Hanns g 28" Monitor Power Supply Question

[Steven_C]

Re: Hanns g 28" Monitor Power Supply Question

Bill/Momanka,

I got the P/S board reworked and have very clean 5V, 12V, and 24V outputs at CN1 and CN3 under load (i.e. monitor turned on). The monitor turns on, I get a picture for 1 second, then the monitor turns off yet the blue led stays lit.

Where would you folks look to next?

[Steven_C]

Re: Hanns g 28" Monitor Power Supply Question

I will do that momanka. You and Bill have been very helpful and I am very grateful. I'm waiting for rework parts as they haven't come in yet.

[momaka]

Re: Hanns g 28" Monitor Power Supply Question

Nice work!
This is by far the most complex controller setup I have seen. It's good that you can make sense out of it.
Keeps us updated how it goes.

[Steven_C]

Re: Hanns g 28" Monitor Power Supply Question

momanka/Bill,

I just figured out my problem. I can't believe it's taken me this long. momanka got me to thinking that there might be another way to turn Q202 on because the polarities of the diodes on that timer chain made it impossible. There is. U202 is a DIP (lm393) whereby pin 1 is an output on that timer chain. It sources the base of Q202 low via R210.

When I probed the op amp output to see if it was pulling the base low, it was behaving as if the output was floating with Vs so I figured the ground was off. All of the other ic's are grounded to the same common yet this one wasn't.

Low and behold ... when I replaced C205 (which is an electrolytic next to U201), a tiny part of the corner pad that connects to R216 which is on that ground chain lifted/came off. It is hardly visible because the pad is immediately adjacent to R210. Thus, one would have to be following the trace from the other side to see that it was broken. It's only a tiny corner but that was enough to remove ground to U201. Hence, no Vs to U102.

I've got some 28AWG flexible tack wire comming in from digikey tomorrow. Once I get that tacked onto the op amp ground, I'll see if the primary is getting voltage. Maybe this will fix things.

[Steven_C]

Re: Hanns g 28" Monitor Power Supply Question

momanka,

The collector of Q202 does in fact also continue through jumper resistor JR9 down to R133 over to R134 then to pin 6 of U102. Those are good size resistors (i.e. 1k+) which makes me think that chain doesn't supply Vs to U102. However, Vs is tied to the emitter of Q202, while the collector is tied to Vs of U102 which makes me think this transistor turns on power to U102. Unless I'm mistaken, until I get power to U102, I'm sort of dead in the water with respect to getting voltage to the primary of the 12V/24V source.

Bill,

The transistor is in fact a PNP. It has diode drops from E-B and C-B. Conformal coating can wreak havoc on probing. There are in fact both diode drops.

I think you were right about the timer as Vs supplies a couple of other ic's on the board directly. This leads me to think that it is in fact a delay.

The designers did things right ... I've just got to understand exactly what they did so I have no plans on replacing anything until I understand exactly what component is not functioning. I'll pester you again once I can get power to that controller ic.

[PlainBill]

Re: Hanns g 28" Monitor Power Supply Question

I tried toggline PSON and I got the same result. The output of that timer seems dead or is not outputting anything when Vs on the timer goes to 14V DC.

One part of this circuit I haven't figured out is that the two diodes between the timer output and base seem to be in reverse to the base-emitter polarity. I get a diode drop between pin's 3 and 1 (i.e. emitter-base) of the transistor which makes me think it's a pnp. There are no other diode drops across any combination of pins on the transistor. Under this configuration, how is the transistor ever turned on? When the timer output goes low, the two diodes in series with base are reversed thus precluding the timer from sourcing any current whereas when it goes high, the base-emitter is np which would turn the transistor off.

I know the timer is mos based. The transistor, however is not because it is labeled as "BE" and not "DS", "DG", or "SG".

I'm a little confused by some of your comments. A good transistor should show diode connections B-E and B-C using a DMM with a diode test range. The fact that the emitter is at ~13 volts, and the collector is connected to Vs of U102 would suggest a PNP transistor.

I believe you indicated the timer output stays high. There are several possible explanations - the timer does not get triggered, the timer is defective, some other component is holding the output high.

My suggestion is to verify that the conditions are being met BEFORE replacing either the transistor or the timer. I spent 10 years teaching beginning electronics techs how to troubleshoot. A common beginner mistake was to note that "IC32, pin 3 is wrong. IC32, pin 3 comes from IC15, pin 6, so I will replace IC15" without verifying the inputs to IC15 were correct.

PlainBill

[momaka]

Re: Hanns g 28" Monitor Power Supply Question

Based on the pictures that member heli2reg provided in the other thread, it looks like the output of Q202 doesn't just go through JR9 and then pin 12 of U102. If you look at JR9, there's a trace that also goes under D107 and continues down, possibly going under D105 as well. I haven't been able to trace it further, but maybe this is where the power for U102 comes from?
Again, this is just an observation, though. The pictures that heli2reg posted can be found here if this helps (PSU is the same):
https://www.badcaps.net/forum/showpo...6&postcount=64

[Steven_C]

Re: Hanns g 28" Monitor Power Supply Question

I tried toggline PSON and I got the same result. The output of that timer seems dead or is not outputting anything when Vs on the timer goes to 14V DC.

One part of this circuit I haven't figured out is that the two diodes between the timer output and base seem to be in reverse to the base-emitter polarity. I get a diode drop between pin's 3 and 1 (i.e. emitter-base) of the transistor which makes me think it's a pnp. There are no other diode drops across any combination of pins on the transistor. Under this configuration, how is the transistor ever turned on? When the timer output goes low, the two diodes in series with base are reversed thus precluding the timer from sourcing any current whereas when it goes high, the base-emitter is np which would turn the transistor off.

I know the timer is mos based. The transistor, however is not because it is labeled as "BE" and not "DS", "DG", or "SG".

[PlainBill]

Re: Hanns g 28" Monitor Power Supply Question

Bill, I think I might have figured out the problem but I'm not sure. U202 mosfet based 555 timer.



It's output (i.e. pin 3 of U202) connects through two resistors and two diodes to pin 1 of Q202 (i.e. the base). When Vs to U202 is 14V, since no current is drawn at the transistor base, 15V DC is seen right up until the timer output. It is 15V except at the output of U202 which is 0V. I ohmed the output of U202 with respect to ground and it comes out to 300 or so ohms but slowly creeps up.

I think this timer ic might be bad. With no output, I'd expect it to be open and float with the other diodes and resisters in that chain as I'd expect the ic's output to a totem pole driver of sorts.

Does this sound logical? This timer isn't outputting anything at all.

That sounds possible, but don't stop there, you've been doing a great job.

My theory (OK, silly wild assed guess) is that the timer is there to insert a slight delay when the PSON signal goes high. This would allow the PFC time to fully charge the main filter cap. If my theory is correct, one of the pins of U202 is changing state at the same time PSON does. Check it out.

PlainBill

[Steven_C]

Re: Hanns g 28" Monitor Power Supply Question

Bill, I think I might have figured out the problem but I'm not sure. U202 mosfet based 555 timer.



It's output (i.e. pin 3 of U202) connects through two resistors and two diodes to pin 1 of Q202 (i.e. the base). When Vs to U202 is 14V, since no current is drawn at the transistor base, 15V DC is seen right up until the timer output. It is 15V except at the output of U202 which is 0V. I ohmed the output of U202 with respect to ground and it comes out to 300 or so ohms but slowly creeps up.

I think this timer ic might be bad. With no output, I'd expect it to be open and float with the other diodes and resisters in that chain as I'd expect the ic's output to a totem pole driver of sorts.

Does this sound logical? This timer isn't outputting anything at all.

[PlainBill]

Re: Hanns g 28" Monitor Power Supply Question

The collector is connected to Vs of U102.

The base and emitter are at the same voltage (i.e. ~ 13V DC). So Q202 is not being turned on. Normally, Vcc is tied directly to Vs of an ic. Perhaps my problem lies elsewhere ... I'm just not finding another direct connection to Vs of U102.

Two points - you are correct - the transistor is not being turned on if this is a PNP transistor. Do check for a base-emitter short, then trace the source of the base drive.

There are several ways to shut down a SMPS power supply. One is to turn off power to the controller. This controller has enable inputs, but it might be that the designer did not wish to use them.

As an aside, troubleshooting monitors is a form of detective work. Most times you open them up, find a number of bulging capacitors, replace them and you have a working monitor. Sometimes the answer is more involved, and you wind up kicking yourself for ignoring an obvious sign. Sometimes the answer becomes apparent only after many hours.

PlainBill

[Steven_C]

Re: Hanns g 28" Monitor Power Supply Question

The collector is connected to Vs of U102.

The base and emitter are at the same voltage (i.e. ~ 13V DC). So Q202 is not being turned on. Normally, Vcc is tied directly to Vs of an ic. Perhaps my problem lies elsewhere ... I'm just not finding another direct connection to Vs of U102.

[PlainBill]

Re: Hanns g 28" Monitor Power Supply Question

Whoa!!! you're moving too fast for me!!! Good work.

The pinout of SMT packages is somewhat standardized. Note the attached drawing of the SOT89 package. If a device is a transistor (Q designation) pin 1 is the Base, pin 2 is the Collector, pin 3 is the Emitter. Pin 2 and the tab (if any) are identical.

Of course, the pinout for dual diodes (D designation), voltage regulators (U or IC designation), and other strange devices will be different. As always, look up the datasheet.

PlainBill

[Steven_C]

Re: Hanns g 28" Monitor Power Supply Question

With respect to Q202, I made a mistake. The the center pin (i.e. pin 2) and tab (other side of the transistor) appear as a short. Pin's 1 and 3 are independent electrically. Pin 1 is connected to pins 4 and 8 of U202, while pin 3 connects to a surface mount transistor. I'm guessing that pin 3 is the on/off pin?

[Steven_C]

Re: Hanns g 28" Monitor Power Supply Question

After looking at that datasheet for the controller, it looks like the zener on pin 12 internal to the ic could be shorted to ground. The only way I'd know if that were the case would be to remove U102 and probe pin 12 again.

[Steven_C]

Re: Hanns g 28" Monitor Power Supply Question

It looks like 3 of the 4 pins from Q202 source Vs on U102. One pin on Q202 is sourced from pins 4 and pins 8 of U202. That pin is reads ~ 14.5V DC. So is Q202 configured like a diode even though it's a transistor? If so, then I'm guessing Q202 is open.

[Steven_C]

Re: Hanns g 28" Monitor Power Supply Question

I used some rubbing alcohol to get the conformal coating off of the ic. I can then read the P/N with a flashlight at the "right angle". It's P/N for U102 is L6598D.

https://cdn.badcaps-static.com/pdfs/...1b218cda6e.pdf

I have no power to pin 12 with respect to pin 10. What sources the required 10V to this controller?

[PlainBill]

Re: Hanns g 28" Monitor Power Supply Question

Bill, I don't have power to U102. There is no DC voltage at any of the 16 pins.

I have no idea what the part number is. Try using different lighting, looking at an angle, etc.

PlainBill

[Steven_C]

Re: Hanns g 28" Monitor Power Supply Question

Bill, I don't have power to U102. There is no DC voltage at any of the 16 pins.

[Steven_C]

Re: Hanns g 28" Monitor Power Supply Question

Bill, do you have a p/n for U102? It's a 16 pin surface mount ic which I don't see on your schematic. A p/n cannot be read off the surface of the ic so I'm uncertain what I'm looking in probing the various pins.