Repair SXJ2010 power supply board for learning

[harp]

Re: Repair SXJ2010 power supply board for learning

Do i put black negative probe of dmm on ground and red positive probe on the output rail, then set dmm to ohm resistance scale?

No. You first set dmm to ohm resistance scale and then put black negative probe of dmm on ground and red positive probe on the output rail. Doing that device is not conected in main.
Resistance wil be low like 20 ohm, 50 ohm... but if you see 0-1-2... ohm it is shorted on output.

[mikey5791]

Re: Repair SXJ2010 power supply board for learning

No. You first set dmm to ohm resistance scale and then put black negative probe of dmm on ground and red positive probe on the output rail. Doing that device is not conected in main.
Resistance wil be low like 20 ohm, 50 ohm... but if you see 0-1-2... ohm it is shorted on output.

By setting dmm to ohm resistance scale 200 ohm, and put negative probe on ground and red
positive on output yellow rail (+12v), i got reading larger than 200 ohm. If the red positive on output blue rail(-12v), i also got reading larger than 200 ohm.
In addition, if the red negative on output orange rail(3.3v), i got reading of 15.6 ohm. If the red negative on output red rail(5v), i got reading of 10 ohm. Can we conclude that there is no short on output? The problem of no output could be due to other cause.

[stj]

Re: Repair SXJ2010 power supply board for learning

do you have a scope?

[R_J]

Re: Repair SXJ2010 power supply board for learning

On pin 12 vcc TL494 ic, i got +13.7vdc stable.

Sorry i am not able to identify and relate the drive transformer (T2 in schematic) as i tried to trace each of the transformer pin to the schematic one. It is strange the pin connection to components on board also do not match what was on the schematic.
But voltage supplying to the middle transformer marked YW EE-16-10(primary side 5 pins and secondary 3 pins) is less than 5vdc.
While voltage supplying to the biggest transformer marked DX EC-35-10-02 (primary side 2 pins and secondary 5 pins) also measured less than 5vdc. Is this normal or something not right?

When you measure voltage on the PRIMARY side you must use HOT ground, this is the negative of the bridge rectifier or in this case the negative of the large capacitor nearest the corner of the board (not the one closer to the middle)

The voltage on the small transformers (3 pin side) is too low, I suspect there could be a shorted transistor or diode , that voltage should be around 10~13 volts. This transformer supplies the drive signal to the primary side transistor bases.
The YW EE-16-10 is the drive transformer and is the same as T2 in the schematic

[mikey5791]

Re: Repair SXJ2010 power supply board for learning

do you have a scope?

Stj,
No, i do not own or have access to a scope.

R_J,
Done measured voltage on the primary side by putting dmm black probe to negative side of large cap nearest to corner of the board and red positive probe to the middle trans(YW EE-16-10). Those pins nearest to primary side(nearer to main cap) reads from 0.7 to about 1.1vdc while those pins on the secondary side(nearer to second heatsink) reads around 102vdc.

In addition, i also got about the same voltage reading on the big trans (DX EC 35-10-02); low voltage about 1vdc on primary side and about 102 vdc on secondary side. Will recheck for any shorted transistor or diode.

[harp]

Re: Repair SXJ2010 power supply board for learning

By using a multimeter, i got about 100vdc on each of the main cap of 470uf, 200v. There is no other voltage on the secondary side apart from the 5vsb purple wire.
...
In addition, i also got about the same voltage reading on the big trans (DX EC 35-10-02); low voltage about 1vdc on primary side and about 102 vdc on secondary side.

Have you or have you not voltage on secondary side?

I not shure, but maybe, directly on transformer pins you should check with dmm in vac mode rather then vdc, it is pulsing dc?

Do you check voltage on green wire?

[R_J]

Re: Repair SXJ2010 power supply board for learning

Stj,
No, i do not own or have access to a scope.

R_J,
Done measured voltage on the primary side by putting dmm black probe to negative side of large cap nearest to corner of the board and red positive probe to the middle trans(YW EE-16-10). Those pins nearest to primary side(nearer to main cap) reads from 0.7 to about 1.1vdc while those pins on the secondary side(nearer to second heatsink) reads around 102vdc.

In addition, i also got about the same voltage reading on the big trans (DX EC 35-10-02); low voltage about 1vdc on primary side and about 102 vdc on secondary side. Will recheck for any shorted transistor or diode.

When you measure voltage on the PRIMARY side you must use HOT ground, this is the negative of the bridge rectifier or in this case the negative of the large capacitor nearest the corner of the board (not the one closer to the middle)
When you measure voltages on the SECONDARY side you must NOT use the HOT GROUND. you MUST use the secondary side ground (Black wires on the plug)

I want only one measurement and that is using COLD or secondary GROUND, measure the voltage on BOTH SIDES of what I have marked as R10, If you do not have around +10vdc on both sides there is likely a shorted Q3,Q4,D10 or D11.

You need to understand the schematic as it is very close to the one you are trying to fix, the only thing that may be different is the component numbers ie: R10 may be R?? on your board

I Re-read post #9 and I would suggest replacing the electrolytic caps with there original values, Higher capacity is likely going to change the operation of those circuits, and the may not work correctly.

[R_J]

Re: Repair SXJ2010 power supply board for learning

Have you or have you not voltage on secondary side?

I not shure, but maybe, directly on transformer pins you should check with dmm in vac mode rather then vdc, it is pulsing dc?

Do you check voltage on green wire?

Most dmm's will not read a 40Khz signal, it is too high a frequency. That's why we use a oscilloscope
OP is not using the proper ground when checking DC voltages

[mikey5791]

Re: Repair SXJ2010 power supply board for learning

When you measure voltage on the PRIMARY side you must use HOT ground, this is the negative of the bridge rectifier or in this case the negative of the large capacitor nearest the corner of the board (not the one closer to the middle)
When you measure voltages on the SECONDARY side you must NOT use the HOT GROUND. you MUST use the secondary side ground (Black wires on the plug)

I want only one measurement and that is using COLD or secondary GROUND, measure the voltage on BOTH SIDES of what I have marked as R10, If you do not have around +10vdc on both sides there is likely a shorted Q3,Q4,D10 or D11.

You need to understand the schematic as it is very close to the one you are trying to fix, the only thing that may be different is the component numbers ie: R10 may be R?? on your board

I Re-read post #9 and I would suggest replacing the electrolytic caps with there original values, Higher capacity is likely going to change the operation of those circuits, and the may not work correctly.

Yes noted, i am using the COLD secondary ground and measured R10 dc voltage. It reads 1.5vdc and it is likely shorted Q3,Q4,D10 or D11.
I will look for electrolytic caps with the original values.

[R_J]

Re: Repair SXJ2010 power supply board for learning

Yes noted, i am using the COLD secondary ground and measured R10 dc voltage. It reads 1.5vdc and it is likely shorted Q3,Q4,D10 or D11.
I will look for electrolytic caps with the original values.

Is that 1.5 volts on R10 on both sides? Is it a resistor or just a jumper wire? as I can't tell from the pictures.
If the two small transistors or there diodes check ok, then check the voltage on R13 (1.5kΩ) it might be open or went high in resistance or D15 may be bad (open).
This PWM circuit must be working to drive the main power supply to get any of the voltages besides standby. This 13v standby voltage also supplies the power on circuit etc. via D15, R13,D14,R54,R51

[mikey5791]

Re: Repair SXJ2010 power supply board for learning

R_J,
The pwm pin 12 vcc doesn't seem to output any voltage. So, i found a burnt resistor of smallest type i think is 1 ohm 0.125w(brown, black,gold,gold) underneath the first heatsink. The problem is i am not able to find such rating of resistor at our local online store. Attached picture shows the length is about 3mm.
The local store are selling 1 ohm 0.25w, which rating slightly higher (length about 6mm).
Can this type 0.25w be used as replacement? Or it will affect the operation of the psu board.

[stj]

Re: Repair SXJ2010 power supply board for learning

red purple brown? thats 270 ohm not 1 ohm.
why did it burn though?

[mikey5791]

Re: Repair SXJ2010 power supply board for learning

red purple brown? thats 270 ohm not 1 ohm.
why did it burn though?

Found 3 mini resistors burned and desoldered from board, tested for resistance but all 3 reads open. All 3 resistors are the smallest type 0.125w rating(3mm long) but the color are charred. I tried to figure out the resistor color as best possible.
As per the picture, R8 (brown,black,gold,gold) and R5 seems to be 1 ohm while R6(red,violet,red,gold) should be 2.7k ohm. Not sure what causes the resistors burned, there is spark during switch power on but fuse is still good.
Tracing R8, one end connects to left pin of transistor E13007 while other end connects to negative leg of 10uf 50v cap. Tracing R5, one end connects to left pin of transistor E13007 and other end connects to D5, diode IN4148. Tracing R6, one end connects to left pin of transistor E13007 while other end connects to right pin of that same transistor, then goes to 4th pin of the middle drive transformer.
Since all the 3 resistors are open, can i replace them with 0.25w rating(about 6mm long) and not affect the operation? Kindly advise.

[R_J]

Re: Repair SXJ2010 power supply board for learning

The resistors burnt because the transistor shorted, you can use o,25 watt if you like and they fit, I would replace the 4 1N4148 diodes as well.
Check the two diodes across the transistor E~C.
Your R6 = R7 (on schematic) R5=R5, R8=R4 (on schematic)

[mikey5791]

Re: Repair SXJ2010 power supply board for learning

Is that 1.5 volts on R10 on both sides? Is it a resistor or just a jumper wire? as I can't tell from the pictures.
If the two small transistors or there diodes check ok, then check the voltage on R13 (1.5kΩ) it might be open or went high in resistance or D15 may be bad (open).
This PWM circuit must be working to drive the main power supply to get any of the voltages besides standby. This 13v standby voltage also supplies the power on circuit etc. via D15, R13,D14,R54,R51

Today i changed the burned 0.125w resistors at R8,R5 and R6 using the 0.25w rating. In addition, i also replaced c24 and c22 to their original 2.2uf50v and 1uf50v caps respectively.

Upon mains power on, R10(100 ohm resistor) has no voltage reading. First, using my dmm being black probe on cold ground and red probe on pin 12 vcc of TL494 shows zero voltage. I know this pin is supposed to show about 13vdc standby. But strangely,. D15 the IN4148 diode had 12.54vdc while R13(1.5 k resistor) also has 12.54vdc. Both resistor and diode tested good and not open or high in resistance.
How is it possible that pin 12 vcc has no voltage but there is voltage on diode and resistor? That really puzzled me.

[R_J]

Re: Repair SXJ2010 power supply board for learning

You had the voltage back in post #20, what happened? The only thing between D15 anode and pin 12 of the ic is the circuit trace, Also follow the trace from the 13v source to the pin, there might be something between the two points that is a problem?

[R_J]

Re: Repair SXJ2010 power supply board for learning

Here is the +12v stby path traced out from the source diode to the ic vcc

[mikey5791]

Re: Repair SXJ2010 power supply board for learning

You had the voltage back in post #20, what happened? The only thing between D15 anode and pin 12 of the ic is the circuit trace, Also follow the trace from the 13v source to the pin, there might be something between the two points that is a problem?

Back then i had 13.7vdc at pin 12 of ic. Just now i checked there is no continuity between pin 12 and D15 anode. Found old solder no contact and i resolder again. Now pin 12 vcc of ic had 13.7vdc, thru anode of D15 diode, then to R13 (1.5k ohm resistor).
Following the trace, R13 actually connects to collector pin(right side pin) of transistor 2222A but the voltage up to here is just 1.6vdc. Setting my dmm to diode mode, and putting black probe to middle pin of this transistor, red pin to the outer pins gives some reading. Is there a proper way of testing instead?

[R_J]

Re: Repair SXJ2010 power supply board for learning

It is a transistor, place one probe on the base and check the collector and then the emitter, it should read .6xx with the probes one way and open the other way. (like checking a diode). Then check between the emitter and the collector, this should read open both ways, but in this circuit it will read the external diode that is across the e-c of the transistor.

[mikey5791]

Re: Repair SXJ2010 power supply board for learning

Hi all,
Been busy with other personal errands for the past few days. Yesterday i found more failing 0.125w resistors reading "1"open around pwm IC TL494. I guessed those resistors must have gone thru lots of stress and over a decade of use, thereby causing most of the failure. Due to limited time and short of full components, i think is time for me to stop diagnosing any further on this partially working board. Will keep it for spare part though.
Overall, it has been a good learning experience with helpful kind guidance and enriching my electrical knowledge especially in looking at schematic and tracing to pcb board. Thank you to all whom had contributed their time to respond and to guide.