Optiquest Q9 No secondary Voltages

[alexanna]

I have a Optoquest Q9 monitor that has no secondary voltages.
It uses a SG5841 SMPS controller.
I have a steady 162volts D.C. at the large capacitor.
I can follow what I think are a series of startup resistors R812 R814 and R819
The voltage starts up stable at 162 volts on R813.The voltage through R814 starts to become unsteady jumping from 59volts up to 62volts,and the voltage is really unsteady through R818 jumping from 12volts to 17 volts very rapidly.
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.
There is 50v 10Uf C824 that has been replaced. The voltage on it is jumping from 11 to 15 volts rapidly
Because of the unsteady voltage at the Vin pin#3 I suspected a problem with the SMPS controller, I have replaced it with one I had from a spare board that was working. It did not make any difference the voltages stayed the same.
http://pdf1.alldatasheet.com/datashe.../SG5841SZ.html
What am I missing?

[smason]

Re: Optiquest Q9 No secondary Voltages

Is this with a load or no load?

[alexanna]

Re: Optiquest Q9 No secondary Voltages

No logic board is hooked up,so no load.

[PlainBill]

Re: Optiquest Q9 No secondary Voltages

No logic board is hooked up,so no load.

Don't forget the inverter is a load. We had one like this with a bad transistor on the inverter Also, check the tertiary winding which provides power for the SMPS controller.

PlainBill

[alexanna]

Re: Optiquest Q9 No secondary Voltages

Don't forget the inverter is a load. We had one like this with a bad transistor on the inverter Also, check the tertiary winding which provides power for the SMPS controller.

PlainBill

Please correct me if I am wrong but, I wouldn't think the inverter would be suspect with no logic board connected.
I did remove the capacitor for the inverter section so I guess that will disable it?
I have another identical power supply that works; I measured the resistance of the tertiary winding. I did not look it up but I guessed that's the SMPS coil. There was no difference noticed in the readings on either board.
I do not have a ring tester so I swapped the good coil to the board I am having problems with, No difference in the voltages were noted.

[jetadm123]

Re: Optiquest Q9 No secondary Voltages

There's also a diode and resistor off the tertiary winding that provide voltage to the 10uf cap. Try checking those two devices.

[PlainBill]

Re: Optiquest Q9 No secondary Voltages

Please correct me if I am wrong but, I wouldn't think the inverter would be suspect with no logic board connected.
I did remove the capacitor for the inverter section so I guess that will disable it?
I have another identical power supply that works; I measured the resistance of the tertiary winding. I did not look it up but I guessed that's the SMPS coil. There was no difference noticed in the readings on either board.
I do not have a ring tester so I swapped the good coil to the board I am having problems with, No difference in the voltages were noted.

A properly working inverter would not cause problems. But if one of the drivers were bad, that could cause problems.

If you look at the schematic of the power supply / inverter of just about any monitor you will see the primary winding, the secondaries, and there is a third winding that provides run power for the SMPS controller. If that is open or shorted, or if the diode is open or shorted you will get the symptoms described.

PlainBill

[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

[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

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.

[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

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

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

[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

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

^ 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

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 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

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

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