Can someone explain for me?

[lilshawn]

I'm having an issue with the regulation circuit of a monitor. It's not a normal SMPS style B+ but instead some kind of transistor/zener type regulation. (I think)

can someone explain how this circuit operates so I can get to figuring out what it's issue is?

please, thank you, as always...

lilshawn

[KhronX]

The group made up of Q901, D991, the inductor and C906 reminds me a lot of what you see in buck converters. Some sort of self-oscillating circuit, i'd reckon.

Something along the lines of http://alignment.hep.brandeis.edu/electronics/A2080/HTML/Breadboard.gif

https://en.wikipedia.org/wiki/Buck_converter

[lilshawn]

see, I kind of thought that, but it doesn't have a tank circuit as such. the one you see there, (T901) is a small transformer used to drive another circuit (unless it's also being used as such). there is a small inductor on the B+ output (inline) that filters some high frequency noise out of the b+ to smooth it a bit.

I think I have an issue with the zener because i'm getting almost no regulation at all. (running in 130v range instead of 90v range.)

[KhronX]

You'll see a second winding on the main inductor in example circuits for (more "fancy") PFC controllers, using that for waveform sensing, or supplying the controller chip itself.

And it doesn't necessarily need a tank circuit, as such.

http://www.romanblack.com/smps/smps.htm

The only (low-resistance) path the current can take, to get to the output, is through the primary of that T901, so chances are good it's basically a buck inductor. I mean, the way the "main" components are arranged would be too much of a coincidence for this whole thing to NOT be a buck converter, imho

[N2IXK]

Looks like a pretty straightforward linear regulator to me.  My guess is that the T901 transformer is being used to generate a "kick start" pulse when power is first applied, perhaps to start the oscillator in the horizontal sweep/high voltage circuitry?

If you are getting a high voltage output with no regulation, I would suggest checking the series pass transistor (Q901) and associated circuitry first.  As a general rule of thumb, the most likely components to fail are those that dissipate a lot of power....

[T3sl4co1l]

It's some kind of self-oscillating converter, yes.  R905 gives positive feedback, making the Q901-Q903 area a power Schmitt trigger (with the current capacity of Q901!).

It's not going to be very fast, because the transistors are only turned off with base-emitter resistors.  (Q901 is wired as an emitter follower and doesn't fully saturate, so at least it won't go terrifically slow.)

It won't be bistable, because the inductor will eventually charge up with more current than the circuit can handle, pulling it off.  That is, if current rises to a few amperes, the switch node's voltage is pulled down, turning off Q903, and then Q902 and Q901.  This incurs a large gulp of switching loss, as Q901 would dissipate on the order of 4A * 160V for a few hundred nanoseconds, and Q902 a smaller amount.

Anyway, bistable operation wouldn't make sense, because there's nothing else in the circuit to switch it on and off.

I don't think it will be monostable either, because when the transistors turn off, the switch node voltage falls slightly below GND (clamped by D991).  When inductor current reaches zero, D991 begins to reverse bias.  But it doesn't turn off instantly, it remains on for ~100ns (reverse recovery time).  This causes the inductor current to reverse, so when the diode does turn off, the switch node voltage flings upwards again.  Which turns on Q903, and so on.

So, without plugging it in or simulating, my guess is it's an astable free-running oscillator, with the time constants defined by the inductance and the transistors' operating point, and maybe C907.

Q904 is the error amplifier.  R908-R910 are the voltage sense divider (and R912, too), ZD902 (plus Q904 Vbe) is the reference voltage, and Q904 collector is the gain node.  C907 may be for compensation, but the loop gain isn't very high anyway (maybe 20?) so it's probably not terribly unstable to begin with.

R902, R906 and R907 seem too large to do much, compared with the small value of R905.  (R905 looks drawn in.  Is it the correct value?)  If as shown, then, R905 will keep Q903 quite close to the switch node voltage.  In the 'on' state, D902 is reverse biased, and Q903 (base and emitter) is up around, say, 150V.  In the 'discharge' state (D991 conducting), D902 is forward biased and ZD901 is dropping ~12V, keeping Q903 emitter around (B+) - 13V, so it will E-B avalanche with a rather large current (50mA?), which isn't good.  Yeah, I don't think that resistor value is quite right.

Also, that should probably be D901?

Anyway, the overall operation is probably much like an astable multivibrator, with a bias tweak to its feedback arm, so the frequency and duty cycle are under control.  Sort of a simplified version of modulating a 555 timer with pin 5.

Whether this wall of paragraphs actually helped your understanding, I don't know.  In any case, check the most likely suspects: cap ESR, blown transistors, burned or out-of-spec resistors.

Tim

[xavier60]

R902, 150k is likely to be for inputting synchronizing pulses from the Horizontal deflection stage.
Carbon resistors often drift high in high voltage circuits. R904 maybe.

[Armadillo]

Looks like a pretty straightforward linear regulator to me.  My guess is that the T901 transformer is being used to generate a "kick start" pulse when power is first applied, perhaps to start the oscillator in the horizontal sweep/high voltage circuitry?

If you are getting a high voltage output with no regulation, I would suggest checking the series pass transistor (Q901) and associated circuitry first.  As a general rule of thumb, the most likely components to fail are those that dissipate a lot of power....

I agree with N2IXK, its a straightforward linear regulator.
Please measure and confirm the following voltages.
First make sure series pass transistor is not short-circuited. Turn-off the power, then discharge the capacitors. Use Multimeter, Continuity Ohm range, Check across series pass transistor, make sure its not short circuited.
If not shorted then, turn on power and measure across the components shown, these to ensure the resistors are not opened. [Remember to turn multimeter to dc volt range to measure the voltage.]
Please tell us your findings.

[lilshawn]

alright, you have all given me lots more to look at and things to research. Thank you muchly.

this is a highly unusual system for us, as most of the monitors in north america tend to use either a linear regulator (like a STR30130) or a plain old SMPS to supply the B+ for the monitor. This monitor is quite popular in japan, not so much here, so information and people who know about them are few and far between.

of course I have 3 here that need to be fixed.

I'll have another poke at this in a few days and i'll report back. I have to head out of town tomorrow and cash out some of our games about 4 hours out of the city... gonna be a long day.

[xavier60]

1K seems too low for R903.

[lilshawn]

I can confirm the value is correct. (and also the resistor that looks hand drawn is correct)

I touched up some parts in MSPAINT as I was troubleshooting because the schematic quality is absolutely horrible and it is one of only two schematics available online and the other is even worse. You cannot read any part numbers and can hardly tell the difference between a resistor and a capacitor.

incoming barf schematic for shits and giggles (two of two available online. this is full size...and not re-sized)

[Andy Watson]

I can confirm the value is correct. (and also the resistor that looks hand drawn is correct)

Could it be 1M ? At 1k there is a danger the  circuit will latch on. Also, T901 forms a supply for something else - so this "regulator" must be oscillatory.

[T3sl4co1l]

I can confirm the value is correct. (and also the resistor that looks hand drawn is correct)

Could it be 1M ? At 1k there is a danger the  circuit will latch on. Also, T901 forms a supply for something else - so this "regulator" must be oscillatory.

I discussed the design above.  It is a power Schmitt trigger.  1k seems awfully small; 10k or 91k might be more reasonable.  1M would be too much.

If it were linear, there would be no need for a buck diode, or filter inductor, or a largish capacitor on the output.   

Tim

[xavier60]

The secondary winding on T901 powers the deflection jungle IC. So even though the HT rail is over voltage, the monitor should be in a safe state if there is no chopping activity in the power supply. I mean that no extra damage is likely to be occurring.
It needs to be noticed that the power supply and deflection section are all MAINS LIVE. The EHT transformer functions as an isolation transformer to supply power to the video input and CRT circuitry.

[Armadillo]

I can confirm the value is correct. (and also the resistor that looks hand drawn is correct)

Could it be 1M ? At 1k there is a danger the  circuit will latch on. Also, T901 forms a supply for something else - so this "regulator" must be oscillatory.

I discussed the design above.  It is a power Schmitt trigger.  1k seems awfully small; 10k or 91k might be more reasonable.  1M would be too much.

If it were linear, there would be no need for a buck diode, or filter inductor, or a largish capacitor on the output.   

Tim

I think that is a B+ rail noise reduction circuit for the higher scan rate used for that "display monitor" as evident in the primary side half-wave rectified & limited and filtered network, coming from the Deflection circuit assembly.

[xavier60]

I can see now that R902 just goes to the unregulated HT. It starts the operation of the power supply.
I wonder what the value of R907 is. It looks like 68K?

[Armadillo]

I can see now that R902 just goes to the unregulated HT. It starts the operation of the power supply.
I wonder what the value of R907 is. It looks like 68K?

Yes, R902 (150K) merely start the linear regulator, fire up transistor 903, then Tr902 and then the series pass transistor 901. After which R902 can be out of the circuit.

[Anks]

what exactly is the symptom with the monitor. i've fixed a few nanao ms8's if the problem is you cant get the b+ in range then i would be looking at all the resistors.

[lilshawn]

what exactly is the symptom with the monitor. i've fixed a few nanao ms8's if the problem is you cant get the b+ in range then i would be looking at all the resistors.

this particular monitor (for sure) blew out the bridge rectifier and R901 (1 ohm 5 watt). I replaced both of these items, powered on the monitor and I get deflection for a 1/2 second then it goes into shutdown. B+ was at ~130 VDC

I isolated the B+ from the rest of the systems by lifting out the diode supplying the power to the vertical deflection circuit IC (D905) and the main B+ fuse (F902) and instead, loading the B+ with a 75 watt lightbulb.

I checked all the resistors and they all look good and measure within specification.

I think I have an issue with ZD901 because it seems to read 0.099v in both directions.

I believe this part is a 12 volt zener diode (it's marked "B2" in one direction and "12" in the other (printed 90 degrees to each other)) the part number on the schematic is hard to read.

[xavier60]

If you are testing it with the Diode test range, that reading indicates the zener  is faulty. It would also have a low resistance reading in both directions. Check the series diode also.
I still cant make complete sense of the circuit. What is the value of R907?

[lilshawn]

I still cant make complete sense of the circuit. What is the value of R907?

yeah it's an odd bugger.


R907 is 68K - (Blue Gray Orange) checks out at 68.1k (within error spec.)


0.589v for the D902 diode (should be okay)

[lilshawn]

I've mad a mistake on my touch up of the schematic, R905 isn't 1K... it's 1M ohm

[xavier60]

That makes better sense. Let us know what happens with the new zener.
Although I can't see how the faulty zener will cause the over voltage.

[lilshawn]

this is about as far as I got into diagnosing.

I did replace the diode (found one elsewhere on the board in the video section.) I tested this diode and found it to be about 0.6v so the one in this section is definitely bad.

it didn't solve the problem though. I'm suspecting Q903 because of this...

[xavier60]

The voltages in red look normal. The voltages in orange don't make sense. Are they in the correct places?

[lilshawn]

I probed a bunch of places and got this:

I double checked all the resistor values and they are within a few percent of nominal.

the transformer (t901) provides voltage through D905 provides voltage (about 20-30 volts) for the vertical deflection circuit. it's shown in the schematic snippet, but I don't think it has a bearing on what's happening on the B+ circuit side. I have lifted this diode out of the circuit for testing.

I also do not read any voltage output from this diode. I'm thinking i'm not getting a pulsation in the B+ output (instead just straight DC) which is not allowing the transformer to do tranformer things and produce electricity. (the transformer needing a changing electric current to produce the magnetic field to produce on the secondary)

[Andy Watson]

Q902 is dead. There should not be 12V difference between its emiiter and base.

[lilshawn]

I figured as much. I noticed that the voltage was much too high there. also... no oscillation or pulsation in the B+ to drive the secondary circuit through T901. no voltage output.

of course q902 was the only part I was unable to immediately source new...I had to take one from another chassis. i'm betting that one is dead to then (it does check fine with the low voltage a B&K 510 transistor checker supplies so I thought it could have been okay)

thanks for the insight, I'll get some replacements and have another go at it.

[T3sl4co1l]

If Q902 is open, Q901 shouldn't be on.

Think you're going to have to find a new one of that, too...

Tim

[lilshawn]

Q901 I have. I have also replaced it again. (unless it has gone wrong again now in my all my testing.)

I just got a new Q902 and replaced it. the output at f902 is now matching bridge output. I also noticed a faint tick in the circuit when powered up so now I think something else has gone. 

[Armadillo]

Q902 is dead. There should not be 12V difference between its emiiter and base.

+1

Q02 is shorted and Q903 is off, which means at least 36mA goes into the base of Q901 turning it full on. Though Q901 is able to withstand max base current 5A, other parameters may have been exceeded, so Replace Q902 and still need to check Q901.

[Armadillo]

Q901 I have. I have also replaced it again. (unless it has gone wrong again now in my all my testing.)

I just got a new Q902 and replaced it. the output at f902 is now matching bridge output. I also noticed a faint tick in the circuit when powered up so now I think something else has gone. 

You are getting near, probe the voltages again.

[Armadillo]

Just 4 transistors, I think is high time you get one of this;

http://www.ebay.com/itm/LCR-T4-ESR-Meter-Transistor-Tester-Diode-Triode-Capacitance-SCR-Inductance-M328-/161932407715?hash=item25b3ec73a3:g:k3wAAOSwCQNWgllX

[lilshawn]

I've replaced Q901, tested bad. the replacement of Q902 could have pushed it over the edge and killed it.

Also dropped my lightbulb load down to 40 watts.

B+output is ~96 volts now. which from what I read elsewhere is about right for weird lightbulb load apparently.

also getting oscillation in the circuit so now i'm getting volt output at D905 from the transformer.



hopefully now I can hook up the rest of the circuits and it NOT blow up (fingers crossed.)

Just 4 transistors, I think is high time you get one of this;

http://www.ebay.com/itm/LCR-T4-ESR-Meter-Transistor-Tester-Diode-Triode-Capacitance-SCR-Inductance-M328-/161932407715?hash=item25b3ec73a3:g:k3wAAOSwCQNWgllX

I sort of looked something similar about a year ago when I was in the market for an LCR meter (specifically for inductance)... but since I already transistor checker, and voltmeter that does capacitacneI figured I wouldn't bother.