Technosonic LCD3251 picture fault

[notallbad]

PC, HDMI and Component In inputs work fine producing a perfect picture but the 2 Scarts, Side A/V jack inputs and the Ant In produce no visible picture, its very distorted with green horizontal flickering/strobing lines covering the whole screen. Shall add a picture.
On power up the panel red led flashes 2 quick, 7 slow, 1 quick, 5 slow. The panel then lights up and displays as per picture.
OSD displays fine regardless of which source is selected and am able to access the service menu.

Service manual found here for a model that seems to have identical boards:
https://www.scribd.com/doc/56718559/Lcd-Profilo-Telra-PT1000

The rear panel is removed ready for checking/testing if one could be pointed in the right direction.
Thanks

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[Mr RB]

Interesting! Obviously there is nothing wrong with the screen or screen driver as the menu appears fine.

It looks to be a fault with the RGB (component) inputs, or the associated switching circuitry. I would take a first guess it's had ESD damage etc on the input plug and replace the IC that the signal first goes through. Since you said it does it on RF in it could also be the switching IC that switches between component inputs and the signal derived from the tuner/decoder. Again, that switching IC could be the IC closest wired to the input connectors.

 

[notallbad]

Thanks, I shall try and locate that I.C and see what I can find.

I have tried to re-tune the tv, freeview, to no avail. I do believe this is a DVB-T set, not exactly sure. Anyway, if it is then I am not getting any channels tuned in, I know the image will not show on the screen but would have thought that they would show up in the programmes list and was hoping I would at least get sound from the channels once it was tuned in. I am beginning to wonder if it is a fault within the Tuner unit.

As a note, when I connect a scart device the tv automatically switches over to that scart input, works on both scart1 and scart2.

 

[notallbad]

I have found a low dropout 3 terminal regulator RC1117-33 that has, measuring across chassis ground and each of the 3 pins, the following voltages:
Pin 1 - 0v
Pin 2 - 4.3v
Pin 3 - 5.4v
Tab - 4.3v
I know the output should be 3.3v so what would cause the output voltage on Pin 2 and Tab to be rated higher? Or is it just likely that the regulator is faulty?

 

[Mr RB]

Yep that is definitely a 3.3v regulator.

There are 3 likely causes of measuring 4.3v on it's output;
1. the regulator is dead.
2. something downstream from the regulator is faulty and feeding in current, making the 3.3v rail rise.
3. capacitors failed giving a massive ripple on Vin or Vout, fooling your voltmeter into reading "4.3v" - so check with a 'scope.

 

[notallbad]

Thanks, I appreciate your help.
I do have an old scope but never used it. I think perhaps I could do with some different probes for this type of testing.
Do I simply connect earth to chassis and probe each point of interest?
Of course settings need to be made such as volts and time.
Thanks again.

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[notallbad]

When measuring with DMM readings are different to readings taken with a scope.

DMM Scope
5.5v 4v
3.15v 2v
12v 9v
23.3v 17.5v

The scope readings are approximate within +/-1v max.

Not forgetting that this is an old scope and a novice driving it!

 

[Mr RB]

It sounds like your scope was not adjusted or zeroed right. Regardless, the main need for the scope was to look for bad oscillation or ripple on the Vin or Vout pins of the 3.3v regulator referenced to it's ground pin (probably the ground plane of the PCB).

If there is no visible oscillation (try on a few different TIME/CM settings) then it looks like you are back to a bad regulator or other circuit fault. Those regulators are cheap and common and should only take a few minutes to replace.

 

[notallbad]

Not sure how to adjust/zero it.

Would the scope need to be set to the ac setting to look for bad oscillation or ripple on the Vin or Vout?
Sorry, new at this!

 

[Mr RB]

Not always, but AC setting can help to view small ripple effects. We were mainly concerned with a massive ripple or oscillation, which should have been pretty obvious even on a 1v/div DC setting.

A tiny AC ripple might be there, but that does NOT explain how you are getting 4.3v on a 3.3v regulated rail.

Really, I don't know this set well and am only going from common sense faultfinding and a couple of sketchy symptoms provided by you.

 

[notallbad]

Well I replaced the 3.3v reg and do now get a 3.3v reading on the out pin and tab but the tv still has the same symptoms, however the picture has now chnged slightly.

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[bryan]

Possibly it has toasted some 3.3v components post regulator.

 

[Mr RB]

Sure looks like it!

At least you now have more info. It looks like time for a schematic diagram and see what that 3.3v rail fed.

 

[notallbad]

Looking at the schematics the 3.3v is labelled +3.3v_vid, this then goes through a 0ohm resistor and label changes to +3.3v_off_ss which connects to a resistor R776 (resistance not specified), all on P.40 of the manual provided. Please advise how to check further. Thanks

 

[notallbad]

Thanks for your help so far, I think I should start from the start and begin with the PS.
I hope this is easily understood:

------------------------------When the TV is first switched on
PS Connector----------------------by the toggle switch--------------TV in standby----------------TV powered up by front panel stby/on button
Pin 1 stby - 4.73v------------------------------------------------------------------0v----------------------------------------4.73v
Pin 2 Gnd - 0v-------------------------------------------------------------------0v----------------------------------------0v
Pin 3 Gnd - 0v-------------------------------------------------------------------0v----------------------------------------0v
Pin 4 +5v main - 5.53v drops to 0.36v and remains-----------------------------0.36v--------------------------------------5.44v
Pin 5 +5v main - 5.53v drops to 0.36v and remains-----------------------------0.36v--------------------------------------5.44v
Pin 6 +5vstby - 5.56v---------------------------------------------------------5.56v------------------------------------5.52v
Pin 7 Gnd - 0v-------------------------------------------------------------------0v----------------------------------------0v
Pin 8 Gnd - 0v------------------------------------------------------------------0v-----------------------------------------0v
Pin 9 Gnd - 0v------------------------------------------------------------------0v-----------------------------------------0v
Pin 10 +12VL - 11.83v steadily drops down to zero------------------------------0v-----------------------------------------11.8v
Pin 11 PWM_DIM - 0v---------------------------------------------------------0v-----------------------------------------3.17v
Pin 12 BL_ON_OFF - 0v-------------------------------------------------------0v-----------------------------------------3.16v

I don't fully understand how this switching works but it seems a bit odd that pins 4 and 5 have this floating 0.36v so I removed just pins 4 and 5 from the PS with the following results:--TV switched on by toggle switch-----TV in standby----------TV powered up by front panel stby/on button(tv does not start up)
----------------------PS---------Main A/V bd------------------------PS------Main A/V bd---------------PS-----------Main A/V bd
Pin 4 +5v main -------5.56v-----0.27v and remains------------------5.56v------0.27v-------------------5.56v--------0.27v drops to 0.25v when the front panel 'ON' button is pressed
Pin 5 +5v main -------5.56v-----0.27v and remains------------------5.56v------0.27v-------------------5.56v--------0.27v drops to 0.25v when the front panel 'ON' button is pressed

If I probe on the Main A/V bd I get readings of 0.27v on the input pins of regulators U704 U715 U714 U705, on reg U716 (the reg that I swapped out) 0.27v on the output pin and 0.8v on the input pin.

Also, Pin 1 Stby is reading 4.73v which according to the service manual should be 3.3v, which I'm guessing is the PS_On signal from the Main A/V bd, this reads 5.54v when Pin 1 is removed from the PS.

I'm hoping someone can direct me towards the source of this voltage or advise better diagnosis.

Just to confirm when the power toggle switch is switched on the red led comes on solid for 2 seconds, then flashes once quickly then 6 times then on the 7th flash stays on solid, this then never changes regardless of how long the tv is left, i.e does not flash again.
When the tv is then switched on by the remote or front panel on/off standby button the red led flashes
flashes are not consistent
7 flashes 1 quick flash 6 flashes
7 flashes 2 quick flash 5 flashes
7 flashes 1 quick flash 5 flashes
1 quick 8 flashes 1 quick 5 flashes
1 quick 7 flashes 1 quick 6 flashes
1 quick 14 flashes

P.S. I do know the TV will not start up with Pin 4 and 5 disconnected.

 

[Mr RB]

I don't think we can diagnose an unknown TV set schematic based on some voltages on a pin connector!
Your pin voltages seem to indicate that the 5v and 12v rails are ok, after the set is turned on.

You definitely had a blown 3.3v regulator, so most likely some IC(s) after that regulator on its 3.3v rail are also blown.

Since you found a blown regulator, you need to chech the schematic for all ICs semis etc that were running from that particular 3.3v rail.

Also keep in mind there is a definite possibility that in the long term you may not have the required information, parts or experience to fully repair the set.

 

[notallbad]

Sure, appreciate that!
It may be that I cannot repair the tv due to lack of equip/experience etc but it would be nice to find the source of the fault.

So, after the set is on the voltages check out ok, but why does the led flash an error code? And what does this error code mean?

 

[notallbad]

So followed the schematics from the 3.3v reg U716 back to K6R4008V1D pin 15 via resistor R368 rated 47k, reading in-circuit is 12.41kohm, urm well it is in-situ so could be right and its an smd so may have to chance lifting one end to measure. Interestingly pin 15 is labelled WE (Write Enable), this I.C connects to VCT49xyi (TV processor) so looks like I may be getting closer to the fault, perhaps. Now the question is, and I know that replacing the VCT49xyi is going to be beyond my abilities, but how do I determine what is at fault? Possibly the K6R4008 or the CVT49, would either of these being faulty correspond to the fault I am having???
What and how can either of these be checked?

I would doubt that the resistor had been damaged and suspect it will still be within spec's.
Updated : Resistor removed and checked and OK.

The K6R4008V1D has 3.3v on pins 11, 15 and 33 not that it means anything I guess.

Though really is 1v rise going to really damage something? OK, if it had risen to 6, 9 or 12v I would expect some damage. And most devices are resilient to a rise in voltage and operate at higher/lower voltages within reason.

 

[Mr RB]

If the 3.3v rail that failed directly drives that IC, and the IC is a "TV processor" that is a likely candidate.

I have no idea if the 1v rise itself damaged anything, it may have been the IC that failed first, which then blew the 3.3v regulator.

 

[notallbad]

Whilst looking at this tv in silence, no children running around screaming and shouting, I noticed a high pitched buzzing noise coming from the T-Con board, main I.C. This buzzing disappears when the source is selected to those that work i.e. PC input and HDMI, buzzing comes back when Tuner and scart inputs selected.
Not sure if this is relevant, could it be the T-Con board is causing the fault and needs replacing?

Its not actually the main I.C but in fact one of the large black round things with .220 marked on it (the bottom one) or at least as far as I can make out, what is that? A resistor???

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