I would check what looks like a thermal fuse (1A/250V?) that is taped to the yellow tape on the transformer. I've had one of those go due to "stress", and not thermal issues.

Looked closer at these wires. This is indeed a sensor, but not a thermostat, rather looks like it is an ammeter mounted on the transformer coils. The sensor's wiring runs into the IC controller's sense pin, which uses the sensor's reading to determine if it should shutdown the PWM for overcurrent protection. And that could be exactly what is happening!

While this amp sensor certainly could have gone bad, it also could be doing its job correctly and alerting the IC that there is a short detected in the transformer, which is one of the likely things that we suspect could be the problem with the power supply. Bypassing the sensor without first proving that it is actually not working is therefore probably a bad idea.

 

A shorted winding on the transformer is indeed a likely issue - that's one reason why checking the aux power feed back to the chip is needed. It can also be a diode failure there.

Taking various points from previous posts:

Pls do check this Resistor

Is there a burnt hole in tht 100K SMD.........I see 104 on the marking somewhat

If I am not mistake tht R could be part of the Vcc PWM IC Supply or ??

R3a&b, C2 and the diode on the topside are the snubber network from Q1 drain to VHT+, catching the reverse polarity spike when Q1 turns off. It is true they are fundamental to the energy transfer from primary to secondary but are unlikely to be the issue.

Looked closer at these wires. This is indeed a sensor, but not a thermostat, rather looks like it is an ammeter mounted on the transformer coils. The sensor's wiring runs into the IC controller's sense pin, which uses the sensor's reading to determine if it should shutdown the PWM for overcurrent protection. And that could be exactly what is happening!

Hmmm, you may be right in some respects but current sense is unlikely since most of the magnetic field is in the core, temperature sensing of windings more probable. My gut feeling is it is a thermistor, but its hard to see the wiring around pin 4 (sense) and the connection to the MOSFET source. It seems to be connected to D6/R6 but its hard to tell, can you sketch that out? Also measure the resistance of that at cold.

Upon power-up with no load and the power supply seeming to be working, current comes out of the bridge rectifier at 168VDC. It then passes through the first 754 resistor and drops to 66VDC. It then passes through another 754 resistor and drops to 15VDC at the spot on the board that you asked for. The final output of the power supply is 29.8VDC, as it should be.

After duty load has been applied by connecting to the printer and the power supply shuts down, there is indeed a change in these voltages, which I did not expect so early in the circuit. While the bridge rectifier is still putting out 168VDC, after the first 754 resistor, voltage is now 62VDC. After the second 754 resistor, voltage is now 4VDC. The final output of the power supply is now also 4VDC, insufficient to power the printer.

If these two 754's are supposed to be 754k ohm resistors, they don't measure quite right. With no power, they seem to have 850k ohm with current flowing in one direction and 650k ohm if I test in the opposite direction. But I'm aware that may not be an accurate observation because these are still installed into circuit, and there could be side effects of the other components on the board (like capacitors) inadvertently charging up using the meter's test current.

The connection point you asked about is also an interesting location on the circuit. It's the spot where the 50V 10 micro-farad capacitor has its positive side connected, where the diode immediately next to the transformer (buried under the white goop) takes its current, and also a lead point powering the components under the metal shielding.

I'm not sure if the results we just saw indicate the shutdown is actually a bad resistor, or if a short elsewhere that gets triggered by the load, or a safety measure is being triggered somehow to protect the circuit from damage. How is the voltage being dropped so significantly unless a lot of current is being re-routed through a low resistance emergency exit somewhere?

So the chip takes around 100uA in both situations, but if we look at the datasheet we see...

So to go from startup to run the aux winding needs to generate some 20v at 5mA - either its not doing that or something is loading the output winding so much the transformer can't get enough magnetic flux in the core to generate the required volts. Is it a symptom or the cause?

What are the markings on Q2? This seems intimately linked with that voltage generation area and is unusual but I'm not getting what its for yet, but it might have a bearing on the matter.

 

My gut feeling is it is a thermistor, but its hard to see the wiring around pin 4 (sense) and the connection to the MOSFET source. It seems to be connected to D6/R6 but its hard to tell, can you sketch that out? Also measure the resistance of that at cold.

I already did that. I guess a thermistor would also work for the purposes of the sense pin. The wires to the sensor are the crossing from R9 at the edge of the metal shield to the D6 Zener diode. 0 ohms resistance across the sensor cold. After D6 it goes straight to the controller's sense pin, but you can't see the line on the board very clearly in the photos, but it is there.

What are the markings on Q2? This seems intimately linked with that voltage generation area and is unusual but I'm not getting what its for yet, but it might have a bearing on the matter.

Q2 looks like another transistor similar to the one of those under the heat shield. It is powered by the outer diode next to the transformer (which in turn is powered normally with 15V at the point you marked yesterday). It seems to read "ZE" and then has "24" vertically. One leg of the transistor isn't attached to anything, while the other leg is attached to two things.

This area under the transformer does seem quite busy, here's a close-up which includes the label on the transistor, along with the sense pin of the controller:

 

Ok, Q2 turns out to be a zener diode but the circuit design isn't obvious. Here's the circuit... In theory D8 shouldn't allow it to work unless its a zener but its not labelled as such..



The circuit around Q3/Q4 seems to be some sort of over-voltage protection latch for the controller, hence the 22v zener at ZD1, but it needs a connection to VDD. I'm not sure of the direct connection from Q3 emitter to FB, maybe that actually goes to VDD, but somehow FB needs to get from R11 to pin 2 of the controller, I just can't see it from the pics.

 

If your Sperry has a diode test function (think it has from a brief read of the manual) can you check D8 polarity,.

Also is there a connection between the 2R0 resistor R7/transformer pin junction and the Q2/big diode junction? Some pictures say no, others maybe.

Either way, can you check the voltage at that Q2/diode junction off load and on load...

 

Hi Irving,

Before I start my next round of responses, I would just like to acknowledge the amazing level of experience and skill that you demonstrate to me while working on this project. Your circuit diagram is looking fantastic! I sincerely appreciate the help and sharing your knowledge, so that others like me can also get a taste of what this trade is like and learn how to analyze these problems objectively.

Ok, Q2 turns out to be a zener diode but the circuit design isn't obvious.

I don't get why someone would install a transistor and leave the emitter pin disconnected. What do the base and collector accomplish by themselves, create some special kind of zener diode? Q2, Q3, and Q4 all seem to have left the emitter totally disconnected!

In theory D8 shouldn't allow it to work unless its a zener but its not labelled as such..

I thought the little D8 after the Q2 transistor was a zener diode. Why else is it clear red like that? Am I missing something too obvious to mention?

I'm not sure of the direct connection from Q3 emitter to FB, maybe that actually goes to VDD, but somehow FB needs to get from R11 to pin 2 of the controller, I just can't see it from the pics.

Honestly, even looking at the board in front of me, it looks like the FB pin 2 is totally disconnected just like all Q emitters. But it is connected directly to R11 just like you predicted! The path is not easy to see, as it appears the line makes the connection from underneath the IC controller! Ok, that should give it away, if you look at the R11 again, you'll see the line coming up from under the R11 and going under the R16, R13, C9, then making a 90 degree turn left, going under the corner of Q3 (no connection made) and then turning right 90 degrees to go under the IC2, and finally I guess it makes a last 90 degrees right turn to connect to pin 2 in a hidden fashion. What a roundabout way to make that journey!

There is no connection from the Q3 emitter to FB, VDD, or anything else as far as I can tell!

 

If your Sperry has a diode test function (think it has from a brief read of the manual) can you check D8 polarity,

The D8 readings are unexpected to me. Maybe we found something, or maybe it's a false test. In the correct direction, the voltage drop is 0.693 volts under the diode test mode. In the reverse bias, it's only a little higher at 0.811 volts. Switching to ohmmeter, it is 3.11k ohm in both directions. Doesn't behave like a diode at all, but with C8 right next to it and connected to both ends, I don't think that I'm really testing the diode at all but rather actually testing the capacitor mostly.

Also is there a connection between the 2R0 resistor R7/transformer pin junction and the Q2/big diode junction? Some pictures say no, others maybe.

No, there is no connection like that here, sorry not this time.

Either way, can you check the voltage at that Q2/diode junction off load and on load...

You mean just across the big diode from where we tested yesterday? I tested that diode too with the meter, and the polarity is positive at Q2. So with power on, voltage was 0V at the Q2 transistor base, both before and after the shutdown. Diode held up strong!

P.S. I'm wondering if all those unconnected transistor emitters are like this so as to serve as a dead end for the current when the controller commands a shutdown?

 

Before I start my next round of responses, I would just like to acknowledge the amazing level of experience and skill that you demonstrate to me while working on this project. Your circuit diagram is looking fantastic! I sincerely appreciate the help and sharing your knowledge, so that others like me can also get a taste of what this trade is like and learn how to analyze these problems objectively.

You're welcome.

I don't get why someone would install a transistor and leave the emitter pin disconnected. What do the base and collector accomplish by themselves, create some special kind of zener diode? Q2, Q3, and Q4 all seem to have left the emitter totally disconnected!

That's because its not a transistor. Its a diode in a SOT-23 case, which is a standard part. Calling it Q2 instead of Dx is a little odd though, but its not unknown - they used Q as an ident to signify case style.

I thought the little D8 after the Q2 transistor was a zener diode. Why else is it clear red like that? Am I missing something too obvious to mention?

No, its a glass cased diode - could be a normal diode or a zener diode, no way to tell from the case style. But if its a normal diode the bar indicating the cathode puts it the wrong way round.

Honestly, even looking at the board in front of me, it looks like the FB pin 2 is totally disconnected just like all Q emitters. But it is connected directly to R11 just like you predicted! The path is not easy to see, as it appears the line makes the connection from underneath the IC controller! Ok, that should give it away, if you look at the R11 again, you'll see the line coming up from under the R11 and going under the R16, R13, C9, then making a 90 degree turn left, going under the corner of Q3 (no connection made) and then turning right 90 degrees to go under the IC2, and finally I guess it makes a last 90 degrees right turn to connect to pin 2 in a hidden fashion. What a roundabout way to make that journey!

Welcome to the wonderful world of PCB layout which is often more art than science! Finding a way through the maze can be very taxing. This indicates, to some extent, the age of the design but also the building down to a price. A double sided board would make it easier but costs more in copper and processing time; when you are building hundreds of thousands or millions of them it adds up fast!

There is no connection from the Q3 emitter to FB, VDD, or anything else as far as I can tell!

There will be... it just need more investigating!

The D8 readings are unexpected to me. Maybe we found something, or maybe it's a false test. In the correct direction, the voltage drop is 0.693 volts under the diode test mode. In the reverse bias, it's only a little higher at 0.811 volts. Switching to ohmmeter, it is 3.11k ohm in both directions. Doesn't behave like a diode at all, but with C8 right next to it and connected to both ends, I don't think that I'm really testing the diode at all but rather actually testing the capacitor mostly.

No, you're seeing the resistance of R12 and R14 in parallel = 6.2k/2 = 3.1k, because current is flowing through the winding of the transformer. So inadvertently you've validated that the winding isn't open circuit! Now its not clear to me which way round the voltages you measured were. The lower voltage is the diode forward voltage, the higher is the test current through the parallel resistors.

You mean just across the big diode from where we tested yesterday? I tested that diode too with the meter, and the polarity is positive at Q2. So with power on, voltage was 0V at the Q2 transistor base, both before and after the shutdown. Diode held up strong!

You mean it conducts with the red probe on the Q2/diode junction, which is correct as per my diagram.

Now the question in my mind is should you have measured 0v, and I'm thinking no... but I need to think more on that...

P.S. I'm wondering if all those unconnected transistor emitters are like this so as to serve as a dead end for the current when the controller commands a shutdown?

Nice thought, but the laws of physics don't work like that. With the exception of the pin on Q2 they will be connected, it just may not be obvious.

 

That's because its not a transistor. Its a diode in a SOT-23 case, which is a standard part. Calling it Q2 instead of Dx is a little odd though, but its not unknown - they used Q as an ident to signify case style[

a glass cased diode - could be a normal diode or a zener diode

From my layman's perspective, it's marked as a Q and has "ZE" printed to indicate it's a 2N5401S "EPITAXIAL PLANAR PNP" transistor, not to indicate it's actually a zener diode. If they are using a standard case for a different purpose, I would have thought that nothing would be printed on it.

It's very troubling to me if components are not necessarily they say they are. But if that's true, then how can you be sure the other two apparent transistors are not just diodes in disguise as well?

I'll try and test these to verify if they are diodes or not, though I'm unclear how an actual transistor would respond differently.

With the exception of the pin on Q2 they will be connected, it just may not be obvious.

I don't know, I had the board and ohmmeter in front of me and tried touching everywhere in the vicinity and found nothing to indicate any type of connection. Using both low and high ohm settings. One transistor case appears to have no connection to the collector, the other nothing on the emitter. I'll try again though and also check if they demonstrate any "diode-like" properties...

 

That's because its not a transistor. Its a diode in a SOT-23 case, which is a standard part. Calling it Q2 instead of Dx is a little odd though, but its not unknown - they used Q as an ident to signify case style.

I'll try and test these to verify if they are diodes or not ...

There will be... it just need more investigating!
...
With the exception of the pin on Q2 they will be connected, it just may not be obvious.

One transistor case appears to have no connection to the collector, the other nothing on the emitter. I'll try again though and also check if they demonstrate any "diode-like" properties...

The collector on Q3 is connected to R18 via the underside, so that connection is resolved.

However, I'm sorry, Q2 does not behave in any way like a diode (rather, it's next to a diode), and the the emitter on both Q2 and Q4 is certainly not connected to anything. And neither checks out as a diode either. So that's weird to me, I know there is some internal connection between the base and the collector in a transistor, but I'm not aware of any practical uses for this, but maybe you or somebody else here following this can think of one?

What do you suggest looking at next?

 

However, I'm sorry, Q2 does not behave in any way like a diode

so how does it behave?

In the absence of any other evidence ..

 

I take your point on the 2N5401S manufactured by KEC



But then the circuit would be as the second one below - unless the base -pin 2 - is connected somehow that makes no sense...



OK, using diode mode on your multimeter, red probe on pin 2, what do you get for pins 1 and 3, then repeat for black probe on pin 2.

 

Just noticed something odd about the KEC datasheet...

Here is info from another manufacturer.................................. and here from KEC... note the pin # order...

 

OK.. I went back to the picture and looked closer...


So there IS a connection to "pin 2" - or pin 1 emitter as per the KEC datasheet... and that changes everything...

now the circuit - using normal pin numbering - looks like this, which makes much more sense! This is basically a soft-start circuit - it slows the start up by about 30mS - 50mS.

 

OK.. I went back to the picture and looked closer...

So there IS a connection to "pin 2" - or pin 1 emitter as per the KEC datasheet... and that changes everything...

now the circuit - using normal pin numbering - looks like this, which makes much more sense! This is basically a soft-start circuit - it slows the start up by about 30mS - 50mS.

Ok, that's a good find, and one of those cases where a photo shows something that the naked eye almost can't see!

Confirmed the emitter on Q2 is connected to R7 and C8.

And then I went to Q4 and see the same thing happening, an under-side connection. In that case, Q4's emitter is connected to J2 and C7.

So do these discoveries now explain the 0V at Q2 and the big diode?

 

So do these discoveries now explain the 0V at Q2 and the big diode?

Well, it's hard to say. The problem is that the voltage at q2 collector is a pulsed signal not DC so its quite likely your meter will read it as 0v, but because it's at 70kHz it's likely to read 0v on the AC ranges too.

Do the diode mode tests on Q2 I suggested in post #52 while I think further on how to proceed.

 

Do the diode mode tests on Q2 I suggested in post #52 while I think further on how to proceed.

Referring to Pin 2 on Q2 as the emitter that we originally thought was disconnected, the diode test with red on Pin 2 is .806V to Pin 1 and OVERLOAD (infinity) to Pin 3. Reversing with black on Pin 2, we get .688V to Pin 1 and 1.730V to Pin 3.

 

BTW - the big diode on the top side connected to Q2 that we keep referring to is actually D5. I guess that makes the other one right next to the transformer D7. Finally, the 10 micro-farad capacitor is C5.

 

ok, I think you need to do the diode tests with pin 1 (base - by the 'Z') as the common point...

 

ok, I think you need to do the diode tests with pin 1 (base - by the 'Z') as the common point...

Not a whole lot changes. Negative on Pin 1 base to Pin 3 collector is .79V. Positive on pin 1 base to Pin 3 collector is again OVERLOAD (infiniti).