Panasonic TX-37LXD85 Inverter SOS

[tn245]

Hi

This TV is doing one blink which I believe on this model means inverter_SOS. It is getting 24V to the inverter and the on signal. I have verified the bulbs work by firing them up with an inverter on another TV. Have also tried to fire up the other TV's bulbs with this inverter and got nothing, so pretty sure its the board. I haven't got much experience with inverters other than replacing fuses so I'm here for help and to learn!

The third picture shows my numbering of the transformer pins. Resistances read as follows. T1 being on top.

    
   T1    T2
1-2 6.84M  6.88M
1-3 6.93M  7M
1-4 22.3R  21.9R
2-3 0.1R   0.1R
2-4 -     -
3-4 -     -

It uses the BD9897FS chip. Resistance across all of the FETs is 2.8K or very close to. F1 and F2 are not blown.

So what does it look like? Should I take out the transformers and test out of circuit or is it more likely the BD9897? Or something else? This board seems pretty rare so I'd like to repair it rather than replace.

Thanks!

[tom66]

Re: Panasonic TX-37LXD85 Inverter SOS

Do you get even a brief flash from the backlight? Or nothing at all?

[tn245]

Re: Panasonic TX-37LXD85 Inverter SOS

No flash from the backlight.

[tn245]

Re: Panasonic TX-37LXD85 Inverter SOS

Any ideas?

[tom66]

Re: Panasonic TX-37LXD85 Inverter SOS

Need a pin out of that chip to think of any - perhaps it has an error output pin?

[tn245]

Re: Panasonic TX-37LXD85 Inverter SOS

Datasheet and pinout are here:
http://pdf1.alldatasheet.com/datashe.../BD9897FS.html

The reason I mentioned the resistance across the FETs is after reading this post about a different inverter, but using the same controller chip:
https://www.badcaps.net/forum/showpo...4&postcount=29

[tom66]

Re: Panasonic TX-37LXD85 Inverter SOS

Can you measure pin 26? Do you have 6V or more for the inverter chip?

[tn245]

Re: Panasonic TX-37LXD85 Inverter SOS

24V at pin 26

[tom66]

Re: Panasonic TX-37LXD85 Inverter SOS

A few more checks:
Pin 28 - 5.8V ± 0.5V
Pin 23 - Briefly high before SOS
Pin 18 - FAIL (logic LOW)
Pin 16 - Briefly high before SOS

Also, is ON/OFF toggling, from main board? Otherwise, backlight may never turn on.

[tn245]

Re: Panasonic TX-37LXD85 Inverter SOS

Pin 28: 0.02V - jumps briefly to about 0.4V once, then again just before shutdown.
Pin 23: 3V - drops to almost 0 once, then again just before shutdown
Pin 18: 0V - fluctuates a bit to about 0.02V
Pin 16: 3.22V - drops to 0 once, then again on shutdown.

ON/OFF is 3.2V for about 3 secs then drops to 0V, goes back to 3.2V for 3 secs, then off as PSU shuts off to error code - which explains the other readings dropping out. Is the main board trying the inverter twice before deciding to give the error code?

[tom66]

Re: Panasonic TX-37LXD85 Inverter SOS

I think it is trying twice, makes sense in case of random ignition failure which does happen sometimes.

It looks like the chip is defective. The BIAS pin 28 should have around 5.8V on it as long as the chip is not locked out (UVLO condition met, Vcc > ~6.5V.)

[tn245]

Re: Panasonic TX-37LXD85 Inverter SOS

Ok, thanks! Just double checked and I'm only getting millivolts on pin 28. The chip is cheap enough but I have a feeling it's really going to challenge my desoldering/soldering skills with all those other components so close. Will order the chip and see how it goes.

Thanks for the diagnostic help!

[tn245]

Re: Panasonic TX-37LXD85 Inverter SOS

So.. soldered in a new control IC and we now have a working TV! The chip cost £3.10 so a pretty good result on this one!

Thanks again for your help tom66!

[tom66]

Re: Panasonic TX-37LXD85 Inverter SOS

Great to hear!
Can you verify pin 28 to confirm diagnosis?