Panasonic TX-25MD1 CRT TV.

[welbeck5]

Can I just ask will this tv have an inbuilt isolation transformer?
The tv will be switched off anyway but I've read about being careful with live circuits and current might be still lurking.
On the vcr's I've learnt to fix, the power supply has the AC to DC transformer for primary to secondary is that
basically the same process with tv's ?

[Welsh Anorak]

Not exactly, that's why TV repair is a bit more involved.

However TVs of this era and later use a switch mode power supply which does effectively isolate the chassis from the 'hot' part of the circuit.
This is good news for you.
If we were talking about a TV from the seventies or Eighties then things may well be different and potentially (no pun intended) more hazardous.

The main hazard is the fact that, unlike a VCR, there are lots of high and very high voltages on the same board and, as you obviously appreciate, they can linger after switch off. However you are looking at a relatively low voltage area which you will be dealing with cold, so just observe the usual precautions mentioned by Nick above.

[Nickthedentist]

As a general rule, if a telly has SCART sockets, the circuitry will be safely isolated from the mains EXCEPT FOR THE POWER SUPPLY COMPONENTS ON THE "MAINS" SIDE OF THE SWITCH-MODE TRANSFORMER

If you look on the print side of the PCB, it's usually pretty obvious which section of the board has live mains on it, and there's a wide gap between it and the "safe" side.

[welbeck5]

Originally Posted by Welsh Anorak

Not exactly, that's why TV repair is a bit more involved.

However TVs of this era and later use a switch mode power supply which does effectively isolate the chassis from the 'hot' part of the circuit.
This is good news for you.
If we were talking about a TV from the seventies or Eighties then things may well be different and potentially (no pun intended) more hazardous.

The main hazard is the fact that, unlike a VCR, there are lots of high and very high voltages on the same board and, as you obviously appreciate, they can linger after switch off. However you are looking at a relatively low voltage area which you will be dealing with cold, so just observe the usual precautions mentioned by Nick above.

cheers

[welbeck5]

Originally Posted by Nickthedentist

As a general rule, if a telly has SCART sockets, the circuitry will be safely isolated from the mains EXCEPT FOR THE POWER SUPPLY COMPONENTS ON THE "MAINS" SIDE OF THE SWITCH-MODE TRANSFORMER

If you look on the print side of the PCB, it's usually pretty obvious which section of the board has live mains on it, and there's a wide gap between it and the "safe" side.

cheers

[welbeck5]

I'm afraid it hasn't worked
Changed the 220uf and resoldered the IC 451 chip and exactly the same!
Please see photos. Also tried the AV3 scart to rca and it's the same too.





Any other ideas? Get a new chip?

I notice when I list the AV channels AV2 is set to AV2S
is this the SVHS someone mentioned earlier ?


I can't seem to find a menu option for the tv and the remote works but
maybe it's not a perfect match for the tv and so I can't really
change or check settings.


Latest:

Just realised there's a panel that lifts up on the remote and it has a menu option. I have just put the tv away,
so will get it out again tonight and see if any of the menu options can fix this

[Norman Raeburn]

If I recall correctly to get rid of the av2s you press the green button on the remote and press it again. That should get you just av2 and should bring the colour back.

[Michael Maurice]

I don't think menu options will help, probably a replacement chip is needed

[welbeck5]

Originally Posted by Norman Raeburn

If I recall correctly to get rid of the av2s you press the green button on the remote and press it again. That should get you just av2 and should bring the colour back.

I'll give this a go later, cheers

[welbeck5]

Originally Posted by Michael Maurice

I don't think menu options will help, probably a replacement chip is needed

[welbeck5]

Does anyone on here sell these IC chips? The one I'm after is TDA8175-3.

I've seen one for sale elsewhere but it's TDA8175 ST (SGS) and not sure if it's the exact match.
Cheers

[welbeck5]

I've took the chip out, note the 3 on the end of the cat number which matches the parts list in the service manual.





Does anyone know the significance of the 3 or will all TDA8175 chips work?


Would it be worth my while cleaning the pins and resoldering to see if it resolves the problem?

[Welsh Anorak]

I wouldn't think that will do it.
I don't think there's much difference between the chips - however I could be wrong.

If a new IC doesn't work then it's time to check the parts around it, especially the diodes.

[welbeck5]

Originally Posted by Welsh Anorak

I wouldn't think that will do it.
I don't think there's much difference between the chips - however I could be wrong.

If a new IC doesn't work then it's time to check the parts around it, especially the diodes.

Thanks mate, re: the diodes, I was speaking to member 'Red to Black'
last night and that's exactly what he said.
I've ordered the chip

[gallowfields]

Originally Posted by welbeck5

I'm afraid it hasn't worked
Changed the 220uf and resoldered the IC 451 chip and exactly the same!
Please see photos. Also tried the AV3 scart to rca and it's the same too.

Attachment 315660

Attachment 315661

Any other ideas? Get a new chip?

I notice when I list the AV channels AV2 is set to AV2S
is this the SVHS someone mentioned earlier ?


I can't seem to find a menu option for the tv and the remote works but
maybe it's not a perfect match for the tv and so I can't really
change or check settings.


Latest:

Just realised there's a panel that lifts up on the remote and it has a menu option. I have just put the tv away,
so will get it out again tonight and see if any of the menu options can fix this

going by the pictures in this post, it looks like you have cured the frame fault now [top lin and flyback fault ] so you dont need a tda 8175 . just got the coloured lines to cure
brian

[welbeck5]

Originally Posted by gallowfields

going by the pictures in this post, it looks like you have cured the frame fault now [top lin and flyback fault ] so you dont need a tda 8175 . just got the coloured lines to cure
brian

Hello mate,
What are you seeing different since I don't know what the different faults describe?
To me it looks the same, maybe 8-10 white lines along the top and
the colour line(s) (pulses) about a third of the way down.
It's difficult to capture it on camera, but in the latest photo I can still see these two faults along with the black and white picture.
I've ordered a new chip now and if it doesn't fix it, I've been advised to switch the set straight off to avoid damaging the new chip and start looking at the diodes in there like D453. Cheers Brian

[Red to black]

The fault symptoms are slow frame flyback and/or blanking, the RGB 'test lines' these lines are inserted for the purpose of auto grey scale tracking calibration and shouldn't normally be seen as they are inserted during the blanking interval off the top of the screen in a normally operating frame stage.

[Red to black]

Just to add, this type of frame chip uses a flyback generator system, which with the associated capacitor/diode effectively makes a voltage doubler arrangement to ensure rapid frame flyback, if this network is defective in some way (usually the cap first. but also the diodes can be responsible) this causes significant over current dissipation to the frame chip and will damage it in short order.

[welbeck5]

Well I've got the new chip and soldered it in and I'm afraid it still hasn't
worked but now I'm getting something different which is even less picture.
Please see photo it's basically lines near the middle of the screen
jumping around with what appears to be nothing top and bottom.
When I first switched it on the picture was there and it may have briefly been
in colour (but it was so quick) but then it deteriated to the wiggly lines through the middle.
I switched the set off after a few seconds, per RED TO BLACKS advice to avoid any chip damage but the way it deterioated on screen suggests something failed?
I could resolder it and start to check the diodes, it's such a cramped space which makes it so difficult.

[welbeck5]

I've put the old chip back in and I'm getting the same problem above,
so maybe I've not soldered it correctly.
It's in a right awkward place up against the plastic frame of the TV.
Or something else has gone.
I did slip the soldering iron causing a scratch across the board but
it's not very deep, hopefully it's not that!
I will take the chip back out and give that area of the board a clean
and make sure none of the pads have come off.
I did also notice that one of the diodes, D453, which is referenced as JW in the service manual is just a bit of wire. And in the list of components D453 is not mentioned so this must be by design.