Optiquest Q9 No secondary Voltages

[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"

[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

[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

[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.

[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

[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

[alexanna]

Re: Optiquest Q9 No secondary Voltages

The part number and revision can be seen in the first photos.
If you need any thing else let me know

[Rtech]

Re: Optiquest Q9 No secondary Voltages

I just don't know,with a diagram suggestions are easier,i don't think removing the coupler,would have removed the secondary load.I may have a scrap Optiquest board come to think of it....will have a look tomorrow....please confirm the ID on your Board to ensure I am looking at like for like.

[alexanna]

Re: Optiquest Q9 No secondary Voltages

I have substituted a new octocoupler,And checked the ground on pin #1 of SG 5841 The ground is at most 6MV. There is no change in behavior.
When I removed the octocoupler, would that have disabled the secondary side to rule out a excessive secondary load causing the fluctuating voltage I am seeing?
Or is there another way I should go about doing this?

[Rtech]

Re: Optiquest Q9 No secondary Voltages

Well,then you diconnected the Optocoupler,so perhaps you need to swap it ??Also Earth/Ground problems,can also cause the odd thing,so it must be worth checking the Ground on pin 1.Apart from those I cannot think of anything else at the moment.

[alexanna]

Re: Optiquest Q9 No secondary Voltages

^ I was really trying to keep the swap to a minimum, But so far I've done a good job.
Working tertiary has been substituted
The 2 diodes near the power mosfet
The sense resistors check Ok
A working mosfet
A sg5841 that was working from another board
A new run cap
There are some glass diodes that appear to be involved D804 and Z802. I have not done anything with these other than compare from board to board.
Is there anything I have missed?

[Rtech]

Re: Optiquest Q9 No secondary Voltages

In that case. looks like a 'swap' job,piece by piece until the culprit can be found.Most input sides have only a few components,so try 2 at a time ,and you should soon locate the faulty bit.Although these transformers rarely go wrong,I certainly would not ignore it,as the tertiary winding is an integral part of the start up.

[alexanna]

Re: Optiquest Q9 No secondary Voltages

Unfortunately I have not found a circuit diagram I am just trying to compare things between a working board and the one that's not working

[Rtech]

Re: Optiquest Q9 No secondary Voltages

What cct Diagram are you looking at ???,send a url if possible.

[alexanna]

Re: Optiquest Q9 No secondary Voltages

My understanding of how the SMPS works if very fogy .Pin #3 of the SG5841 is fed from R819, And the voltage is jumping all over the place.
I have tried to isolate to one side of the P/S, I am unsure if I did it the correct way. Earlier in the process I did remove the photocoupler to see if voltages would stabilize on pin #3 of the SG component.
Other than getting grand Uncle Blackbeard's blunder buster out, I'm unsure on what to try.
Is there any thing I have missed on the primary side that could be causing this problem?
You say The SG5841 and SG6841 have similar pin outs, Could SG6841 be used as a substitute, I notice that part on a lot of used power supplies that I have sitting around?
I am totality up to suggestions so if anyone has ideas I am willing to try.

[Rtech]

Re: Optiquest Q9 No secondary Voltages

I recently has a fault caused by a bad SG6841,same pinout as your SG5841,and the fault symptom on the controller was 4.52volts on the Vin(pin3),indicating it was shot.On replacing it,and the monitor now working fine, I again measured the Vin to confirm my diagnosis,and then it read 16-18v varying.So your Vin, although varying,the range of that variation seems grossly excessive.

[alexanna]

Re: Optiquest Q9 No secondary Voltages

Clicking means the PSU is in protect mode. Be *very* careful - removing capacitors will NOT shut a power supply down guaranteed and could very well fry stuff! It will shut down a well-behaved power supply, true. However, IF the power supply shuts down, ALL rails go down since they're regulated by the same controller, i don't know what you are aiming for here.

If you want to disable a voltage rail remove the switching diodes (this will still kick it in protect mode), or better, just unplug whatever is feeding off that rail or cut the trace after the capacitors. If it's not possible to unplug the load, cutting the trace will remove any load and the power supply should come up.

Well what I'm trying to do I figure out why I have unstable voltages on the primary side of the power supply, and why the standby power will not start.
I really just don't know he correct way to go about it.

[Th3_uN1Qu3]

Re: Optiquest Q9 No secondary Voltages

Clicking means the PSU is in protect mode. Be *very* careful - removing capacitors will NOT shut a power supply down guaranteed and could very well fry stuff! It will shut down a well-behaved power supply, true. However, IF the power supply shuts down, ALL rails go down since they're regulated by the same controller, i don't know what you are aiming for here.

If you want to disable a voltage rail remove the switching diodes (this will still kick it in protect mode), or better, just unplug whatever is feeding off that rail or cut the trace after the capacitors. If it's not possible to unplug the load, cutting the trace will remove any load and the power supply should come up.

[alexanna]

Re: Optiquest Q9 No secondary Voltages

A little bit of an update,
I have replaced diodes 802 and 808 these are on either side of Q801 mosfet ,There are two large resistors R821 and R803 and these check OK.I also have substituted a good mosfet Q801.
I have removed the 24v, 12v and 5v capacitors on the secondary side to try to rule out something on the secondary drawing an excessive load, I am still noticing the voltage fluctuation to pin 3 of the SG component.
I am noticing something different today. If you look at the picture of the bottom side of the power supply just to the right of the A.C. plug. CR904 and CR901 are clicking, its very noticeable.
These are BAV99 which are switching diodes, Could this be a clue of anything else to check