Why Q660 on Panny SC/SN board may kill buffer ICs when failed

[tom66]

Didn't realise this until looking at a modern (TX-P42U30B) SN board schematic.

Normally VFG is the SUS output, VSCN_F is the SUS+VSCN output. It follows SUS but ~150V higher for buffer ICs to select the right row in the addressing phase.

A lot of Panny boards have the equivalent of Q660, it connects VSCN_F to GND for the ramp down portion of the initialisation waveform. If this fails short, but the VFG is stuck HIGH (due to being at that part of the sustain waveform, for example...) this would reverse bias the buffer ICs with 200VDC!!! (As VFG would be at +200VDC, and VSCN_F would be at 0VDC, rather than VFG at +200VDC and VSCN_F at +350VDC.)

This would clearly cause fatal damage to the buffer ICs, AND since the energy path is from VSUS rather than VSCN_F there's a whole lot of energy to dump into the ICs...

As a bonus it would also probably damage the rest of the SC board, due to the pulse energy of discharging the VSCN capacitor...

I have not seen this circuit on modern LG/Samsung sustain boards... only Panasonic.

Just a thought/musing.

[Alastair E]

Re: Why Q660 on Panny SC/SN board may kill buffer ICs when failed

Prob just as well that Q660 Isnt one of the Usual Suspects on say, TNPA5349!

[tom66]

Re: Why Q660 on Panny SC/SN board may kill buffer ICs when failed

Yes, given the integrated buffer ICs that would make for a very tricky repair! I don't think a single buffer IC would survive...

[Alastair E]

Re: Why Q660 on Panny SC/SN board may kill buffer ICs when failed

I'm guessing under This fault condition, It depends on exactly how fast the O/Current protection/SOS 7/10 shuts down the PSU--as to just how many buffer-chips survive--If any!
Guess its a bonus there's not That much capacitance on the V-SUS lines of these Pan sets, in comparison to some....

[tom66]

Re: Why Q660 on Panny SC/SN board may kill buffer ICs when failed

I've had SOS10 trip out so fast I could see the screen pink out for about 1/2 second as Vsus collapsed and sustain boards kept going. I would imagine SOS7 could trip out pretty fast but on the semiconductor scale a reverse bias of 200V is damaging beyond a millisecond. Internally the buffer ICs look like an array of multi-parallel two-series reverse diodes, which effectively means they conduct very well in reverse...

Only real hope is the little SMD diode series with Q660 fails O/C... really it'd be nice to see a little SMD fuse there but Panny aren't going to do that when they cost down everything else.

[tom66]

Re: Why Q660 on Panny SC/SN board may kill buffer ICs when failed

One possible resolution to this serious flaw (IMHO) is to install a reverse-parallel diode across the scan IC supply, or a series diode in-line with VSCN_F.

[Alastair E]

Re: Why Q660 on Panny SC/SN board may kill buffer ICs when failed

Series-diode with VSCN_F would be best IMHO.....

--Reverse parallel diode may not offer sufficient protection--Before it is exploded by the V-SUS!

[tom66]

Re: Why Q660 on Panny SC/SN board may kill buffer ICs when failed

A correctly sized reverse diode could easily dump all vsus energy with no failure.

[Alastair E]

Re: Why Q660 on Panny SC/SN board may kill buffer ICs when failed

Aye--True, But the extra expense --Pan aint gonna do That-- for a Possible protection against something (That in their mind) shouldn't happen anyway, that still makes the set--Not Work, as all this addition would do is make the fault--Less destructive IF it happens!

Lets face it, these makers (I include--ALL of 'em) couldnt care less (May even actually Want the set to fail at this--what WE think is an early death), if the board fails two or three years after sale, OR how much is destroyed on it when it does, All they do is

--Sell another board, Or a replacement set anyway!

Those that dont buy a new Pan may buy say, a Sony and those in same situation with a faulty Sony may buy a Pan Lets not delude ourselves that the set makers Don't Talk to each other over such important topics as set Longevity There's probably an unofficial Cartel controlling their activities.....

Win-Win for them Collectively, Planned Obsolescence Built In.....
--Ever watched, 'The Lightbulb Conspiracy' If not, Worth watching.....

[tom66]

Re: Why Q660 on Panny SC/SN board may kill buffer ICs when failed

I have watched that documentary, it's quite fascinating.

I work for one of the largest set-top box manufacturers in the world. (I won't mention which but you can probably guess.) We test all of our widgets to get a 7 year expected lifespan with at most 3% failing before this 7 year lifespan.

This is a consumer electronics business and we've recently been pressured by customers (who we supply boxes to) to reduce the cost, and thus the lifespan, to just 3 years...

This is why there needs to be an new law requiring a minimum 5 year warranty (covering ALL costs upon failure, excluding life-limited devices like lamps and batteries however) on every consumer item sold. The amount of landfill is getting ridiculous.

[Alastair E]

Re: Why Q660 on Panny SC/SN board may kill buffer ICs when failed

Ah--Well done Tom....

I'll bet that behind that pressure from your corporate customers--there's summit like the 'Phoebus Cartel' or its modern equivalent behind it....

I'm a realist--There'll be no such law ever forthcoming.--More is the shame, I agree though There SHOULD be a minimum expected life of any consumer-electronics item of at Least 5 years, (7 is better) with parts and service info available to at Least 10 years from end of production of that particular item.

Additionally, ALL Service Information should be made freely available to anyone.

The Reason such a law will never happen, is it goes against the Govt ethic of promoting Growth,-- MONEY is king, nothing else matters in this world, to business or to Govts.

Call me cynical old fart-- Maybe so, but life's experiences and observations over the past 40 odd years haven't proved me wrong in this.

I am foresighted enough to see that its quite impossible to have continuous growth, Business and Govt also know--but don't care, they are in it for the dosh.

--When problems arise, as we are beginning to see, they'll just cross that bridge when they come to it, Hence the more extreme recycling legislation we are beginning to see....
--That way, They get to keep their cake--And eat it

Recycling is BIG Business with BIG players involved in it, VERY little if ANY WEEE (Waste Electrical and Electronic Equipment) goes to landfill, Too much precious and non precious metals etc in it!
--Only stuff that goes to landfill these days is black-bags, even then these are picked through to remove anything thats recyclable like certain Plastics, Glass, Paper, Cardboard, and Metals. Working as I did at a Waste Transfer-Station, I saw all this going on, on a day to day basis.

Local Councils are under strict instruction to minimise waste to landfill hence the picking-lines for paper and black-bags.....
The newest regulation round here is that as a householder, You cannot put out More than 3 black refuse-bags every 2 weekly collection......
If Council fall short of their EU Imposed Targets, they face very serious fines......

I have a theory for you. IF a more dishonest manufacturer wanted--Whats to stop him in-building into the sets Firmware, a destruct code, activated at some random time when the set has done say, 5000 (or number of hours to suit them) plus hours,--Bit like Hewlett-Packard did with that printer?

Easy enough to do, All the Control-Board needs to do is turn on both the earthy and the live IGBT's of the SUS boards at the same time, and also reset the time-counter to say, 1000 hours, so even then if set is repaired it wont last a year....

Welcome to the 21st Century

BTW-- Sorry to mess up your thread.....