resistor is the one that seems to be the one that is going to get the voltage change that is needed.

OBSERVATION:
Made from post #9 's photos.

In looking all around the board, we see the use of resistors using the common 3 band color coding.
In the case of the R6 and R7 pair, which are being used to produce a more precise trimming in of of the zener voltage created with that IC2 . . .K431 . . .shunt regulator.
They are using more precise value resistors, with those two resistors seeming to be using a 4 band color coding.
HOWEVER that is not what my eyes color perceptions seem to be on those two resistors.
When I look at R7 color codes, I immediately want to see it as a 100 ohm unit with my eyes only seeing
BLACK and BROWN bands . . .with no RED's or ORANGES ?.
Now with you ohmming it out with your meter, you are saying it is being 10,000 ohms.
Now I know that my "read" of 100 ohms is far too low or even up it to 1010 ohm reading, if using 4 bands.
However, that 1010 value could then be a possibility at the low end of possible values.
I would expect it to be up in the 1000's of ohms, with that 10 K being on a possible high ranges end. Which seems to be the case.

So what needs to be done, is to initially pull your next resistor to be involved, which will be R6 and use your ohmmeter to read its value.
I see color bands of ORANGE BLACK RED YELLOW.
Which could read out as 3K if using 3 band color coding
I would expect it also to be up in the 1000's of ohms . . . . which that would be . . .but let's just find out what you actually are reading..



73's de Edd

 

[73's de Edd]

Sir Nauman Muhammad . . . . . . .

AN AFTERTHOUGHT:

You need to go back and reread my previous post for an add on.
Relevant to this post now, is the suggestion that you go to some " known resistances " on the power supply and test your ohms reading capabilities with your new meter.

R8's color coding reads as 820 ohms.
R5's color coding reads as 220 ohms.
R3's . . . .I think it is . . . its the larger resistor beside the one to be mentioned just below . . . . its color coding reads as 220 ohms.
The hidden identifier of the resistor to the side of RW1 precludes my reading it, BUT, that resistors value is 1200 ohms.

Concentraating on R7 again, a top view of that resistor, appears to either have either a slight dark shadow or a fine line between the YELLOW and RED markers.
Since that marking band is placed along the slope of an end cap, that is why it appears to be so fine, from either a top or side shot.

A VEWY-VEWY *** close observation of the side shot seems to confirm a fine VIOLET line, which would read out in making that unit a 4700 ohm unit, which would certainly be within suspected reasoning of that units value.

Take your meter and ohm out all of those prior resistors, to be sure that you are reading a value correctly.

A Future, VEWY-VEWY VALUABLE tidbit:

At the AC power line input, you will find RF1 immediately in line with one wire of the A.C. line.
That is a being a "blowable" fuse / fusible resistor, for use in opening the power line circitry due to an EXCESS power pull from the power supply or for a sustained over voltage AC line input.
Since it is unmarked, NOW is the time to take a reading of its resistance and mark it down somwhere nearby.
NOW you can know . . . . but AFTER one is blown . . . . . . you have no idea.
If replacement is required in the future, you just use a marginally wattage rated, common carbon film unit to replace it with.


***
Quote 1 from:
http://www.buzzle.com/articles/classic-elmer-fudd-quotes.html

Example:
http://www.barbneal.com/wp-content/uploads/elmer01.mp3


73's de Edd

 

[Nauman Muhammad]

Oh I see.. But there is bad news for me...
Although I tinned my tip and it became shiny, but whenever I am trying to solder its not melting from tip.. So I ordered a new one from Aliexpress..

However I will follow the instructions of above posts on Saturday as its a weekend and I am no more on leaves from my office

 

[73's de Edd]

Sir Nauman Muhammad . . . . . . .

Although I tinned my tip and it became shiny, but whenever I am trying to solder its not melting from tip.

The only thing that I might consider, is that if it was tinned and was left running hot so long that it lost its tinning.

If it is still shiny, it should still melt solder when it is being applied to it.

Also . . . . . . try dampening a cellulose sponge and squeezing out, such that most all of the water is lost,and it is just left being moist.
Then, you drag your soldering iron tip across it, and immediately see if solder applied to the then shiny tip will melt and adhere to the tip.
If so move on over to solder your connection, with a little more solder added.


The re-tinning procedure that I gave you should ALWAYS work right down to the condition such that the tip is too thin or too short to use.


73's de Edd