Magnavox 42mf231d/37 vscan not adjustable

[freakaftr8]

Well I got all but one bug worked out on the power supply after replacing ic561 f501 and zd565. Vscan is operating at -173v and the pot will not change the voltage. I already tried replacing the photo coupler and varistor controller transistor. No luck. What else might I'd be overlooking. Power supy is Samsung model v4c or lj44-00092e
Thanks guys. Your input would be
Much appreciated.

[freakaftr8]

Re: Magnavox 42mf231d/37 vscan not adjustable

Edit: when I mentioned varistor controller I means shunt regulator controller transistor.

[freakaftr8]

Re: Magnavox 42mf231d/37 vscan not adjustable

Any ideas what to look for. It's for sure a feedback problem. It's got to be.

[freakaftr8]

Re: Magnavox 42mf231d/37 vscan not adjustable

Hey does anyone have a schematic on the v4c power supply? Lj44-00092e

Thank you. Preferential the feedback circuit for the vscan that feeds to ic561 for vscan voltage control.

[freakaftr8]

Re: Magnavox 42mf231d/37 vscan not adjustable

Alright I'm really stumped now. All the components from the optocoupler to the varistor for vscan all read -173 v. Even the top of the caps!! The other feedback and voltage adjustment circuits on the board using the same components are only like 12v.
Could use someone's input
Thank you

[tom66]

Re: Magnavox 42mf231d/37 vscan not adjustable

Can you post a picture of Vscan circuit?

[freakaftr8]

Re: Magnavox 42mf231d/37 vscan not adjustable

Hey Tom. Sorry I can't upload a pic at the moment. But I found if I short the opto on the vscan circuit the voltage is still there it doesn't shutdown the circuit so the problem is before the opto

[freakaftr8]

Re: Magnavox 42mf231d/37 vscan not adjustable

Well another observation shows that even though the vscan and ve and vset circuits are all virtually identical there seems to be current drawn from the negative side after the t561 that is deferent from other circuits. The shunt regulator middle pin is direct with the vscan output to the ysus. In turn the varistor had -173vdc going thru it and everything after the transformer is all -173v. This blows my mind how its bot shorting out while in operation.

[freakaftr8]

Re: Magnavox 42mf231d/37 vscan not adjustable

Ok soon pulled the vscan pin out of the circuit feeding from the power supply to the ysus. The -173v is coming from the ysus. Vscan is dead on the power supply. What do you think the chances the transformer is damaged?

[tom66]

Re: Magnavox 42mf231d/37 vscan not adjustable

There is a Vscan pin on Y-sus board and you are telling me this is an output?

[freakaftr8]

Re: Magnavox 42mf231d/37 vscan not adjustable

Yes I have never seen this before. The vscan input on the ysus is actually generating its own voltage. Just to see what would happen I went ahead and pulled the ve pin off the zsus and it seems to generate voltage too. So essentially I disconnected vscan and ve off the power supply and still had an image!!

But the power supply still generates ve voltage as it is disconnected. Vsus does not. I thing it's just feedback from vs being generated through the panel.

[tom66]

Re: Magnavox 42mf231d/37 vscan not adjustable

OK, I would really need to see a pic of the Y-sustain to advise further.

[freakaftr8]

Re: Magnavox 42mf231d/37 vscan not adjustable

Diagnosed problem. I have a similar Philips. If I pull vscan pin tv still works. It's backfeed from the vs. so back to original problem. No vscan output from power supply. It seems as if 205v is getting to the top pin of the vscan transformer but nothing comes out. I ohmed the transformer and it seems good.

[tom66]

Re: Magnavox 42mf231d/37 vscan not adjustable

Check if the diode near the transformer is open. Sometimes there's a fuse, too.

[freakaftr8]

Re: Magnavox 42mf231d/37 vscan not adjustable

F501 was open. I replaced it upon first inspection. That fuse is the master fuse for vset vscan and ve. Vs is a seperate circuit.

[tom66]

Re: Magnavox 42mf231d/37 vscan not adjustable

Great; so does it work now?

[freakaftr8]

Re: Magnavox 42mf231d/37 vscan not adjustable

No unfortunately. The fuse I replaced when I replaced ic561 and shorted zd565

[tom66]

Re: Magnavox 42mf231d/37 vscan not adjustable

Hmm, I would like to offer more help, but I'm a bit lost now...

[freakaftr8]

Re: Magnavox 42mf231d/37 vscan not adjustable

Ok I know I got a little confusing. Ill try to re summarize what this tv is doing.

I picked it up from a gentleman that said blue light comes on no picture. Ok so I got it home and opened it up. Buffers test good and ysus good. So next I took voltage tests. Good vs and ve. No vset and low vscan. Vscan was a -173. Supposed to be -190. Ok so I pulled power supply and visually found a blown ic. Ic561 and zd565 nest to it shorted along with open f501. The ic is a Fairchild A5L0365R. 4 pin with feedback sensing. Replaced all and powered It up. Came on with a nice picture so I thought ok problem solved. Not quite. Even though that brought back vset voltage vscan was still reading -173v. I tried adjusting vr561 for the vscan and made no change. I started researching the circuit and it seems as if the circuit is identical in retrospect to ve and vset circuits getting a common feed from the same source hence only one fuse labeled f505. So after searching tirelessly on the board coming to no conclusion what would cause. Low voltage in the vscan circuit I decided to disconnect vscan pin from the power supply that feeds into the ysus. Up on power up I was stunned to see the pic come up working! I thought to myself how can this be? Vscan is disconnected and what is feeding the buffer boards? So I probed the disconnected vscan wire Nd to my surprise it reads -173v disconnected! So something on the ysus is back feeding the vscan circuit. Mind you now that vscan is disconnected from the ps there is zero volts output from the ps.

Then I decided to disconnect ve from the power supply to see if it still outputs disconnected and it does. But oddly even with ve disconnected the screen still works and has a good picture! I'm totally stumped. The ve pin off the zsus was reading a voltage as well. Lower like 70v instead of needed 84 but that's what vscan pin does also. I replaced opto in vscan and some caps And checked every single component on the secondary side of the previously blown ic561. Everything g is in spec.

I can literally run the tv and watch it with vscan disconnected completely. The only explanation I would have as to why the screen works with vscan and ve pulled is the panel is generating some voltage from vs and vset to actually power these circuits from the back side.

Any ideas?

[tom66]

Re: Magnavox 42mf231d/37 vscan not adjustable

That is really odd! Is there any problem with the picture, for example sparklies (random flickering pixels), unstable pixels, lag/sparkles when changing from bright to dark?