Re: LG LCD LED TV No Standby Light Won't Turn On Pwr-On Relay Clicks On Then Off

[Fixin Stuff]

UPDATE: Title updated 28th June 2020 - skip to page 3 to get to current issue. Thanks! 

Hi Peeps! Welcome to my first EEVBlog post! 

Been fixin' stuff all alone for years... but then in my quest for answers, I stumbled upon 'the promised land' that is EEVBlog!

I'm an electrician and a tinkerer. Got some basic electronics knowledge but am new to troubleshooting SMPS. This is the most ambitious repair I've undertaken so far. Doing it more for the fun/learning/lolz than the result. TBH, I don't know what I'd do with this TV if I get it working 

While in lockdown a month or so ago and stricken with boredom, I discovered an LG47LE7500 47inch LCD LED TV (https://www.lg.com/au/tvs/lg-47LE7500-lcd-tv) in the trash room of my apartment building. After seeing it sitting there for a week and noticing the panel seemed to be in good condition, I managed to convince my girlfriend to allow me to drag it's heavy a$$ (the TV, not my GF) up to our apartment.

I plugged the TV in anddd...nothing. No noises, no smoke (maybe someone already let it out?), no smells, no standby light. Power button didn't seem to do anything.

I decided to look inside, and it was only then that I realised someone had already opened it, because most of the screws on the back cover were missing. An ominous sign perhaps?

Anyway, I took the back cover off and had a look inside (see pic of my ACTUAL board attached). It didn't really look like anything had been touched internally. So either:
- They knew what they were doing so their work is undetectable, and they couldn't fix it  or;
- They replaced entire parts, eg: a whole board, or;
- When the TV broke, the previous owner took a look to see if they could fix it, decided it looked too scary/difficult/expensive to attempt a fix and left it for dead.

I'm hoping it's the latter!

The board details are:
- Model: YP47LPBL
- Part number: EAY60803401
- Revision: 1.12
- Date: 2010/07/05 (Looks like QA was done on 2010/08/28)

You can see some super high detail pics of an earlier revision of this board than mine here, which seems to have an extra relay/contactor thingy compared to my board, but otherwise seems very similar:
Pic 1: https://assets.shopjimmy.com/media/catalog/product/cache/1/image/f8f28dc4a19c21aeb55ee5774693fb5d/s/h/shopjimmy-eay60803401-top_1.jpg

Pic 2: https://assets.shopjimmy.com/media/catalog/product/cache/1/image/f8f28dc4a19c21aeb55ee5774693fb5d/s/h/shopjimmy-eay60803401-bottom_1.jpg

I spent A LOT of time trying to find a schematic for this board but to no avail. Only a crappy tech manual which only seems to have schematics for everything BUT the power supply.

I watched many hours of YouTube vids and found that it's best to first disconnect the power supply board from the main board and test what the low voltage output is when powered up. So I disconnected all cables from the power supply board except the power cord. I've got a fairly basic UNI-T multimeter, but the LV output was pulsing up to around 3.5v. So that made me assume the power supply board was at fault.

Once the incoming AC goes through the first 3x diodes (D100, D101 & D102 - follow the circuit in the 'Pic 2' linked above) the voltage after the last diode is pulsing from 0v up to about 180v at a consistent 1-2 second interval. Not sure if that interval has much to do with the refresh rate of my meter. I also measured a voltage drop across the last diode of around 100v (from memory, haven't tested this for a while). All of the diodes test ok when tested in diode mode.

Tests I've done so far:
- There are no shorts to ground on the LV outputs
- Seems like good continuity between all GND pins and GND on the LV outputs
- Seems to be no shorts to GND on the incoming AC, or on the incoming DC after the above-mentioned diodes.
- Have checked/tested for other shorts around the place and no signs so far.
I've tested most (if not all) of the large capacitors for shorts to ground or being short circuited and they all seem ok. No visible problems with any of the caps! (Not that that means anything).
- I don't have an ESR meter so haven't checked caps in that way yet.
- I've tested all of the large diodes on diode mode and all seem to test ok, although it's been too long since learning electronics at school to know how the zener diodes should test. They all just seemed to work/test like the other diodes.
- I've tested all of the large resistors with 4/5 band ratings. All seem to test pretty much perfect. There were a couple of resistors that I couldn't for the life of me work out the colour coding/rating of (used Digikey's guide). Where these were in pairs/threes, I compared their resistance with other identical resistors and they all seemed to be very closely matched in resistance, nothing out-of-range that would suggest an issue.

Things I've changed so far:
- I thought one of the optocouplers (IC 503) under the centre transformer tested funny/different to the one next to it (IC502), so I ordered a replacement and swapped it. It made no difference, so I swapped the other one too, and still no difference.
- I thought one of the MOSFETs tested weird (in circuit), so replaced both of those (Q102 & Q103). That was fiddly! Even added new thermal paste. But it made no difference.
- I read an EEVBlog post about another SMPS that was pulsing, and someone said that it was probably an issue with the 'snubber' cap/resistor/diode. The resistor (R501) tests almost perfect for it's rating, and the diode (D501) tests fine on diode mode, so I replaced the capacitor (C502), but again, it made no difference.
- I decided to replace any other caps related to the voltage converter chip, and replaced:

• The 5x little 18uf electrolytic caps at C506 - C510
• The 471k ceramic cap at CY 105, and then cos I had lots left, the other 6x of these at CY 101, 102, 103 & 104 near the incoming AC,  CY 106 near the pair of transformers, and CY 107 in the corner by itself.

Still no difference!

The only other caps I haven't replaced that, as I learn more I probably should have done as one of the first things  , are the pair of electrolytic caps by the diodes I mentioned at the start and next to the snubber, which are caps C501 & C511. I have replacements for these arriving next week.

The 4x light blue metalised polypropylene caps near the incoming AC all look perfectly fine, and no shorts etc, but again I haven't tested them with an ESR meter. Could consider changing these out for a few $$$.

I was also wondering what the chances were that the voltage converter IC needed replacing? It's an ST Viper27h (see attached pic). Full details here: https://docs.rs-online.com/0422/0900766b813e568b.pdf .

That's my next guess if the pair of caps I've ordered doesn't fix it. BUT, that's why I'm posting here, because my guesses so far have sucked haha! Grateful for any suggestions, and feel free to ask any further questions etc.

Thanks for reading my novel... 

[TheMG]

Only a crappy tech manual which only seems to have schematics for everything BUT the power supply.

That's not uncommon at all. LG did not design the power supply, it was manufactured by a third party. A lot of companies do this.

- I thought one of the optocouplers (IC 503) under the centre transformer tested funny/different to the one next to it (IC502), so I ordered a replacement and swapped it. It made no difference, so I swapped the other one too, and still no difference.

Quite rare for optocouplers to go bad, and when they do, one of two things will happen: very little to no output from the power supply, or far too much output due to open feedback loop and everything on the secondary side releases the magic smoke.

The 471k ceramic cap at CY 105, and then cos I had lots left, the other 6x of these at CY 101, 102, 103 & 104 near the incoming AC,  CY 106 near the pair of transformers, and CY 107 in the corner by itself.

All of those blue capacitors with the "CY" designation actually are not even needed for the power supply to operate. They are strictly there to reduce electromagnetic (radio) interference by providing a path for high frequencies back from the secondary to the primary side. Also note that because these capacitors bridge the low voltage and high voltage sides (which is conveniently indicated by a dashed line on the PCB), they are safety critical, and must only be replaced with the special (often blue) safety-rated class Y capacitors. These capacitors are special in that they are designed so they will not fail short, which would potentially create an electric shock risk if that were to happen.

The 4x light blue metalised polypropylene caps near the incoming AC all look perfectly fine, and no shorts etc, but again I haven't tested them with an ESR meter. Could consider changing these out for a few $$$.

These caps also do not affect the operation of the power supply. They are there to filter out noise (EMI) from the power supply from being conducted out through the AC line cord.

Have you at all measured the 3.5V with the power supply connected to the rest of the TV? Reason why I'm asking, is that some power supplies require a minimum load in order to operate properly, which could be the reason it's pulsing.

[TheMG]

Maybe this will help... I've taken your picture and identified the various sections of the power supply and what they do.

Ignore every component for now that is not in the standby power supply section, until you've confirmed the standby supply is operating correctly. Again, it may need some load on it to work correctly, what does the 3.5V output measure when connected to the TV main board?

[TheMG]

Was browsing the forums again and something in the title of your thread caught my eye... you say both the primary and secondary are pulsing?

Unless it's your multimeter's auto-ranging playing tricks on you, that would be a dead giveaway as to what the problem might be: a high resistance path somewhere between the AC mains input and the capacitors C501/C511, so the capacitors charge up to 180V, the power supply starts up but as it draws current and there is too much resistance, the primary capacitors quickly discharge and it shuts down, then they charge back up, and the cycle repeats.

A bad solder joint, a failure of NTC thermistor TH102, or even a blown fuse that is still just slightly conductive, could be possible causes. Heck, for that matter, even a faulty power cord could theoretically cause such a problem.

What I would do is measure the resistance between the live pin of the mains AC input connector and the first rectifier diode, and also the resistance from the AC neutral input to the negative side of C501/C511. The resistance for both paths should be very low, less than one ohm for the neutral path, and around 10 ohms for the live path due to the NTC thermistor. However, for the standby power supply to exhibit a charge/discharge cycling behavior, there would have to be a resistance of well over 1kohm, so it should be fairly obvious, don't worry too much about the exact resistance readings as they are approximate.

[Fixin Stuff]

Hi TheMG

Thanks so much for the detailed responses and info. Very grateful for you going to so much effort!

All of those blue capacitors with the "CY" designation actually are not even needed for the power supply to operate. They are strictly there to reduce electromagnetic (radio) interference by providing a path for high frequencies back from the secondary to the primary side. Also note that because these capacitors bridge the low voltage and high voltage sides (which is conveniently indicated by a dashed line on the PCB), they are safety critical, and must only be replaced with the special (often blue) safety-rated class Y capacitors. These capacitors are special in that they are designed so they will not fail short, which would potentially create an electric shock risk if that were to happen.

Thanks for mentioning this. I of course want this to be safe if/when I get it working. I was quite careful in selecting replacements although I wasn't aware of this point. These are the ones I replaced them with, which were also 471k X1Y1 ceramics, albeit with a higher voltage rating: https://au.rs-online.com/web/p/ceramic-single-layer-capacitors/7368858/

Have you at all measured the 3.5V with the power supply connected to the rest of the TV? Reason why I'm asking, is that some power supplies require a minimum load in order to operate properly, which could be the reason it's pulsing.

I haven't done this yet actually. After reading your later post/points (see below), I get the impression this would be worth considering after trying/testing/confirming the operation of the power supply, right? I can get this put back together so I can test this and report back if you think it's worth doing before addressing the points below.

Maybe this will help... I've taken your picture and identified the various sections of the power supply and what they do.

Ignore every component for now that is not in the standby power supply section, until you've confirmed the standby supply is operating correctly. Again, it may need some load on it to work correctly, what does the 3.5V output measure when connected to the TV main board?

This pic really helps! Especially with no schematic and my limited knowledge of electronics

Was browsing the forums again and something in the title of your thread caught my eye... you say both the primary and secondary are pulsing?

Unless it's your multimeter's auto-ranging playing tricks on you, that would be a dead giveaway as to what the problem might be: a high resistance path somewhere between the AC mains input and the capacitors C501/C511, so the capacitors charge up to 180V, the power supply starts up but as it draws current and there is too much resistance, the primary capacitors quickly discharge and it shuts down, then they charge back up, and the cycle repeats.

A bad solder joint, a failure of NTC thermistor TH102, or even a blown fuse that is still just slightly conductive, could be possible causes. Heck, for that matter, even a faulty power cord could theoretically cause such a problem.

What I would do is measure the resistance between the live pin of the mains AC input connector and the first rectifier diode and also the resistance from the AC neutral input to the negative side of C501/C511. The resistance for both paths should be very low, less than one ohm for the neutral path, and around 10 ohms for the live path due to the NTC thermistor. However, for the standby power supply to exhibit a charge/discharge cycling behavior, there would have to be a resistance of well over 1kohm, so it should be fairly obvious, don't worry too much about the exact resistance readings as they are approximate.

This comes in at:
0 ohms from the AC input connector to before the thermistor TH102, and 14.3 ohms after the thermistor and at the input of the first rectifier diode.
The AC neutral input to the negative side of C501/C511 is about 4.4 M-ohms!

I'm looking at the multiple neutral return path routes coming off the negative terminals of C501/C511...and to say it looks complicated is an understatement haha... 

Also, would that neutral resistance be affected by the power supply being powered on vs off?

I'm open to and grateful for any suggestions on how to start figuring this out!

[TheMG]

0 ohms from the AC input connector to before the thermistor TH102, and 14.3 ohms after the thermistor and at the input of the first rectifier diode.

No problem there, that is right in the ballpark of what I'd expect.

The AC neutral input to the negative side of C501/C511 is about 4.4 M-ohms!

Now you're on to something (or maybe not, keep on reading...).

Also, would that neutral resistance be affected by the power supply being powered on vs off?

No. The standby power supply is always energized as long as there is AC mains to the TV, it provides power to the "brains" of the TV so you can turn it on using the remote control, so that current path should always be near-zero ohms. It's also standard practice to switch the live not the neutral, or to switch both, but there's nothing like that at all happening in this power supply from what I can see.

I'm looking at the multiple neutral return path routes coming off the negative terminals of C501/C511...and to say it looks complicated is an understatement haha...

I see there is something I missed (based on the shopjimmy picture of the back side of the PCB). It looks like the negative side of those caps is in fact NOT connected directly to neutral, it's actually connected to the negative DC output side of the main bridge rectifier, so that the primary high voltage DC of both the standby supply and the main power supply are commoned together. When the TV is turned on and the PFC boost converter is running, power for the standby supply no longer comes from D100/101/102, but instead comes from D10? (blob of silicone over the last digit) adjacent to D601, feeding the standby supply with 400VDC from the PFC converter.

Extra complication for sure, but since it appears the 3.5V supply is responsible for powering the majority of the logic circuits in the TV even while it is turned on, and thus a higher current demand, the manufacturer wanted to make sure it could meet power factor requirements, thus getting power from PFC while on, but directly from half-wave rectified mains while in standby, for power efficiency reasons (albeit at a much poorer power factor). So this design aspect makes sense.

Anyways, what I'm getting at here, is that the 4.4Mohm reading could just be a diode in bridge rectifier in reverse-bias, and not actually an open circuit (a red herring if you will).

I don't know about you, but I'm a visual person, so... I've attached another picture outlining the neutral current flow path, hopefully it helps. Check the path from neutral input to the bridge rectifier, and also check the diode in the bridge rectifier for proper forward/reverse bias reading.

[Fixin Stuff]

Hi TheMG

Thanks again for the follow up and going to the effort with the pics.

I see there is something I missed (based on the shopjimmy picture of the back side of the PCB). It looks like the negative side of those caps is in fact NOT connected directly to neutral, it's actually connected to the negative DC output side of the main bridge rectifier, so that the primary high voltage DC of both the standby supply and the main power supply are commoned together. When the TV is turned on and the PFC boost converter is running, power for the standby supply no longer comes from D100/101/102, but instead comes from D10? (blob of silicone over the last digit) adjacent to D601, feeding the standby supply with 400VDC from the PFC converter.

It's D103 that has the blob of silicone over it (not that it really matters as we have no schematic to refer to). Yes, that was confusing to me how both power outputs seemed to converge. It makes sense now you spelled it out.

Anyways, what I'm getting at here, is that the 4.4Mohm reading could just be a diode in bridge rectifier in reverse-bias, and not actually an open circuit (a red herring if you will).

I don't know about you, but I'm a visual person, so... I've attached another picture outlining the neutral current flow path, hopefully it helps. Check the path from neutral input to the bridge rectifier, and also check the diode in the bridge rectifier for proper forward/reverse bias reading.

This pic, as simple as it is, was actually super helpful haha! I wasn't sure how the bridge rectifier was internally 'wired'. I coulda looked it up, but until now hadn't thought it could be significant.

So I did some tests based on your info:
- From neutral terminal to the bridge rectifier EL115 (through all the chokes) is 0 ohms.
- From negative terminal of bridge rectifier BD101 to capacitors C501 and C511 negative pins is 0 ohms.
- Diode in bridge rectifier measures OL in reverse bias (according to your diagram), and .456v in forward bias.
- Other diode in bridge rectifier measures OL with the positive probe of the meter on the positive terminal and the negative on EL116, and it measures .464v with the meter probes switched over.
- From negative pins of capacitors C501 and C511 to neutral terminal is 5 M-ohms when the meter probes' polarity is correct, and OL if probes are reversed.

Unless I've missed something, this all seems to be testing ok so far and nothing is obviously wrong. Open to any other suggestions 

[TheMG]

Hmmm... maybe it's not actually pulsing and it's just your multimeter acting funky? If it is auto-ranging can you try taking the voltage measurements while manually selecting the appropriate voltage range? I've seen some inexpensive multimeters with auto-ranging do some funny things like this.

Other possibility is the TV's power cord or power connector is bad. Check the 120V AC on the two AC input pins of the bridge rectifier if you haven't already. Should be a solid 120V AC no pulsing.

[Fixin Stuff]

Hmmm... maybe it's not actually pulsing and it's just your multimeter acting funky? If it is auto-ranging can you try taking the voltage measurements while manually selecting the appropriate voltage range? I've seen some inexpensive multimeters with auto-ranging do some funny things like this.

This could be the case. My meter is purely auto-ranging and only has modes for AC/DC voltage, and no ranges. Even cheaper meters than this have ranges.

Other possibility is the TV's power cord or power connector is bad. Check the 120V AC on the two AC input pins of the bridge rectifier if you haven't already. Should be a solid 120V AC no pulsing.

We have around 240V AC mains voltage here. I tested between all of the pins on the rectifier and struggled to get more than 2v between any of them.

Between the rectifier pins and GND there was barely any voltage registering, but between the rectifier pins and the L terminal of the connector on the board side was a solid 250v with no pulsing.

The standby power supply area of the board still has around 180V pulsing on AC voltage mode.

The DC output connector is still pulsing from zero to around 3.9v DC.

It's also making a faint 'tick' which sounds like the tick of a clock at about the same frequency as the voltage pulse spikes which seems to be coming from the area where the incoming AC EMI filtering stuff is. Could be one of the chokes/inductors. Really hard to identify exactly where it's coming from. Nothing seems to vibrate if you rest a screwdriver on it or anything.

My replacement caps for C501 and C511 arrived. Is it worth chucking those in and seeing if it makes any difference? They directly/indirectly connect to the areas of the board that supply the voltage controller IC.

Thanks for the ongoing suggestions. Open to any other ideas from TheMG or anyone else

[TheMG]

Solid 250VAC from L to N on SK101?

[Fixin Stuff]

I've got:

- Solid 245VAC from L to N (pads either side of SK101)
- Solid 245VAC from L to SK101
- Solid 0.06V from N to SK101

 

[sean0118]

Are you getting constant DC voltage across C501 / C511? Should be around 330Vdc.

[Fixin Stuff]

Hi Sean

Ok, I think you've cracked the case of the 'pulsing primary AC volts' (we'll get to the ongoing pulsing secondary DC issue later), and this is SUPER embarrassing for me! But I'm gonna be honest... 

Context: At polytech, the tutor said to always first test circuits with your meter set to the highest AC voltage mode to avoid cooking your meter...and then to switch to other modes or DC when you could be sure it was safe to do so...

So the tests I'd done up till now around the standby power supply had been on AC voltage mode, because when I first tested, I got a fairly high and pulsing voltage which I couldn't be sure was being properly rectified to DC.

But after your message, I was like "what the heck", the man asked for DC so I'll test DC. And whadya know!?

I got CLEAN, smooth 340v DC across the terminals of both C501 and C511! Let the insults fly! 

I gotta apologise to TheMG for sending them on a wild (Canadian) goose chase. So sorry!  I've updated the title of the thread to reflect what's actually happening.

So after some tests with DCV! I get:
- 340VDC coming out of the first lot of diodes D100 - D102
- 340VDC between GND (SK101) and the primary windings (EL165) of the central transformer
- 340VDC between EL165 and EL166 on the central transformer
- On the secondary of the same central transformer I get 0VAC and 0VDC between EL167 & EL168, and between any of the secondary connections actually.
- BUT I still get the DC output connector pulsing between around 2.9VDC- 3.5VDC at around the same regular frequency of 1-2 seconds as the initial pulsing and as the 'tick' coming from the AC input filtering area.

So now we seem to be getting somewhere, please feel free to make any new suggestions of what/where to test next 

[TheMG]

Context: At polytech, the tutor said to always first test circuits with your meter set to the highest AC voltage mode to avoid cooking your meter...and then to switch to other modes or DC when you could be sure it was safe to do so...

That advice is pretty much irrelevant with modern digital multimeters, it was mostly applicable to analog meters.

And yes, I have seen less expensive digital meters have the auto-ranging go berserk and show an unstable reading in AC mode when connected to a large DC voltage (this is actually a common thing to do when you want to measure for ripple on a DC supply voltage). One of the many differences between a cheapo meter and a good meter (never had this problem with the Fluke 289 I use at work).

The good news is, the standby power supply is getting primary input voltage just fine. The bad news: we still don't know if the problem is in the primary or secondary side.

One possibility would be a short or overload on the output causing the power supply to go into "hiccup" mode, but since it's briefly coming up to the full 3.5V as opposed to some lower value, that's probably not the case. Anything getting warm on the secondary side if you leave it powered on for a few minutes?

More likely scenario, is that there is a problem in the primary side circuit that is responsible for providing power to the SMPS controller (IC501: VIPER27), or the chip itself.

The way it works in normal operation is as such:

High voltage present at pins 7 and 8 (DRAIN), the chip allows a small amount of current to flow from this pin through to pin 2 (VDD) which charges up C506. Once the voltage on C506 reaches operating threshold (nominally 14V), the chip begins operation.

After the chip has started operating and is switching the transformer, an auxiliary winding (bottom two pins on primary side of transformer) begins outputting voltage. This is rectified by D502 and charges up C507 via R506. Q503, R505, and zener diode ZD502 form a voltage regulator limiting the voltage being fed to keep C506 charged, this is where the chip gets its operating power VDD during normal operation, after the initial startup.

Aside for the three components Q504, R505, and ZD502 forming an external voltage regulator, this is pretty typical of how a SMPS normally works, and you can also see this in the typical application diagram in the VIPER27 datasheet.

If for whatever reason the charge in C506 is not being maintained from the auxiliary winding, it will quickly discharge until the chip shuts down again, and then start recharging. This cycle will repeat itself indefinitely, leading to a pulsing output as the SMPS starts and stops repeatedly.

Several things could cause this:

-open auxiliary winding, or bad solder connection to it
-D502 open (probably unlikely)
-R506 open
-voltage regulator comprising of Q503, R505, ZD502 not working

I would measure the following next:

-DC voltage on C506: steady/pulsing? what is its maximum reading?
-DC voltage on C507: steady/pulsing? max reading?

[Fixin Stuff]

Hi TheMG

Thought I woulda lost you after the goose chase haha. Really grateful for your patience and ongoing and detailed assistance 

That advice is pretty much irrelevant with modern digital multimeters, it was mostly applicable to analog meters. And yes, I have seen less expensive digital meters have the auto-ranging go berserk and show an unstable reading in AC mode when connected to a large DC voltage (this is actually a common thing to do when you want to measure for ripple on a DC supply voltage)

This is pretty interesting. Wish I'd known! And yes, our polytech tutors got sick of us blowing up their analogue AVO meters 

One possibility would be a short or overload on the output causing the power supply to go into "hiccup" mode, but since it's briefly coming up to the full 3.5V as opposed to some lower value, that's probably not the case. Anything getting warm on the secondary side if you leave it powered on for a few minutes?

It does seem like a sort of 'hiccup mode' in terms of the VIPER chip starting up and turning off like you say, or some similar/related problem. I loved the detailed description of how this circuitry works on this particular board, too. It was great being able to follow it all through.

I did previously have a feel around the board for hot spots but couldn't find anything warm/hot, and nothing looks visually like it's overheated (not that this means much). I don't have an IR camera. I've seen Louis Rossmann splash isopropyl alcohol on boards to see which area is warm by observing where the alcohol evaporates from the fastest. But electronics stores have been sold out of alcohol here for a while as all alcohol was being used to make 'home-brew' hand sanitiser and as a disinfectant  I could try again to see if they have it back in stock yet to test for hotspots/shorts.

I also tested the 3.5v output connector for shorts to GND. All of the voltage/signal terminals are at minimum in the K-ohms to the GND terminals, and the GND terminals are 0 ohms to the main GND terminal. So doesn't seem to be an obvious short to GND at least.

Several things could cause this:

-open auxiliary winding, or bad solder connection to it

If I'm right that you're referring to EL166 and its neighbouring transformer connection, this had a 1.2 ohm resistance when I tested it. I re-soldered it but it didn't change. It usually takes about 5 seconds for the meter to gradually climb to this reading for some reason.

-D502 open (probably unlikely)

This measures 0.424v on diode mode in forward and OL in reverse

-R506 open

This measures 21.9 ohms

voltage regulator comprising of Q503, R505, ZD502 not working

Is ZD502 is meant to be installed in reverse? It currently measures OL from positive to negative, and .646v from negative to positive. This is basing the orientation on the fact that it's connected on one side to the negative pin of C507, and this connection has a 0 ohms reading.

R505 measures 1 k-ohm.
Not 100% sure how to test Q503 or if it can be done reliably in circuit.

I would measure the following next:

-DC voltage on C506: steady/pulsing? what is its maximum reading?

Seems to be flickering between 5VDC-13VDC

-DC voltage on C507: steady/pulsing? max reading?

Seems to be almost identical to C506 above and is flickering between 5VDC-13VDC

Both of these caps (C506 & C507) are brand new too.

I'm starting to get excited about this. Seems like we're getting close! Lemme know what to check next 

[sean0118]

Is ZD502 is meant to be installed in reverse? It currently measures OL from positive to negative, and .646v from negative to positive. This is basing the orientation on the fact that it's connected on one side to the negative pin of C507, and this connection has a 0 ohms reading.

Yeah it would be a zener diode, so it's normal for it to be reversed biased, it and R505 form a shunt regulator. I'm not 100% sure what Q503 does, maybe over current protection? Someone on here will know.

R505 measures 1 k-ohm.

Probably right? Does it have the value printed on top?

Seems to be flickering between 5VDC-13VDC

Good work, this is the problem, now you just have to find what is causing it. 


You've already ruled out some of the possible causes that TheMG mentioned.

Maybe the next step is to have a *really* good look at the soldering, particularly the pins of the transformer and any large through hole components around it.

Could also be the Viper chip is damaged and it's drawing too much current when it turns on, pulling its supply rail too low and making it turn off.

You could try measuring resistance between the VDD pin and Drain and also VDD to GND. You could also try to measure the diode between the GND and Drain pin. But the only way to be sure is to replace it, as it might only break in operation.

[TheMG]

Yeah it would be a zener diode, so it's normal for it to be reversed biased, it and R505 form a shunt regulator. I'm not 100% sure what Q503 does, maybe over current protection? Someone on here will know.

Actually the resistor, zener diode, and transistor all work together to form a very basic voltage regulator. Zener diode is from base of transistor to ground, resistor goes from collector (input) to base, and the emitter to the load/output. It's an emitter-follower configuration.

The good news is, since such a regulator circuit doesn't allow for reverse flow of current, this means the auxiliary winding, D502, and R506 are doing their thing, otherwise you wouldn't have any voltage on C507, only on C506 getting charged through IC501.

This is getting into the realm of things that are difficult to troubleshoot with just a DMM. An oscilloscope with a high voltage isolated probe (or an isolation transformer), would be able to tell a lot more about what's actually going on.

But it is starting to look like IC501 might be at fault. It's under two dollars on Digikey, might be worth a shot.

[Fixin Stuff]

Hi Sean and TheMG, thanks so much for the messages and suggestions!

For R505, and mainly for the transistor, it was super hard to see the markings. But luckily I have a half-decent camera so managed to zoom right in and get a close up photo for a better look (see attached pic).

R505 is marked '102' which seems to mean it should be a 1 k-ohm resistor, so that looks good.

As for the transistor, there are only two characters visible on it ("PZ"?) and I'm guessing there should be a number as well? Even with the original full-resolution image, I can't see any other figure. Is there any way to be able to work out what this should be based on the surrounding components and test results? 

Maybe the next step is to have a *really* good look at the soldering, particularly the pins of the transformer and any large through hole components around it.

Did this. Re-soldered the all of the transformer joints, and also re-soldered the large through-hole stuff nearby, including the pins of the Viper chip. I just powered up the board again to test it and the soldering hasn't made any difference. The DC output is still "hiccup-ing" between 2.5VDC and 3.5VDC.

You could try measuring resistance between the VDD pin and Drain and also VDD to GND. You could also try to measure the diode between the GND and Drain pin. But the only way to be sure is to replace it, as it might only break in operation.

- VDD to either drain pin = OL
- VDD to GND pin = OL
- VDD to negative of C507 = 17 M-ohms
- GND pin of Viper to GND of board = OL
- GND pin of Viper to negative of C507 = 0 ohms

Please let me know if any of the above readings seem out of range.

But it is starting to look like IC501 might be at fault. It's under two dollars on Digikey, might be worth a shot.

Happy to replace the Viper chip if that's the next logical step. Thanks so much for the help. I'll be sure to update when the chip comes in and is installed 

[Fixin Stuff]

Ok, so my Viper chips arrived today (had to order a minimum of 10 from local supplier, which was still cheaper that postage to Australia for a single chip from Digikey)...

I soldered a new Viper chip onto the board, anddddd... *drumroll* ...it's still doing the same thing!    This has got me stumped!

Something about me which is a blessing and a curse is I'm pretty persistent. I still don't want to give up on working this problem.

But I'm already beyond my current electronics knowledge. All I'd really be able to do now is to start doing more 'shotgun' approach (changing everything until it starts working).

I'd be really grateful for any more ideas/suggestions of what to look for/change/do next to keep troubleshooting 

[shakalnokturn]

How about checking capacitors C204, C205, C506 to C510?

[Fixin Stuff]

Hi shakalnokturn

Thanks for offering some suggestions! 

For capacitors C506 t0 C510, I have actually already replaced all of these with brand new capacitors of the same value and unfortunately it didn't help.

But I haven't yet tested/replaced capacitors C204 and C205. According to one of TheMG's first diagrams, these are also part of the 3.5V power supply, so your suggestion to investigate them makes a lot of sense. I will check these out next and report back. Much appreciated

[shakalnokturn]

Sorry about C506 to C510... The topic was already quite long , I did fly over some of it.

[Fixin Stuff]

Hey team. I found something of interest when testing shakalnokturn's suggestion.

Both caps C204 and C205 actually seemed ok from some basic multimeter tests (I don't have an ESR tester). But both had NO shorts to ground, zero ohms continuity to GND on their negative terminals, and almost identical resistance internally. If one was standing out as having a noticeably different reading from the other, I'd probably be more concerned about them, but they seemed ok. I also don't fancy waiting another week for them to arrive, as I can't get them locally. Especially if they don't make any difference haha!

But I decided to test the other components around the VIPER chip. I tested the small SMD caps, but they seemed ok based on same tests as the caps above.

Then I looked at the SMD resistors coming off the VIPER chip. The first one I tested was R502, marked 474 (470k-ohms, see pic). It turns out it's reading just 10.2ohms! If I understand what you guys have been saying previously, could this cause the capacitors that help the VIPER chip to start-up to drain before the chip has completed its startup? Then cause the startup loop 'hiccup'? 

I want to replace this resistor before anything else as it's WAY out of spec. But I have three choices of 470k SMD resistors available locally and I have no idea how to calculate/choose which one to use, OR if any of them are going to be any good and if I need something else.

The 3x options I have locally are as follows, they all have this same package, but have different wattage's (option listed below): https://www.altronics.com.au/p/r1268-470k-.125w-805-metal-film-smd-resistor-pk-10/ (picture on listing is indicative so markings on actual item may be different)

Should I choose:
0.125W
0.25W
1W

Or something else? 

If it helps, you can get the full VIPER chip specs and typical circuit etc here: https://docs.rs-online.com/0422/0900766b813e568b.pdf

Grateful for any suggestions 

[Fred Basset]

Sorry, I could not follow all of it properly as I am pretty ill right now.  But I did see something like this once before.  The pulsing power supply is caused by it trying to close down as soon as it starts up.  Interesting that it seems to be once the logic gets going properly.  What I came across was that it was getting a signal to turn itself off from the remote control board.  Error on the board, leaky IR diode and don't forget potentially a sticky "Power" button on the remote, or the front of the TV.

I hope I have been able to help in some way, after all the work on it you have done, you deserve to get this one going!

[TheMG]

99.9% of the time, resistors fail high resistance or open-cicrcuit.

The chances of a 470k resistor going to down to 10 ohms is pretty much non existent. If you are measuring it while it's in the circuit, it's probably something else in the circuit that you're actually measuring not just the resistor itself.