Samsung DVD Recorder DVD-HR755 Doesn't Turn On

[max.wwwang]

This refers to the internal Vref inside the IC and nothing at all to do with its VCC supply.

Yes the dropper resistor/s allows current to pass from HV DC to C1S13 and VCC until the UVLO is nulled and the IC can start, then the working voltage is maintained from the primary feedback winding (transformer pins 8 & 10) via diode D1S03 and supply current limiting resistor R1S18 to C1S13 and zener ZD1S01 sets the max value VCC can reach so the IC will not be damaged.
I have always found it good practice to replace the VCC electrolytic, in this case C1S13 especially when the SMPS is always ON like this one is......also mentioned earlier. 

I need a good think to understand this. Thanks.

In this circuit zener reverse current spec means very little as the zener connects to circuit ground and not another above ground voltage.

But if R1S14 is not open (i.e. not broken), the mains rectified voltage, around 340V, is constantly between the Zener diode and the 3 resistors in series. I reckon the reverse voltage across the Zener, if not affected by anything else, would always be around 20V. Is this right?

[tautech]

This refers to the internal Vref inside the IC and nothing at all to do with its VCC supply.

Yes the dropper resistor/s allows current to pass from HV DC to C1S13 and VCC until the UVLO is nulled and the IC can start, then the working voltage is maintained from the primary feedback winding (transformer pins 8 & 10) via diode D1S03 and supply current limiting resistor R1S18 to C1S13 and zener ZD1S01 sets the max value VCC can reach so the IC will not be damaged.
I have always found it good practice to replace the VCC electrolytic, in this case C1S13 especially when the SMPS is always ON like this one is......also mentioned earlier. 

I need a good think to understand this.

In this circuit zener reverse current spec means very little as the zener connects to circuit ground and not another above ground voltage.

But if R1S14 is not open (i.e. not broken), the mains rectified voltage, around 340V, is constantly between the Zener diode and the 3 resistors in series. I reckon the reverse voltage across the Zener, if not affected by anything else, would always be around 20V. Is this right?

You need understand when the IC's VCC voltage passes the UVLO current is being consumed by the IC in operation switching the MOSFET gate. The resistor dropper chain can only supply enough current to charge the C1S13 cap for the start then the feedback winding supplies the current to keep the IC running.
You know from the open resistor that current from the resistor dropper is not required once the IC has started.
You made it start with your DMM across the open resistor.

The running IC will help keep the voltage in check and the zener is only there to prevent the IC from excessive input voltage.

You confuse forward (conducting) zener voltage and reverse (leakage) current, they are 2 entirely different things.
For the purpose of further understanding, the zener anode is at 0V so there can be no reverse leakage but only forward conduction when the zener cathode is above 18.6V.

[shakalnokturn]

Don't discount C1S13 is leaky too and may have caused one of the 270K resistors to open.

"Leaky" as in electrically? Or rather leaking juices?
I haven't done the power dissipation calculation for the dropper resistors but wouldn't have expected the 20V difference at the capacitor to have much effect on destroying the dropper resistors.

[tautech]

Don't discount C1S13 is leaky too and may have caused one of the 270K resistors to open.

"Leaky" as in electrically?

Yes but we may never know unless it gets measured.

[max.wwwang]

I didn't measure the cap's leaking. I tested by parallelling a 200k with the faulty one then I went with it and it worked. Normally very low likelihood of two faults happening simultaneously.

[max.wwwang]

You need understand when the IC's VCC voltage passes the UVLO current is being consumed by the IC in operation switching the MOSFET gate. The resistor dropper chain can only supply enough current to charge the C1S13 cap for the start then the feedback winding supplies the current to keep the IC running.
You know from the open resistor that current from the resistor dropper is not required once the IC has started.
You made it start with your DMM across the open resistor.

The running IC will help keep the voltage in check and the zener is only there to prevent the IC from excessive input voltage.

You confuse forward (conducting) zener voltage and reverse (leakage) current, they are 2 entirely different things.
For the purpose of further understanding, the zener anode is at 0V so there can be no reverse leakage but only forward conduction when the zener cathode is above 18.6V.

Thanks that saves some of my brain cycles!

[Shock]

Congrats. If it was me I'd check the resistors, the cap, replace if needed and resolder any joints in that area. Guys that do a lot of repairs on them will also replace the IC but in this case it seems to be working fine and have survived whatever caused the failure. It's common to have one component cause another to fail in electronics. In this case I doubt it but yeah it doesn't mean you wouldn't look.

[shakalnokturn]

I didn't measure the cap's leaking. I tested by parallelling a 200k with the faulty one then I went with it and it worked. Normally very low likelihood of two faults happening simultaneously.

Simultaneously no, but quite often a bad electrolytic will cause a diode to short or resistor to fuse.
I also doubt it is the case here though, high value resistors on high voltage just go open.

[Shock]

Quick lesson when testing with oscilloscopes as I noticed you are learning in this area. First watch that how not to damage your oscilloscope video a few more times. It is not safe to connect most oscilloscopes to mains circuits at all, it warns against this in their manual. Most oscilloscopes have exposed common BNC ground connections on them. Not only do you need to be careful of input voltages you need to be careful about putting voltages onto the oscilloscope ground which creates a dangerous touch hazard as well as damage equipment.

It helps understand that ground, earth, common, negative, neutral are all labels give to points of a circuit that may be at different potentials and create shorts when connected to each other. You have to be really careful connecting the oscilloscope ground lead (real earth/ground in most countries) to points on the PCBs which are not actually the real earth/ground because it can act like a short.

You would think it's safe to hook up ground lead to the secondary, it's low voltage right? No only if the ground lead goes somewhere where it doesn't create a short to mains earth/ground. Think of the oscilloscope ground lead always as a short creating wire to earth/ground.

So then you find a nice low voltage floating voltage/signal and you connect your oscilloscope ground to it (creating a ground to reference your measurements off) and are observing that signal on your oscilloscope. The moment you attach any other equipment you need to consider does it have another ground/earth connection, and are you making a short with it.

Want to use a second channel on the oscilloscope? Since the ground leads are common you have to connect them at the same potential. If you don't and there is a voltage potential between those two points, you have created a short and bad things will happen.

It gets easier once you understand the concept and reading and understanding schematics helps. If you see several components between two points on a circuit that is a dead giveaway there is going to be a difference in voltage potential and they are not electrically common.

Hope this helps.

[max.wwwang]

Simultaneously no, but quite often a bad electrolytic will cause a diode to short or resistor to fuse.
I also doubt it is the case here though, high value resistors on high voltage just go open.

Agreed.

Quick lesson when testing with oscilloscopes as I noticed you are learning in this area. First watch that how not to damage your oscilloscope video a few more times. It is not safe to connect most oscilloscopes to mains circuits at all, it warns against this in their manual. Most oscilloscopes have exposed common BNC ground connections on them. Not only do you need to be careful of input voltages you need to be careful about putting voltages onto the oscilloscope ground which creates a dangerous touch hazard as well as damage equipment.
...

Hope this helps.

This absolutely helps and I'll read this for some good times. Thanks for putting this together - yes, I'm just starting.

[max.wwwang]

Congrats. If it was me I'd check the resistors, the cap, replace if needed and resolder any joints in that area. Guys that do a lot of repairs on them will also replace the IC but in this case it seems to be working fine and have survived whatever caused the failure. It's common to have one component cause another to fail in electronics. In this case I doubt it but yeah it doesn't mean you wouldn't look.

Thanks. It would be too much hassle for me to unsolder the IC without knowing it's down. I felt very lucky I easily took off the faulty resistor with a solder sucker. I expected it would have been more difficult. I was quite satisfied and very encouraged!

[max.wwwang]

I had a question which I have not got an answer yet - why is there no earthing for this machine?

I got a bettering understanding of the classes of electrical appliances from here just now but again this does not justify the absence of an earth wire for this machine. Its case is metal - well, its upper cover may be coated but the exposed bottom of the chassis is clearly bare metal.

Also in the above link about Classes I,II,III, what's the purpose of the Cy capacitors? I also recently learned about the different classes for capacitors (X/Y) but still don't quite understand why there need to be capacitors between the L/N wires and E (this is commonly seen in surge protection/noise filtering stage close to the mains)?

Appreciate if anybody can shed a light.

[tautech]

I had a question which I have not got an answer yet - why is there no earthing for this machine?

Looking at the schematic there is no provision for one as it seems the mains is supplied via a 2 pin connector.



Further, mains reference ends at the SMPS however the LV circuit ground is tied back to the mains via the XY network. Circuit design such as this is not uncommon for the international marketplace where in some counties only 2 pin power sockets are common.
Further reading here:
https://www.justradios.com/safetytips.html

[max.wwwang]

Looking at the schematic there is no provision for one as it seems the mains is supplied via a 2 pin connector.

That's exactly what I don't understand - why is there no earth wire for a machine with a metal case.

The justradios page is very comprehensive and useful. Thanks.

A part of my X/Y capacitors stock --

[Shock]

It's likely Class II which doesn't require an ground/earth from the mains. Look at the rating symbols on the rear of the DVD player. Class II devices have extra requirements to be insulated from their chassis. Is this better than having a earthed chassis? Open to debate that one.

[max.wwwang]

It's likely Class II which doesn't require an ground/earth from the mains. Look at the rating symbols on the rear of the DVD player. Class II devices have extra requirements to be insulated from their chassis. Is this better than having a earthed chassis? Open to debate that one.

Having had a look at its inside, I don't see any extra protection of the machine but I don't know. I need a copy of IEC 61140 (Protection against electric shock — Common aspects for installation and equipment) to get a definitive answer but I don't have one.

[Shock]

Depends on when it was made as well, standards obviously will change and improve over time.