How can I check zener diode?

[bshi02]

hello.
I have 29 inch LG CRT TV(CN-29H3) which won't display any screen.
and it is written "MC-7CA(MAIN) 6870VM0056C(6) 980424 L.W.G(0)" and
"MC-7CA(CPT) 6870VS0167A(1) 971014 K.J.B(0)" on the surface of its PCB.

Frankly speaking,I posted a thread about this CRT TV in the reddit forum.
But It was impossible to repair this CN-29H3 CRT TV.
https://www.reddit.com/r/ElectronicsRepair/comments/1cb7m6y/how_can_i_check_str_6707_regulator/
and I also post a thread in the badcaps forum,and I finally got a clue of this problem.

It may be related to defective zener diode or C3852A transistor.
https://www.badcaps.net/forum/general-topics/general-capacitor-questions-issues/3282862-how-can-i-test-non-polarized-capacitor#post3297183

I attached a pdf files(lg_chassis_mc-7ca_sch.pdf) because I found that Its schematic of Hot area is almost the same to that of CN-29H3.

I measured pin#9 of STR-S6707 by placing red probe of analog meter to pin#9 and placing black probe to negative termainal of C808 the biggest capacitor(400v 470uf) and It was measured 6.4-6.6Vdc.

But In the datasheet of STR-S6707,It says that its On-State Voltage is 7.6Vdc.So I think that there would be a defective component which is undoubtedly adjacent to pin#9 of STR-S6707.

And In the same way,I measured cathode of ZD808 zener diode and Q800(C3852A)base,It was measured 7.1Vdc.

I also measured positive terminal of C805 and cathode of  D807 and Q800(C3852A)collector,It was measured 10.8Vdc.

Judging from pdf files(lg_chassis_mc-7ca_sch.pdf), ZD808 has 7.5V  specification.So I desoldered this ZD808 zener diode and I checked it with zener diode tester which was made with the aid of veteran eevblog users.

It displays 7-7.5V on analog panel meter and 0.5-1V with placing opposite polarity of probe.
and It is written B.75 on its surface.

I desoldered ZD413 zener dioder which has 5.1v specification and also checked it with zener diode tester.
It displays 5V on analog panel meter and 0.5-1V with placing opposite polarity of probe.
And it is written B.51 on its surface.

I think that There is no faulty in ZD808 zener diode with zener diode tester. I don't know why Output voltage of ZD808 zener diode cannot reach 7.5Vdc..

[MathWizard]

I was reading some of the badcaps post, IDK much about TV's or that layout of SMPS, but do you have have a better drawing/picture with the resistor values ? Can you edit a picture like below, and put them in ?

Yeah it sounds like the switching IC Vin pin9, is not getting up to it's full level. But is that just the startup voltage pin? Where do R805/6 go? There normally would be a rectifier diode if they go in series to the mains voltage, before the main 4-way rectifier. Those resistors would be pretty big, and the probably drift over the years, and could easily change that pin9 voltage, so that the chip never starts at all.

I'm not sure if Q800 is part of a low voltage supply circuit, using some of those little windings, and maybe D807. Or if it's related to the oscillator and FB circuits, or maybe it does all of that.  I want to know all those resistor values.

But yeah I don't see that going near pin9, so where's it and R805/6  go ?

So does the TV ever start at all? So you don't have an oscilloscope I take it ? Actually 1 of those $20 ebay scopes should see if 50-100kHz SMPS is running, no problems.

[bshi02]

Thank you very much for reply.
By the way,I soldered IC01(LG8708-08A), IC201(GL3812),IC501(TB1231N) and Turner TU101(TUSHB-A07A),Then It displayed actual picture on screen.
The culprit turned out cracking on the solder joint of IC.
and I confirmed that all of resistance of carbon resistor in the Hot area is exactly the same to schematic of attached pdf file(lg_chassis_mc-7ca_sch.pdf)

[kripton2035]

just to go back to the original title, here are some zener testers you can build yourself.
http://kripton2035.free.fr/semiconductorsre.html

[wasyoungonce]

I built a zener tester from the above link but from the original designer, José González Baena.

http://www.soloelectronica.net/50v_test_zener.html

You need a language translator but that's not hard to do.    Works great I don't use it often but when I need it ...its gold!  Some parts are a litle expensive like the transformer  MYRRA 44240 (I think OP Bish is in SK so uses 220V so you can use same Transformer).

I have eagle PCB drawings of mine and made to fit Hammond 1591 case.  You need a separate power for the meter so I used a mains to 5V DC phone charger in the case for this.    The designer used a

I have eagle PCB files for this OP wants.  Pretty simple diy toner transfer single side PCB.  PM if you want them.   There are cheaper designs out there.  One issue with this one.   The inbuild voltmeter meter I used isn't the most accurate but I made banana jacks you can put a multimeter on the jacks and mease more accurately.

 José has some decent little beginners and further diy testers

cheers