Samsung DVD Recorder DVD-HR755 Doesn't Turn On

[max.wwwang]

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 available here.

[max.wwwang]

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.

[tautech]

Power lead is good and mains power (240V AC) comes in the unit. Apparently the the PSU is a switch mode one.

It certainly is.

I measured a rectified DC voltage 333V on the full bridge rectifier (GBJ2J).  SM controller is ICE2BS01. Do idea what are the right next steps to trouble-shoot.

I 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.

[Falkra]

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).

[max.wwwang]

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 manual here which 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?

[tautech]

The SM IC must have sufficient VCC in order to start so first suspect is the HV resistor/s.

[max.wwwang]

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.

[tautech]

Likely. From the schematics, I suspect the Zenar diode (MTZ-J20B, highlighted) is down (short).

That's certainly possible but not the first thing I would suspect.

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.

The 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.

[max.wwwang]

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.)

[max.wwwang]

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!

[tautech]

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!

Nice, 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 eyes    suspect any joint that has any physical or heat stress.
Good luck. 

[Shock]

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.

[max.wwwang]

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?

[tautech]

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?

It means if you are to scope or measure anything use the common ground and not the chassis as the reference point.

[max.wwwang]

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)?

[max.wwwang]

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?

It means if you are to scope or measure anything use the common mains ground and not the chassis as the reference point.

I'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.

[tautech]

It goes to ready when I measure the voltage across the resistor with this multimeter at the setting of 600V DC.

Which resistor, one of the 270K ones ?

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)?

If no metal parts are accessible to the user it won't need a PE.

[max.wwwang]

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!

[max.wwwang]

Which resistor, one of the 270K ones ?

Yes, R1S14.

If no metal parts are accessible to the user it won't need a PE.

The 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?

[tautech]

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!

Don'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. 

[tautech]

The remaining question is possibly why my multimeter - and only this one - had such magical behaviour?

Its 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.

[max.wwwang]

In my first post I mentioned the dropper resistors to VCC from the HV DC is something to always check. 

That was spot on. Good on you and thanks!

Its internal resistance on the range you used was lower than your other DMM's so conducted sufficient current for the IC to do a single kick or two ...

That was the same thing I could think of.

... then the primary side kickback supply took over to allow for ongoing operation.

I struggled to understand how the working of IC could be maintained. Will have more tests on this to get a better understanding.

[max.wwwang]

I struggled to understand how the working of IC could be maintained. Will have more tests on this to get a better understanding.

According 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.

[tautech]

I struggled to understand how the working of IC could be maintained. Will have more tests on this to get a better understanding.

According 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".

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 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.

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

[max.wwwang]

Bingo, fixed!