Sir pharaon . . . . .
This is what I suspicion . . . . . . over an extended time period of use . . . has happened with that unit of yours . . . .
A RUN DOWN OF ITS OPERATION . . . .
Referencing to the supplied FAIRCHILD tech application note, to then have its reference schematic.
Observing far left bottom corner and rising upwards, you see the origin of the MAIN AC to DC power supply of the unit and it entering into the C102 supply capacitor being across
BLUE A and
BLUE B. The DC voltage is now being upwards of 300VDC, then routes to the right from
BLUE A over to
BLUE C. There is a power flow path from
BLUE C to
BLUE D of the primary of the T101 switch mode power supply transformer . Then there is the
BLUE D connection made to the left to the
BLUE E connection to the drain connection of an internal POWER MOSFET *** built within the IC proper.
*** ( THIS POWER FET can handle upwards of 22 WATTS, so you can see the long time discoloration effect upon the nearby phenolic PCB area around it.)
All is in order for that power supply to spring into operation . . . . being only less the drive pulses into the FET.
BUT . . . that voltage would have already been initiated, at the instant that the unit was plugged into AC line power.
The voltage at
BLUE A would have passed thru R105 dropping resistor and fed into the IC pin 5 where there is an internal zener to feed initial internal supply voltage to get a timed burst of drive pulses to the gate of the power FET.
In this initializing start up procedure, the current consumption of the
BLUE C-D primary winding is sampled, as well as current sensing at I.C. #4 and voltage sensing at I.C. # 3.
Simultaneously the power drain on the transformer by the secondary supplies created by D203-D204-D207 and D205 have to be within established limits.
If all are in order, then the dedicated supply voltage source for IC101 will be active.
That consists of the
BLUE F-G winding and the D102 rectifier and the filter capacitor C106.
( If a sluggish turn on procedure has time evolved . . . expect a capacitance depleted C106. )
If this system now is constantly staying on and running after start up, then we need to inspect your across the board error in regulation.
That concerned area will be within the
PURPLE BOX area.
It interfaces thru 5 interconnects to associated circuitry.
There is the far left 817 optical isolator coupler, with its photo transistor that can progressively start pulling the Vfb voltage at IC pin #3 to the HOT ground.
Less voltage at Vfb tells the pulse width of the drive to the FET to shift as a voltage compensation / correction.
MEANWHILE at the other right half of the optical isolator, with its internal LED its normal brightness is established by its
BLUE I supply voltage and the conduction of the IC301 precision adjustable zener.
You can see a
GREEN circled
I prime supply point that flows down to
BLUE J and encounters its need to then flow thru two divider resistors of a voltage divider bridge consisting of R203 and R205.
That derived voltage fed into IC301, then establishes the conduction /brightness of the internal LED that is fed by
BLUE I .
The constant monitoring and interactions in compensatory shifting the frequency and pulse widths of the drive to the power FET give you a constant voltage from its different supplies and a response to shifts of loads on the supplies, within its design limits .
How you interpret the above info, relates to the fact that the most important section of stabilization relates to the
PURPLE BOX area.
And where is it obtaining its all important power ? . . . . . . The
GREEN CIRCLE areas of the 5VDC supply area.
If those
HARD worked filters . . . . .having had the HELL hammered out of them by incoming 10's to 100's of thousand pulses PER SECOND. . . . have not succumbed and are starting progressively dropping capacitance by incremental drying out of their internal electrolyte.
Then, fully expect a gradual on setting of progressive heating to further speed up the capacitance decay process.
Of the suspect C213 and C214 electrolytics of that 5VDC supply, the first C213 is most subject to the hardest HAMMERING, while the C214 receives some pulse buffering by the inline 4.7 ufd inductor.
So, should you choose to test this unit . . . the approved Mac Guyver technique . . . will need you to do this.
Choose your favorite and most used finger upon one hand . . . . . then revert to actually USING the like finger on the OTHER HAND.
Fire up ye olde power supply . . . . . test the temperature of the IC with aforementioned finger until you can
DEFINITELY detect a warming of the units top case.
Hopefully after this warm up time you can then move over to the COLD secondary side of the power supply and do a fingertip temperature test to the top metal caps of each of the 9 caps on that side of the transformer.
At the end, go to the 1 electroltytic on the HOT side of the board near the IC and check its temperature. ( a 47 ufd 50VDC ? unit)
If you find hot ones, solder a like value of electrolytic capacitor across its connections and do a voltage check of your supplies again.
If this gets you fixed . . . . . a MASS change out of all of the electrolytic capacitors would be in order.
CIRCUIT APPROXIMATION OF WHAT YOU HAVE THERE . . . .
73's de Edd
