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 Betreff des Beitrags: Philips 21TD310A help needed
BeitragVerfasst: So Apr 14, 2019 17:14 
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Registriert: So Aug 05, 2018 21:41
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Hi TV friends
my Philips 21 leonardo volautomatic TV has started getting a very dark picture lately !
in the past , the tv was all repaired by me, new electrolitic capacitors and general maintenance and it worked very well for the last months!

now the sound is good, but the picture is very very dark
without a signal on the antenna, the regelspannung (AGC) voltage is -1,5volts and if i inject composite video on the grid of the video verstarker the tv displays the video
but the moment i touch a signal to the antenna input, the regelspannung jumps to -50 sometimes -90volts and the picture gets very very dark
i have been testing components for 10 hours now and i even replaced all the Rohren by new ones that i have here.
there is a VDR that does the regulation of the regelspannung (R63), i tried disconnecting it and short circuiting it, both with no effect
i really dont know what else to test
i will be very gratefull if you loose some time helping me. i have an oscilloscope and multimeter as test tools
my video source is working well because i tested it on other tv's
the circuit around the contrast and brightness control has been checked, i really know that this is a problem with regelspannung regulation


this is signal through the antenna
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this is video injected on the grid of the video verstarker
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i uploaded the schematic to here
https://www.docdroid.net/fDYfmZg/21td310a.pdf#page=5


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 Betreff des Beitrags: Re: Philips 21TD310A help needed
BeitragVerfasst: So Apr 14, 2019 17:39 
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Registriert: Do Okt 05, 2017 19:39
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Kenntnisstand: Elektrotechnischer Beruf/ Studium
Hello,

The picture ist very dark, but there are bright spots in the picture, too. A wrong DC-Voltage on the picture tube or in the video-stage would not cause this and the good picture with composite on g1 makes this fault impossible.
I had a similar effect of wrong gradation because of a faulty video-rectifier (in your set X6 "OA 70"). I would inject a correct test-pattern over vhf and put an oscilloscop on M7 (output video-rectifier), perhaps you can see the faulty signal with wrong gradation.

The picture is without noise and the level seems to be o.k., too, so I would say, there is no fault in the front-end oder if-amp.

It is offten not easy to change this diode, perhaps you have an output filter in your workshop, it is possible to connect a similar filter on the last if-stage to prove this presumtion. The alignment ist not so critical, if the cores are not way of.

greetings, Ingo


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 Betreff des Beitrags: Re: Philips 21TD310A help needed
BeitragVerfasst: So Apr 14, 2019 18:30 
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Registriert: So Aug 05, 2018 21:41
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hi Ingo
thank you for that idea
i tried the following:

turn on the DVD player, video cable goes directly to grid of videoverstarker. picture is OK

only the audio cable goes to my VHF modulator and i touch it on the antenna...the AGC kicks in and the picture is all black again, the sound plays through the IF and through the speaker of the tv but dark picture.

unplug the modulator from the antenna and the picture starts getting brighter again gradually

DC voltages at the picture tube are about 20v higher both on cathode and on g1 (maybe because the high line voltage) but they seem fine in respect to eachother
g1 voltage varies while turning the helligkleit control as expected
this crt is very weak but the picture was acceptable before this malfunction and is acceptable when injecting video directly in the video verstarker

i dont have a test pattern generator but i will play something on the tv and post now the oscilloscope trace here

i will try to locate the diode, i think it is inside one of the metal cans, can you tell what type of modern replacement is suitable?


edit:
here is the trace on the output of the video gleichrichter (m7)
not much to see i guess without a test pattern
we have the modulation and the sync pulses below like expected
Dateianhang:
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 Betreff des Beitrags: Re: Philips 21TD310A help needed
BeitragVerfasst: So Apr 14, 2019 19:16 
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Registriert: Do Okt 05, 2017 19:39
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...if the picture gets brighter when unplug the antenna, there can be an issue with the AGC eather than with the video-rectifier !
Test: unplug the antenna-coax-plug and the hold the outer contact together with a little airgap between the inner contacts (lose-connection). If now the picture is noisy but o.k. in gradation, then the agc is suspected, more than the video-rectifier.

greetings, ingo

you can replace the video-rectifier, if nessesary, with many types of old GE-Diodes, my Henninger comparison list shows AA133 for instance, look to other TV-shematics for other types.
I have good results with schottkys, too, BAT41 e.g.


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 Betreff des Beitrags: Re: Philips 21TD310A help needed
BeitragVerfasst: So Apr 14, 2019 20:02 
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Registriert: So Aug 05, 2018 21:41
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hi ingo
if i approach the antenna terminals but dont touch it the picture is noisy but correct brightness !!!
its definetely the AGC like you and i suspected
i have tested a lot of components on the AGC path but i cannot find the fault! I suspected the R63 VDR but i tried shunting it or disconnecting it and no good results from that. i also tried a spare vdr of the same colour code


with r63 shorted i read about -20v
with r63 connected i read the same
with r63 disconnected i read -1.5v and the picture gets totally black nothing on the screen

also tried disconnecting the tuner AGC line (A on the schematic)
disconnecting and connecting with the tv on makes no difference...
the tuner AGC line measures -3volts but the schematic says -1,5v,
i tried disconnecting the tuner agc wire and connecting a AAA battery with negative side to tuner AGC and positive side to chassis, the result is the same

also the "tuner snow" looks correct without a signal! good white and black snow

i had the same problem on a nordmende c15 tv but it had an AGC adjustment and tweaking it solved the problem...but this tv has a very complex agc circuit because philips wanted to maintain a very perfect black level independtly of the signal strenght..all is automatic and now we deal with the complexity :wut:

this tv has a circuit called storaustastung or "noise blanker" can that be the problem? because from what i saw the AGC only acts in the tuner or in the first IF tube...both voltages are correct there..


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 Betreff des Beitrags: Re: Philips 21TD310A help needed
BeitragVerfasst: Mo Apr 15, 2019 18:34 
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Registriert: Do Okt 05, 2017 19:39
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Hello,

at this point it is not easy to make diagnosis from distance. But I am sure you can fix this.
With an external agc-mimiking voltage (over AAA-cells or so, better with a Pot. 0... -10V) you have to get an o.k.-picture, see service manual for if-alignment. If not, there is a fault in the if-stage.
My homemade if-stages run all with constant agc-clone-voltages (adjustable over potentiometer) and this works very good.

Pleas tell the progress if you get one ^^

Greetings Ingo


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 Betreff des Beitrags: Re: Philips 21TD310A help needed
BeitragVerfasst: Fr Apr 19, 2019 21:14 
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Registriert: So Aug 05, 2018 21:41
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Hi to all

today i made some valve socket extension adapters so i can slide them inside the valve shields and test the voltages easily with the tv on
i checked all dc voltages on the IF AGC and Tuner
everything is present and good.
i tried making a fake AGC voltage and the result was the same

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I was reading the service manual when i read something like this:
" in order to supress the "Heating hum" the tv tuner remains blocked until the line output stage starts working and the AGC voltage is generated. There is a negative voltage already present at the tuner wich is rectified from the heaters of valve 25 and valve 5... this negative voltage is supplied at the tuner until a positive counter voltage is applied after the line otput stage starts workin, that counter voltage is drived from the Boost voltage thru r93 r94 r97

ok so i tried disconnecting that voltage line that goes to the tuner and i only have a blank raster
connnectin back the wire makes my usual dark picture
applying a stronger negative voltage with a battery also makes a dark picture
applying a positive voltage with a battery also makes a dark picture :| :wut:
very strange!!!

all the dc path of those control voltages and counter voltage was checked, all resistors are good!


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 Betreff des Beitrags: Re: Philips 21TD310A help needed
BeitragVerfasst: Do Apr 25, 2019 18:18 
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Today i grabbed a parts TV set without chassis and put it back to back with this one, i put my yoke in there, connected the anode wire and all the crt neck connections
the crt is also 110º deflection, it is 17inches instead of 21, the neck pinout is the same, this crt tests very strong unlike the original wich is very very weak
the image now is very watchable but still too dark, i can see black streaking to the right of white objects on the screen for example subtitles, this usually happens when the agc is overloading the IF

Dateianhang:
WP_20190425_18_01_48_Pro.jpg
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this image is with contrast and brightness all the way to the maximum


Dateianhänge:
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 Betreff des Beitrags: Re: Philips 21TD310A help needed
BeitragVerfasst: Mo Apr 29, 2019 0:11 
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:bier:
Well after all this work turns out it was a bad CRT indeed...
found a replacement crt with a bit weak emissions but it still produces a nice picture adn the Philips tv is fully working again!!!

i never thought a bad crt could produce such weird problem of wrong contrast!!! i usually see blooming and lack of brightness...


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 Betreff des Beitrags: Re: Philips 21TD310A help needed
BeitragVerfasst: Mo Apr 29, 2019 7:06 
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Registriert: Do Okt 05, 2017 19:39
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Hello,

yes, this was an interesting and difficult issue ! Your description at the beginning said, that the picture was quite o.k. with an external CVBS on the Video output, that fits not so good to a bad CRT, but your success is the prove for your diagnosis, congrats !
Its not so easy in these days to get a decent CRT.

Greetings Ingo


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