Sylvania LC420SS8 shutting down on protect(?)

[iKitsune]

Hey, all. A friend gave me a LC420SS8 because I "like puzzles". Initial symptoms were no picture, and a ~3 hz flashing green "power" light for 10 seconds that turned to a ~6 Hz flash for 3 seconds and then returned to the red "stand by" light. Opened the back and reseated the cable between the digital input board and the T-con. Next power up attempt has the same light pattern, but the backlights fire up and the screen displays "Digital Television" before shutting down. Voltage from the inverter power supply has +24v at +23 with the inverters disconnected, and +24.5 with them disconnected. When initially powered up, the +24v rail comes up to +24.1v and then drops to +23 when the backlights switch on. There is a blue LED that lights on the "A" inverter, and then fades out after the backlights strike. +13v from inverter PSU to main PSU is +13.55v. PROTECT1 and PROTECT3 are ~2.5V. +33v on main PSU is +31v. +3.3v measured at CN201 is +2.75v. +12v-LCD measured at CN201 is +11.9v.

Caps are Su'scon on the main PSU and Yec on the inverter PSU. No obvious bulging or leaking (but I'm aware that caps can silently fail.) All 4 inverter transformers are buzzing slightly enough to be audible if you're near the inverter board, but are not audible from the front of the set with the back on.

I feel like I'm missing something obvious, so I'm hoping someone can help me with this.

PDF of the service manual including schematics is attached.

[Mr Bill]

Re: Sylvania LC420SS8 shutting down on protect(?)

Can you post pictures of your boards? Looking at shop jimmy's site i'm seeing this tv uses the LG Philips 6632L inverters which are known to fail.

[budm]

Re: Sylvania LC420SS8 shutting down on protect(?)

OK, I looked at the schematic.
There are 5 protection detection circuits that I found, when it is normal it should be HI 2.5~3V (it is pulled by the 3.3V power supply), below 1.5v= bad.
I.E. Protect 1, 2, 3, 5, 6 feed IC 3101 Micro controller as shown in page 46.

The protection detection circuits use a bunch of diodes as OR Gate to activate the PROTECT signals. iF any one of these voltage is not present, it will produce LOW signal to the IC 3101 Micro controller Protect inputs.

Protect1 detects lots of power supply outputs (see page 33): Switched +13V, +9V REG, +3V, +4V, +2.7V, +6V, +33V, +3.3V, Switched+12V, Switched +6V.

Protect2 detects +6V (page 32).

Protect3 detects +24V power supply out out for the backlights inverter (page 36).

Protect5 detects Switched +1.2V OUTPUT of IC3601 (page 47)

Protect6 detects Main Incoming AC power feeding the TV power cord (page 32).

So you have a lot of voltages to check. You should look at the IC 3101
Pin 1 (Protect1)
Pin 64 (Protect2)
Pin 63 (Protect3)
Pin 47 (Protect5)
Pin 46 (Protect6)

They should all have >2.5VDC. Just DO NOT SLIP THE PROBES!

[iKitsune]

Re: Sylvania LC420SS8 shutting down on protect(?)

Can you post pictures of your boards? Looking at shop jimmy's site i'm seeing this tv uses the LG Philips 6632L inverters which are known to fail.

They are the 6632L inverters. What's the failure mode on em?

Edit: Can't do good photos right now, my digital camera is charging, and my cell phone's camera "sucks".

[iKitsune]

Re: Sylvania LC420SS8 shutting down on protect(?)

OK, I looked at the schematic.
There are 5 protection detection circuits that I found, when it is normal it should be HI 2.5~3V (it is pulled by the 3.3V power supply), below 1.5v= bad.
I.E. Protect 1, 2, 3, 5, 6 feed IC 3101 Micro controller as shown in page 46.

The protection detection circuits use a bunch of diodes as OR Gate to activate the PROTECT signals. iF any one of these voltage is not present, it will produce LOW signal to the IC 3101 Micro controller Protect inputs.

Protect1 detects lots of power supply outputs (see page 33): Switched +13V, +9V REG, +3V, +4V, +2.7V, +6V, +33V, +3.3V, Switched+12V, Switched +6V.

Protect2 detects +6V (page 32).

Protect3 detects +24V power supply out out for the backlights inverter (page 36).

Protect5 detects Switched +1.2V OUTPUT of IC3601 (page 47)

Protect6 detects Main Incoming AC power feeding the TV power cord (page 32).

So you have a lot of voltages to check. You should look at the IC 3101
Pin 1 (Protect1)
Pin 64 (Protect2)
Pin 63 (Protect3)
Pin 47 (Protect5)
Pin 46 (Protect6)

They should all have >2.5VDC. Just DO NOT SLIP THE PROBES!

PROTECT3 is the first one to drop just before the set shuts down. It gets pulled down to 0.0v.

[budm]

Re: Sylvania LC420SS8 shutting down on protect(?)

That is the 24v power supply, that will be the starting point. But you verify if other voltages are OK also since the 24v power supply is turn on by the main board so it may be told to turn off by the main board. This TV has way too much detection circuits.
There should be backlIghts inverter boards in your TV, do you have the pictures of the whole TV, you can disconnect the 24v outputs that go to the backlights inverter to see if the 24v will stay on.

[iKitsune]

Re: Sylvania LC420SS8 shutting down on protect(?)

That is the 24v power supply, that will be the starting point. But you verify if other voltages are OK also since the 24v power supply is turn on by the main board so it may be told to turn off by the main board. This TV has way too much detection circuits.

Like I said in my initial post, the +24v is around 23v when the inverters fire up. It stays above 24v with no load however. It looks like it's either got a heavy load (failed inverter?) or bad regulation, or something else dragging the 24v line down. The +33v line is pretty low, too.

Edit: That's what makes no sense to me: The inverter PSU (+24v) appears to be *completely separate* from the "main" PSU! I don't get what load could be dragging down both +33v and +24v.

[budm]

Re: Sylvania LC420SS8 shutting down on protect(?)

If you disconnected the backlights inverter and the fault went away, then yes, I would suspect shorts in the backlights inverter board, or the 24v supply just cannot handle the load.

[iKitsune]

Re: Sylvania LC420SS8 shutting down on protect(?)

If you disconnected the backlights inverter and the fault went away, then yes, I would suspect shorts in the backlights inverter board, or the 24v supply just cannot handle the load.

I don't want to just go blindly replacing parts, but could silently failed filter caps on the 24v PSU be causing the problem? I have neither a capacitance meter nor an ESR meter, just a basic DMM.

[budm]

Re: Sylvania LC420SS8 shutting down on protect(?)

Well I am still trying to know if you do not have the inverter connected to the 24v power supply, do you still get the Protect3 error?
We need to narrow down to the board first.

[iKitsune]

Re: Sylvania LC420SS8 shutting down on protect(?)

Well I am still trying to know if you do not have the inverter connected to the 24v power supply, do you still get the Protect3 error?

I was just checking that, and I noticed on retesting that PROTECT2 never goes high - it stays around 0.9v. PROTECT3 drops at the same time as the rest of the power with the inverters disconnected.

[budm]

Re: Sylvania LC420SS8 shutting down on protect(?)

See page 32 then see if you do have +6v from the power supply Transformer T600 Diode D674/675, Caps C656/657

[iKitsune]

Re: Sylvania LC420SS8 shutting down on protect(?)

See page 32 then.

Hang about a bit, I seem to have gone blind and can't find R657, D659, or R656, lol.

[iKitsune]

Re: Sylvania LC420SS8 shutting down on protect(?)

See page 32 then see if you do have +6v from the power supply Transformer T600 Diode D674/675, Caps C656/657

Alright, I'll check those, because I can't find the locations page 32 lists for 6v. Probably from working on a TV at 1 AM.

[iKitsune]

Re: Sylvania LC420SS8 shutting down on protect(?)

See page 32 then see if you do have +6v from the power supply Transformer T600 Diode D674/675, Caps C656/657

I have 7.56v at D674/675.

[budm]

Re: Sylvania LC420SS8 shutting down on protect(?)

What do you have at the Cathode side of the D659.
If you still have <1v, then you need to check the resistance at pin 64 of IC3101 (page 46) and Ground to see what is pulling it down.

[iKitsune]

Re: Sylvania LC420SS8 shutting down on protect(?)

What do you have at the Cathode side of the D659.

0.88v.

[budm]

Re: Sylvania LC420SS8 shutting down on protect(?)

then you need to check the resistance at pin 64 of IC3101 (page 46) and Ground to see what is pulling it down.

[iKitsune]

Re: Sylvania LC420SS8 shutting down on protect(?)

then you need to check the resistance at pin 64 of IC3101 (page 46) and Ground to see what is pulling it down.

Resistance is 1.48 kOhm from pin 64 to ground, tracing the circuit back now.

[budm]

Re: Sylvania LC420SS8 shutting down on protect(?)

I just look at the board layout on page 48, I can see jumpers (protect 2) J134, 135 that go over to the right side of the board and connected to D801 and D228 which are not shown in the diagram, are those section of the board stuffed with parts?