Samsung DVD Recorder DVD-HR755 Doesn't Turn On - Page 1
Forum section: electronics / repair

author: max.wwwang
author_rank: Frequent Contributor
timestamp: March 22, 2020, 07:48:50 am
content: I got a Samsung DVD Recorder DVD-HR755 but it does not turn on. I'm trying to fix it.Power lead is good and mains power (240V AC) comes in the unit. Apparently the the PSU is a switch mode one. I measured a rectified DC voltage 333V on the full bridge rectifier (GBJ2J). SM controller is ICE2BS01. No idea what are the right next steps to trouble-shoot. Any help appreciated.User manual availablehere.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: March 22, 2020, 10:03:13 am
content: The boards are taken off from the chassis.First safety. The cap with high voltage is dangerous - I can discharge it with a resistor when not live. Laser is another one. Are there any other risks handling this puppy? Thanks.

author: tautech
author_rank: Super Contributor
timestamp: March 22, 2020, 10:21:22 am
content: Quote from: max.wwwang on March 22, 2020, 07:48:50 amIt certainly is.QuoteI normally start by finding the datasheet for the controller and if it's available from a few manufacturers download and study them all.They generally have a typical application schematic and often this is replicated quite closely in actual circuits you might work on.Study the controller VCC supply circuits as there's generally 2 supply paths, one from the rectified mains via dropper resistors (usually several hundred K or sometimes a few meg ohms) and these can drift or go open.The other supply is normally a kickback from the primary both rectified and smoothed and both diode and cap should be checked.There's normally a UVLO spec that unless VCC get above this the SMPS will fail to start.Watch what you're doing with this stuff, it bites !Work careful and methodically with live circuits and never when tired.

author: Falkra
author_rank: Regular Contributor
timestamp: March 22, 2020, 11:00:02 am
content: Hello,try to locate the standy voltage rail on the output of the power supply to see if it is present.Be very careful with the high voltage side (and don't touch the heatsinks with the fingers, sometime they are live, depending on the power supplies, maybe not this one but be very careful).

author: max.wwwang
author_rank: Frequent Contributor
timestamp: March 23, 2020, 04:36:08 am
content: Thanks both. I thought about the same thing - first trying to figure out how the power on/off click button relates with the power unit (which are way away). I traced the two pcb traces from the button but they go to the display panel module with a ribbon connector (a black box to me). Then I went back to the PSU side (the output pins) - there should be some idle power rails standing by but I was unable to detect any voltage between GND and any of the pins (there are pin descriptions on the board).Also found this service manualherewhich should be very helpful.Strangely I cannot detect a voltage between pin 6 (VCC) and 7 (GND) of the SM controller chip (2BS01), which should be between 8.5~21V according to the datasheet. Maybe experts are already able to tell where the rub is?

author: tautech
author_rank: Super Contributor
timestamp: March 23, 2020, 04:41:26 am
content: The SM IC must have sufficient VCC in order to start so first suspect is the HV resistor/s.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: March 23, 2020, 06:55:57 am
content: Likely. From the schematics, I suspect the Zenar diode (MTZ-J20B, highlighted) is down (short). As a result there is no reverse voltage (should be around 20V according to the datasheet) which is a prerequisite for the SM controller to function.

author: tautech
author_rank: Super Contributor
timestamp: March 23, 2020, 07:15:43 am
content: Quote from: max.wwwang on March 23, 2020, 06:55:57 amThat's certainly possible but not the first thing I would suspect.QuoteThe zener just limits how high VCC will get if it's working correctly but the 47uF cap, the 270K resistors, the series diode and the 82R resistor from the flyback should all be checked....out of circuit if necessary.The zener can be checked easily with a PSU and a series load resistor. Just check its datasheet to ensure it has enough current passing through it for it meet spec.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: March 23, 2020, 08:44:29 am
content: I checked the power resistors and Zenar diodes, seem all working. I got resistor reading both direction of a diode around 100 or so Ohm I suspected faulty but checked again all good (I should have had a good note of all measured carefully, it's confusing when turning the board back and forth, measuring here and there). Also I used a damp wipe to clean the surface of some components, also the board as far as I can reach (and cleaned the dust over the silk screen print to read the text) - of course very carefully and when not live.The most strange thing was at one point I got a voltage across pins 6/7 of the controller around 10V, and about 200V across R1S14 (1/4W 270k). This was very different from previously (i.e. now seemed working as it should). Surprised (also confused), I unplugged the power and discharged the 400V cap, reconnected the PSU board with the main AV processing board and the front panel (it's needed because it sits between the power switch and PSU). I pressed the button and it worked (the power LEG lit up)! See photo.With excitement I put the boards back to the chassis expecting all will be good now. Just to be cautious, before hooking up the DVD drive, I tested by pressing the button again. It failed to turn on! And the voltage across these jobbies all disappeared! (330V or so DC after the rectified still there, i.e. the power is definitely plugged.)

author: max.wwwang
author_rank: Frequent Contributor
timestamp: March 23, 2020, 08:52:04 am
content: Even stranger, I had another test of R1S14, now I got 200V back and 10V between 6/7, too! My finger went to the button without instruction from my brain. Bang!!! Even the HDD was now working and the front panel lit up.But I can not lay back assured!

author: tautech
author_rank: Super Contributor
timestamp: March 23, 2020, 09:39:39 am
content: Quote from: max.wwwang on March 23, 2020, 08:52:04 amNice, yes I understand.Cracked solder joints could be the cause.Disturbing the PCB might provide a short term connection for it to only fail again.Joints that should be checked and if necessary with magnification are, diode bridge, NTC thermistor, MOSFET, 270K dropper resistors and the transformer pins. Check the interconnect between the PSU PCB and the mainboard too.Wide open eyessuspect any joint that has any physical or heat stress.Good luck.

author: Shock
author_rank: Super Contributor
timestamp: March 23, 2020, 02:13:45 pm
content: Other things to try... you can also go back and try to reproduce the method you used to get it running, it will be very handy to isolate the problem. It's happened before that a screw can cause a bad earth, short or make an intermittent joint on a PCB cause weird problems. If no luck try to manipulate the cables a bit and check the solder joints on those connections like tautech mentioned.You ideally want to know the state of if the power supply is getting a signal to switch on. But it's always a good idea in intermittent cases to rule out the power switch and rig an alternative if you need to as they cause a lot of stuck or open issues. The other cause of that particular symptom is a component overheating or failing intermittently so you can also use the hot and cold testing method. Caps (high esr) and cracked components can also cause weird intermittent symptoms. They can suddenly work after a while of trying or a while of leaving the device off. Just reconnecting cables on it's own can fix devices, doesn't happen all the time, but occasionally that is all it needs.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: March 24, 2020, 07:39:48 am
content: I think the bug is being cornered. I will do more test to figure it out. One question -- from the PSU schematics I noticed this (highlighted in the photo). What's the significance of it?

author: tautech
author_rank: Super Contributor
timestamp: March 24, 2020, 08:24:49 am
content: Quote from: max.wwwang on March 24, 2020, 07:39:48 amIt means if you are to scope or measure anything use the common ground and not the chassis as the reference point.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: March 24, 2020, 08:25:42 am
content: More tests. It seems not a physical problem, maybe still an electronic one. But really really strange!As the photo, this is the standby condition. Under this condition, the unit is ready and could be turned on by either the button or the remote. But when I unplugged and replugged to the power outlet, the panel has no display and the unit is not ready. It goes to ready when I measure the voltage across the resistor R1S14 (270K) with this multimeter at the setting of 600V DC. It took the unit about 2 sec to enter the standby mode (it's now all good to go from this point). This mode can be entered with the 200V DC setting but taking a bit longer (about 8-10 sec). None of my other two multimeters has this magical power.I tried to put physical force on this resistor but no effect. Also strange is when I turn on the power outlet, I can hear a very low click sound from the power unit (like a relay, but much weaker) - before the panel showing anything. I cannot find a relay on the schematics or on the board. Where is this sound from?!Another question, since we mentioned GND, why does this unit not have to have a 3-wire mains lead for safety (it does not have the PE wire)?

author: max.wwwang
author_rank: Frequent Contributor
timestamp: March 24, 2020, 08:28:33 am
content: Quote from: tautech on March 24, 2020, 08:24:49 amI've watched that video of Dave "how not to blow your scope" (and I think I understood); my taking was that a better title for that one is "How not to blow your puppy with your scope". Maybe I missed something.

author: tautech
author_rank: Super Contributor
timestamp: March 24, 2020, 09:02:24 am
content: Quote from: max.wwwang on March 24, 2020, 08:25:42 amWhich resistor, one of the 270K ones ?QuoteIf no metal parts are accessible to the user it won't need a PE.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: March 24, 2020, 09:09:31 am
content: Very probably the problem is that the R1S14 is broken/open (or half). The sound much have come from this broken resistor when high voltage was being applied on it. The cure would simply be a 270k resistor. I don't have one for now (or close at hand). Will try to salvage one from the boards tomorrow. Tautech you are right I believe. I measured its resistance when the unit unplugged; it gave me 'open' while R1S13 and 15 both around 270K!

author: max.wwwang
author_rank: Frequent Contributor
timestamp: March 24, 2020, 09:10:51 am
content: Quote from: tautech on March 24, 2020, 09:02:24 amYes, R1S14.Quote from: tautech on March 24, 2020, 09:02:24 amThe chassis is all metal. Actually, to rule out this little possibility, I once connected the chassis to PE of the power outlet but with no effect (very very unlikely indeed).The remaining question is possibly why my multimeter - and only this one - had such magical behaviour?

author: tautech
author_rank: Super Contributor
timestamp: March 24, 2020, 09:16:52 am
content: Quote from: max.wwwang on March 24, 2020, 09:09:31 amDon't discount C1S13 is leaky too and may have caused one of the 270K resistors to open. Measure each carefully to check they haven't drifted.In my first post I mentioned the dropper resistors to VCC from the HV DC is something to always check.Hope you're all sorted and happy to have helped.

author: tautech
author_rank: Super Contributor
timestamp: March 24, 2020, 09:20:26 am
content: Quote from: max.wwwang on March 24, 2020, 09:10:51 amIts internal resistance on the range you used was lower that your other DMM's so conducted sufficient current for the IC to do a single kick or two then the primary side kickback supply took over to allow for ongoing operation.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: March 25, 2020, 03:54:17 am
content: Quote from: tautech on March 24, 2020, 09:16:52 amThat was spot on. Good on you and thanks!Quote from: tautech on March 24, 2020, 09:20:26 amThat was the same thing I could think of.Quote from: tautech on March 24, 2020, 09:20:26 amI struggled to understand how the working of IC could be maintained. Will have more tests on this to get a better understanding.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: March 25, 2020, 04:21:32 am
content: Quote from: max.wwwang on March 25, 2020, 03:54:17 amAccording to the datasheet of the SW controller the resistor is only for startup, after that "the internal bandgap generates a reference voltage VREF=6.5V to supply the internal circuits".I found a 220k resistor in my stock, the maximum reserve current would be --(340V-20V) / (270*2+220) = .0.42mA comparing to(340V-20V) / (270*3) = .39mA (with 3x 270k resistors)I think it's safe and would work. But I don't fully understand the reserve current IR of the Zener diode (2uA @15V) shown on the datasheet. What do these two numbers mean? I don't think 2uA is its maximum reverse current.

author: tautech
author_rank: Super Contributor
timestamp: March 25, 2020, 06:30:41 am
content: Quote from: max.wwwang on March 25, 2020, 04:21:32 amThis 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.QuoteIn this circuit zener reverse current spec means very little as the zener connects to circuit ground and not another above ground voltage.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: March 25, 2020, 08:47:00 am
content: Bingo, fixed!

author: max.wwwang
author_rank: Frequent Contributor
timestamp: March 25, 2020, 08:52:24 am
content: Quote from: tautech on March 25, 2020, 06:30:41 amI need a good think to understand this. Thanks.Quote from: tautech on March 25, 2020, 06:30:41 amBut 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?

author: tautech
author_rank: Super Contributor
timestamp: March 25, 2020, 09:22:30 am
content: Quote from: max.wwwang on March 25, 2020, 08:52:24 amYou 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.

author: shakalnokturn
author_rank: Super Contributor
timestamp: March 25, 2020, 10:29:57 am
content: Quote from: tautech on March 24, 2020, 09:16:52 am"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.

author: tautech
author_rank: Super Contributor
timestamp: March 25, 2020, 11:08:09 am
content: Quote from: shakalnokturn on March 25, 2020, 10:29:57 amYes but we may never know unless it gets measured.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: March 26, 2020, 04:54:17 am
content: 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.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: March 26, 2020, 05:36:01 am
content: Quote from: tautech on March 25, 2020, 09:22:30 amThanks that saves some of my brain cycles!

author: Shock
author_rank: Super Contributor
timestamp: March 26, 2020, 09:33:34 am
content: 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.

author: shakalnokturn
author_rank: Super Contributor
timestamp: March 26, 2020, 09:35:56 am
content: Quote from: max.wwwang on March 26, 2020, 04:54:17 amSimultaneously 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.

author: Shock
author_rank: Super Contributor
timestamp: March 26, 2020, 10:21:20 am
content: 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.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: March 27, 2020, 06:37:06 am
content: Quote from: shakalnokturn on March 26, 2020, 09:35:56 amAgreed.Quote from: Shock on March 26, 2020, 10:21:20 amThis absolutely helps and I'll read this for some good times. Thanks for putting this together - yes, I'm just starting.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: March 27, 2020, 07:04:00 am
content: Quote from: Shock on March 26, 2020, 09:33:34 amThanks. 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!

author: max.wwwang
author_rank: Frequent Contributor
timestamp: April 01, 2020, 04:39:56 am
content: 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 fromherejust 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.

author: tautech
author_rank: Super Contributor
timestamp: April 02, 2020, 08:35:18 am
content: Quote from: max.wwwang on April 01, 2020, 04:39:56 amLooking 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

author: max.wwwang
author_rank: Frequent Contributor
timestamp: April 04, 2020, 07:19:42 am
content: Quote from: tautech on April 02, 2020, 08:35:18 amThat'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 --

author: Shock
author_rank: Super Contributor
timestamp: April 04, 2020, 08:40:24 am
content: 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.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: April 06, 2020, 08:41:53 am
content: Quote from: Shock on April 04, 2020, 08:40:24 amHaving 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.

author: Shock
author_rank: Super Contributor
timestamp: April 06, 2020, 10:05:45 am
content: Depends on when it was made as well, standards obviously will change and improve over time.