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Old 05-22-2021, 11:32 AM   #1
yuuki47
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Post VESTEL 17PW26-5 Problem

Hello!

I have this old JVC LT-42TG30J TV with a flashing blue light. It does not turn on, but weirdly, if I leave the power switch on, the backlight of the TV randomly comes on though the blue light continues to flash and I can't bring up something like a volume control, which makes me think that the backlight just randomly turns on even if the system isn't. I can hear this mechanical hum from the inverter board as well. It will turn off and on itself randomly after that. Weird. The first time it happened it scared me to death lmao.

My first test was to see if the power supply was OK. It has a 17PW26-5 type PSU, which has no schematics D: Though the older revision (17PW26-4) does and the power connector is similar.

Here are the results:
12_V1 OK @ 11.80
5V STANDBY OK @ 5v sharp
3.3V OK @ 3.3v

VCC_3V3_ON OK @ 3.3v

24VA OK @ 24v

PIN_11 OK @ 11.80V
PIN_13 OK @ 3v
PIN_15 OK @ 3.3v

PIN_17-21 OK @ 5v

PIN 23 OK @ 24v

ST_BY is... weird. It's at 0.00v volts but when I cut power to the board it goes 3.3v. It could be normal behaviour though...
SW_SWITCH... I'm unsure of what that is. It fluctuates between 8.02v and 9.35v. Something tells me that this isn't normal. Maybe this is the line that turns on the backlight? The inverter board is connected to the power supply, so the logic board must send a signal through somewhere to turn it on, which also makes me believe that the logic board is the problem.

What do you think? I say bad logic board... especially because of the random backlight on thing... though I didn't test everything on the PSU. Some lines like PIN_27 KEY I'm unsure of what it does and PIN_1-9 seems to be nothing as well as BLK_ON/OFF (wtf is that?) which is at 0.4 volts constantly.

I just don't want to get a logic board only to find out that nothing is wrong with it

Here are some pictures if you need 'em. Both boards look free of capacitor corrosion and bulging.





BTW love the retro design of the forum. Please don't ever change it.
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Old 05-22-2021, 12:55 PM   #2
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Default Re: VESTEL 17PW26-5 Problem

I’ve had a shorted MLCC on this main board similar symptoms just flashing the blue led, good luck hunting

Edit: Disconnect main board see if psu brings 24v up.
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Old 05-22-2021, 02:57 PM   #3
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Default Re: VESTEL 17PW26-5 Problem

Welp... if the main board was broken before... it looks like the PSU followed. It suddenly doesn't put out 24v at all... disconnected or not.

Why would it matter if it could put out 24v if PSU was disconnected?

Also if the board is disconnected, isn't the PSU supposed to stop putting out voltages? How can I fool into doing so?

Last edited by yuuki47; 05-22-2021 at 03:00 PM.. Reason: Forgot to mention something
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Old 05-22-2021, 03:44 PM   #4
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Default Re: VESTEL 17PW26-5 Problem

Update:

I feel like this board is a lost cause at this point, but I took your MLCC suggestion into consideration, and started doing some probing on the logic board. I only have a multimeter, so I turned on OHM mode and set it to 2k, I then started probing the caps to check if they were charging.

Some did not. There were some caps in certain region of the board that had a constant resistance no matter what, which told me that maybe you weren't so wrong. As I tried more caps, I found that a chain of them had constant resistance, which was either lower or higher. I then started going upwards which cap by cap arrived at a very low resistance. The best I could go was at 0.287 ohms, so that was good enough. I started probing everything in that region from MLCCs to SMDs and I found an SMD cap that literally had 0.001 resistance, which is a dead short. So its either this cap or some other close to it. And its bloody tiny too... I've replaced one on a dead raspberry pi before but it wasn't trivial to do with a normal-sized soldering iron

Additionally, there's no 24v supply anymore. The blue led comes on when you apply power to the TV for one second and then that's it. It could be that the cap just gave up completely which causes a short to ground and prevents the 24v line from coming online at all.

But its not coming online if I disconnect the main board... is it supposed to or is it not? This is what I need to know at this point. Maybe this entire mayhem is caused by the mainboard.
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Old 05-23-2021, 07:38 AM   #5
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Default Re: VESTEL 17PW26-5 Problem

I replaced the 47uf 10v SMD cap with a big electrolytic one with the same spec (Should be alright, right?)

But no 24v supply still, so the mainboard cannot work without that. I still don't understand if the main board is supposed to signal the PSU to bring that up or not. I have schematics from similar boards but not mine exactly

ST_BY is also missing, but even when the main board had a healthy 24v supply, that line was missing unless I cut power to the PSU, which brought it up. From what I have seen online that's normal... and so I can assume that this is not brought up anymore as the mainboard isn't working properly without 24volts.

I wish more documentation was around *sigh*... Not having Right to Repair sucks.
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Old 05-23-2021, 03:07 PM   #6
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Default Re: VESTEL 17PW26-5 Problem

ST_BY is the line that turns the power supply On and Off. So without any voltage on that line the power supply is in standby.
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Old 05-23-2021, 04:51 PM   #7
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Default Re: VESTEL 17PW26-5 Problem

That's odd then, because the PSU sends out voltages perfectly when I have the PSU disconnected from the logic board. 24v is the only one missing.

I read somewhere that I can try a test where I place a 1k ohm resister between ST_BY and 5v standby rail to see if 24v comes up. It does not. And the PSU makes a clicking noise, additionally, most power rails like 5v and 12v disappear with 3.3v being the only one that remain. I tried the 3.3v standby rail instead just because this is what I saw when the main board was partially working, and voltages don't disappear, but drop significantly. Not normal is it?

I got the two 13N50 MOSFETs out (Q813 and Q814) to test them, and... the drain always seems to be open @ 340 ohms whether I charge the MOSFET or not. Is that normal? I was also certain that you are supposed to put positive voltage on Gate and negative on Source, but the reverse is true for this MOSFET (negative on Gate, positive on Source) and if I move negative from Gate to Drain, it is open, as it always is. Whether I discharge it or not. That can't be normal... right?

If it isn't, then it can't be that both MOSFETs decided to give up on life, there must be a reason they died.
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Old 05-23-2021, 05:28 PM   #8
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Default Re: VESTEL 17PW26-5 Problem

Have you checked the secondary diodes for the +24 & +12 volt supplies, If any of the diodes are shorted the power supply will try and start and shut down (the clicking sound) Mosfets can be hard to check but they usually fail shorted.
Using a 1k between ST_BY and +5v will turn on the power supply (do this only with the main board disconnected)
Quote:
That's odd then, because the PSU sends out voltages perfectly when I have the PSU disconnected from the logic board. 24v is the only one missing.
If you are missing +24v you must also be missing +12v as they come from the same transformer
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Old 05-24-2021, 04:00 AM   #9
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Default Re: VESTEL 17PW26-5 Problem

So if I shorted 5v to ST_BY the PSU turns on right? I should have the voltages but I do not.

12v rail does show up if ST_BY and 5v are *not* shorted, only 24v doesn't. But it does not if they are, either not at all or as 5v which is dropped voltage. I think the transformer is working but some cap at its output is messed up, I think that's the only way to explain how I have 12v without 24v, especially cause 12v drops when I short ST_BY and 5v.

I checked all of the board's big diodes, and they seem fine? I checked if they are leaky and shorted, and they are not. The two diodes near the mosfet put around 1,5m ohms just fine. But I'm guessing that's not the most reliable test... is it... thing is I only have a cheapo multimeter ;d

I guess next I need to check the capacitors, but checking ESR might be difficult cause I don't have anything that produces low-voltage AC D: I can only check leakage current probably.

Last edited by yuuki47; 05-24-2021 at 05:04 AM.. Reason: Typo
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Old 05-24-2021, 02:45 PM   #10
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Default Re: VESTEL 17PW26-5 Problem

So um... a bit of clarification.

I had dissassembled the board slightly (taken out transformer to access the MOSFETs and diodes) and then a few minutes ago when I reassembled everything, I tested what I said again.

shorting 3.3v to ST_BY via 1k ohm resistor (cause that's what I saw in that rail when the main board was working to begin with) 12v disappears... sometimes slowly... sometimes suddenly.

5v_stby is still around, but the three-chain of 5v rail disappears... sometimes dropping to 3.3v

3.3v always up.

24v missing completely... I see -0.24v which is really -240 millivolts.

When I disconnect 3.3v from standby, I hear rapid clicking... almost like a crackling sound... and all rails except 24v come back.

I also tried shorting 5v to ST_BY just to see what happens, and same results (but with 5v st_by dropping to 3v or completely this time)


I can't do much right now, I don't know the integrity of the capacitors (their tops don't look exactly flat) so I'll get a LCR-T4 and test all of this out. A simple multimeter won't do anymore.

the 24v rail looks shorted though... why -240 millivolts? Something must be dead... maybe a cap because diodes look ok as far as I can tell.
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Old 05-24-2021, 03:47 PM   #11
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Default Re: VESTEL 17PW26-5 Problem

are you using the correct ground reference ?
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Old 05-25-2021, 12:49 PM   #12
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Default Re: VESTEL 17PW26-5 Problem

Correct ground reference...? All ground pins on the connector are common, so I'm not sure where the correct reference would be. And besides... why was I seeing 24v before? So I must be... right?
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