Philips 42PFL9632D/79 2-secs-to-black

[pedro]

Re: Philips 42PFL9632D/79 2-secs-to-black

The Samwhas on the two inverter boards have been replaced with Panasonic FR's. Recall that on the ESR meter the Samwhas measured 0.02 out of circuit (the same spec as the FR's) and I wasn't expecting the FR's to make any difference. No surprise - they didn't. But at least that variable has been removed from the equation.

Decide that while I'm in here I may as well also fit the Philips/LG mod #2 kit, which has the Philips part# 996500095005. Philips are still trying to look it up ....

[pedro]

Re: Philips 42PFL9632D/79 2-secs-to-black

Still no response from Philips service on their "repair kit" for these inverters.

Have spent a lot of time (probably too much) searching for a schematic for these boards, using search terms LC420WU5, PNEL-T707A etc but no joy to date. Has anyone ever seen one?

Without a schematic, and with the heat-testing not being able to identify *which* component is responding, this set looks destined for the dumpster shortly. I'm not thrilled by the prospect of a $50 chinese clone board as a long-term solution.?

[tom66]

Re: Philips 42PFL9632D/79 2-secs-to-black

You don't need a repair kit; I thought it was two 4.7 ohm resistors and two TVS devices? Plus maybe a replacement of the IC?

[pedro]

Re: Philips 42PFL9632D/79 2-secs-to-black

Actually I should have said "mod kit #2" but we are on the same wavelength. (The repair kit is the four FETs and the driver IC). No I don't *need* the mod kit but was intending on fitting it while the set was here *IF* the cold start issue was resolved.

Interestingly, selecting a TVS diode for this application isn't as straightforward as some might expect. The ratio of clamping voltage to standoff voltage creates an issue, as anything with a >24V standoff will NOT have a clamping voltage less than the driver's specified 36V absolute max and so won't provide the presumed protection against transients.

(I'm more familiar with the 1.5KE series transorbs, where 18V standoff devices 1.5KE22A are sometimes fitted to 12V mobile radio equipment. Their role is to go hard short and take out the line fuse if the equipment is inadvertently connected to a 24V vehicle. I was given one Motorola set as "beyond economic repair" that just had a shorted transorb!)

On one German forum they were referring to a 100V device!

The other idea - a zener - was part of mod kit#1 which wasn't a success, as the zener wasn't intended to handle the sort of transients that a true TVS does.

Back on the heating subject, I've never liked the idea of using a soldering iron for this, and especially so on an epoxy case of a semi. But unless I can come up with a scheme to localise the heating to SMD-sized areas I don't see a way to progress this.

Thanks for hanging around on this thread.

[Recyclojunk64]

Re: Philips 42PFL9632D/79 2-secs-to-black

Hi
I am having a simmilar problem with a LC420WU5 inverter board as the OP. I don't have the back cover of the TV with me at the moment so can't give the exact model number, but I believe it's the same / simmilar to the one in the thread title.

When the TV is turned on, the blue LED turns on on the inverter board which then emits a buzzing sound from the transformers. About two seconds later the LED fades off and the backlight cuts out. Then the backlight starts turning on and off about once every second, with the transformers still buzzing reasonably loudly when it's on.

Firstly I checked the voltage outputted from the powersupply board and it's a clean 24volts going out to the inverters. I did check the caps on both the inverter and powersupply boards for ESR and they appear fine. I've also checked all the diodes on the inverter board and none were shorted out. Also the mosfets are fine aswell.

I've looked at the voltages on the test pads for the master-slave interconnect cable, and found that the voltage on the SoftStart test pad (connected to pin26 on the main IC) is at about 3 volts while the backlight is on, and drops down to zero while it's off. I am not sure if this is just a symptom of the IC resetting for some reason or part of what's causing it to not work.

I thought I'd have a peek at the flourescent lamps in it (easier said than done) and saw that they were external electrode lamps, and looked in good condition so I'm quite sure that isn't the problem.


So does anyone have any idea of what might possibly be wrong with them? I I'd prefer fixing it myself than ordering a new set of cheap chinese clones. Is it likely to be one of the control ICs or transformers?

Thanks for any help

[Recyclojunk64]

Re: Philips 42PFL9632D/79 2-secs-to-black

In the last post I meant SoftStart was pin 23 instead of pin 26. I must have miscounted them.

Anyway I think I've found the problem.

The BD9897FS controller IC on the slave was getting very hot (whereas the master one was cool). Between Vcc (26) and SW1 there was 10 ohms, whereas on the master the two pins had no connection between. (I desoldered the IC and there was still 10 ohms between them). SW1 is the "lower rail voltage for HN1 output". As far as I can tell it is a reference input for the high-side n-mosfet gate-output, and is connected to the source of the mosfet that HN1 drives. So it is of course also connected to the discharge of one of the low-side mosfets, meaning that alot of current will flow out of it as that mosfet is conducting, heating up the main IC. But more importantly the high-side mosfet driver won't be driving the gate at the correct level.

I've ordered a replacement chip (plus a few spares) under the assumption that nothing else on the board is damaged. Will post back when they arrive as to whether it works again.

[pedro]

Re: Philips 42PFL9632D/79 2-secs-to-black

You're in Perth too? Want this set to play around with? Please. It's gathering dust right now and I need the space it is taking up.