Optiquest Q9 No secondary Voltages

[alexanna]

Re: Optiquest Q9 No secondary Voltages

There are 3 fairly large schotty diodes on this power supply .With one leg lifted from the PCB there is what I feel is a big difference in the voltage drop across them,1.5v on the first 2.5v on the second and 4.5 volts on the last one.I will compare them with the good board in the A.M.
Wonder if that may be my problem

[Rtech]

Re: Optiquest Q9 No secondary Voltages

Unfortunately, the board I had was not the same as yours.If I remember right however,the last time I saw the three diodes as yours are,and had a non working board, the problem was in the Inverter side,and voltages did vary a great deal.I presume the voltage drops you are measuring are DC ??therefore you are getting something from the power side to get these readings,so if that is the case,then the secondary side needs to be investigated.

[alexanna]

Re: Optiquest Q9 No secondary Voltages

There are 3 fairly large schotty diodes on this power supply .With one leg lifted from the PCB there is what I feel is a big difference in the voltage drop across them,1.5v on the first 2.5v on the second and 4.5 volts on the last one.I will compare them with the good board in the A.M.
Wonder if that may be my problem

The large diodes D801-D803-and D805 were removed from the circuit and measured with the diode test of the DMM.
D801=4.5v D803= .162v and d805= .227v
These are the same reading on both working and non working board.
Back to square one

[Lumberjack777]

Re: Optiquest Q9 No secondary Voltages

Pin #3 of the SG is the Vin at its very unsteady jumping from 12 to 17 volts rapidly.
Pin 7 VDD is jumping from 11 to 15 volts .
Pin #2 is FB and its jumping from 0 to 4.5 volts rapidly.

Hey Alex. Did you ever get that resolved? I'm having the exact same problem with an Acer 1916 power/inverter board. Voltages are jumping all over the place on my SG6841 PWM Controller.
Pin#3 goes like 5.XX, 8.XX, 11.XX, 14,XX, 17,XX. Then starts again at a low number and bounces up to around 18v. Same with Pin #7 though I noticed it doesn't get much over 15v.

Sure would be nice if we could get this resolved. Does anybody out there know what causes these voltage fluctuations on pins 3 and 7 of an SG6841 PWM Controller? Thanks.

[PlainBill]

Re: Optiquest Q9 No secondary Voltages

Hey Alex. Did you ever get that resolved? I'm having the exact same problem with an Acer 1916 power/inverter board. Voltages are jumping all over the place on my SG6841 PWM Controller.
Pin#3 goes like 5.XX, 8.XX, 11.XX, 14,XX, 17,XX. Then starts again at a low number and bounces up to around 18v. Same with Pin #7 though I noticed it doesn't get much over 15v.

Sure would be nice if we could get this resolved. Does anybody out there know what causes these voltage fluctuations on pins 3 and 7 of an SG6841 PWM Controller? Thanks.

It's either the cap on pin 7 (Vdd), a shorted output diode, or a short on one of the secondaries. The most common cause is the cap; that is why we recommend ALWAYS replacing that cap.

PlainBill

[alexanna]

Re: Optiquest Q9 No secondary Voltages

It's either the cap on pin 7 (Vdd), a shorted output diode, or a short on one of the secondaries. The most common cause is the cap; that is why we recommend ALWAYS replacing that cap.

PlainBill

No I was not able to get that one repaired.
I did have a Samsung that was doing something very similar, fluctuating voltage going to the SMPS.
I got lucky finding the problem, every time I would touch my DMM lead to one of the output diodes the voltages would stabilize .The problem turned out to be a bad connection at the solder pad one of the output diodes

[Lumberjack777]

Re: Optiquest Q9 No secondary Voltages

It's either the cap on pin 7 (Vdd), a shorted output diode, or a short on one of the secondaries. The most common cause is the cap; that is why we recommend ALWAYS replacing that cap.

PlainBill

Bill, excellent troubleshooting. We found the culprit. Thanks for your help too Alex.

1. First off I replaced the cap leading from the #7 pin. (vdd). It was a 4.7uF 50v that I had already tested out of circuit with my blue esr meter. The reading was 1.0 exactly. But it was *gasp* .... a "crapxon". I went ahead and replaced it with a rubycon with an esr of .88
I doubted the esr difference, would make a difference. But I replaced it to make up for my laziness the first time. I replaced the 47uF 50v nearby for the exact same reason.
2. Tested all the diodes. They all tested good. So I plugged in the power to take those readings from the SG6841 again. Still fluctuated
3. Went back to read Bill's notes. "or a short on one of the secondaries". I took out my 10X jewelers loop and started examining the board. Much to my amazement, I spotted a "solder bridge"(to nowhere)
I had purchased this monitor after a friend had "attempted repair". Dummy me. I figured he knew how to solder.
A little solder wick in 2 suspect areas and those bridges came a tumbling down. Time to test again.
Pins 3 and 7 had stable voltage! Slapped it together and it worked!

Lesson learned...... Always, ALWAYS check for solder bridges, or solder splash. Especially after somebody ahead of you "attempted repair".

Thanks again Bill! As the younger kids would say, "you da man"