crt tv does not like white screen signal over compoite - Page 1
Forum section: electronics / repair

author: kokodin
author_rank: Regular Contributor
timestamp: April 14, 2017, 06:51:08 pm
content: This is going to be a wierd topic, because everything works, but not quite :]My long term "fix my childhood tv odysei" has gone to the point i was trying to fix screen geometry. After replacing all capacitors on the main pcb and most important modules, tv works. Why i want it to work is another story.Anyway i encoutered a problem with screen getting wierd whatever i try to display mostly white picture on the full screen,but only over composite video. Picture visably shift to the left a little bit from middle to the bottom and get a bit darker or shaky. It also have a little problem with catching synchronisation at the beggining, for example when i turn on the NES and try to play Kirby title screen always is distorted over composite. It takes about 5-6 secunds to display it corectly, but not if the screen is fully white, then it stays distorted.I can send the picture over rf and then it is fine, but via rf i have blank synchronised screen even when game isn't running and console is under power , white screen dosn't distort via rf . When i send white picture via rgb-in screen is also stableI tracked down the problem to the av input/output module i have made in order to get other inputs than rf to this specyfic tv. i took the schematics and pcb leyouts out of service manual for this tv, so the way i connected my module and module itself are mostly correct. and now i am hitting the wall.The only thing i did differently than in the manual schematics are different transistors i used. they seems to be good replacement parts, most of the parameters in datasheets match (bc547b insread of bc238b and bc557b instead of bc307b)i have no idea how to debug this thing. i can probably post full tv schematic or just the module i am stuck with, photos of the distortion and so on, but i doubt that you would like to dig around 30 years old polish construction without the clue how it supoded to work. . so i would like to sak for general pointers what i could mess up and maybe with that help i get to the problem.as i see it now, tv is losing synchronisation i think horisontal because it stays in place only distort to the left on the bottom, like if signal itself was getting overdriven/distortedi tryed my module on other tv and it is doing similar things, only that tv have better synchronisation and recover faster

author: edavid
author_rank: Super Contributor
timestamp: April 14, 2017, 07:10:06 pm
content: DC restoration isn't working. Post schematic of the composite input module.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 14, 2017, 08:54:34 pm
content: Yes, DC restoration, or sync separators, or supply voltages.A white picture draws maximum beam current; if this is pulling down other supplies, it might explain the effect on sync.(Usually, high voltage problems are, well, high voltage problems -- the picture enlarges ("bloom") because the acceleration voltage drops, making the deflection angle larger.)Tim

author: Armadillo
author_rank: Super Contributor
timestamp: April 14, 2017, 09:34:43 pm
content: Replace all those small electrolytic capacitors [from 1 to 22uF] around the sync/deflection IC location.

author: kokodin
author_rank: Regular Contributor
timestamp: April 14, 2017, 09:39:35 pm
content: Well i don't know about high voltage stuff, it works from RF or RGB inputs just fine, only AV make problems Also power suplu seems to work for all rails on the main board, 214V 140V and 18V rails seems ok and 12V is regulated down to 12V from 18V rail. AV board is powered by this 12V railSince this tv is kind of wierd i explain how it suposed to workWhen tv is operating normaly composite signal is taken stright from the tuner block output, and then it is buffered with t923 and attenuated with resistor devider straight to the output wy wideoWhen composite signal is inputed to the WE Wideo input signal goes trough 3 transistors where it is buffered and amplified by the factor of 2,4 and outputed to the composite output on tuner block, at the same time synchronisation pulses are detected on transistor t910 which is used to switch off audio and video outputs on tuner block, it also light av ledSignal is also buffered and transmited back to the video outputWhen rgb signal is detected via synchronisation or video signal inputed to WE IMP SYNCHRO input audio is blocked at the tuner block and av led is light. Since rgb mode uses r-y b-y and Y signals r g and b signal are converted into r-y ,b-y nad y signals in another moduletv does not have av switch, all av modes are recognized and executed from av board which take over the video output line from tuner block or color decoder outputs via anther input switch modulei hope that make sense i can google translate exact paragrapf about umm2002-4 module operation modes if it does not make sense.tv is a polish unimor neptun d505 with neptun m558 av module and neptun m557 rgb module (same basic mainboard,crt tube and most of modules) but that is probably only a strange name for you :]By the way, voltages at the test points of this module all match the red printed ones in all operation modes, but only when module is connected to tvi say match if tolerance is less than 0,2V

author: kokodin
author_rank: Regular Contributor
timestamp: April 15, 2017, 09:30:52 am
content: ok so i tested stuff again "in circuit"some voltages do not match in av mode. strange , they all matched manual specs outside the tvi am not really bothered by few floating pins on g913 and g914 conectors , because my setup does not use those pinsbut i have 3 transistors that act wierdlyt916 instead of E 5,9V have 1,5V there and B should be 6,5V and shows only 2Vt917 instead of E 6,5V ther is 2V and B should be 7,24V and i have 2,7Vt921 instead of C 10V i have 0,1, but again pinon g914 is not used in m557 tv so i did not used this pin, same gows for pin 2 on g913i probably need to dig deeper into differences between m557 and m558 av modules and how they are connected to the main boardalso the transistor i drawn in , where schematic was smired should be t916 i think not t923, because there are now 923 now sorry for thatedit*******************************ok i messed up a little bit when mixing m557 and m558 elements into my d505 model because i first made av module from m558 and then the rest od m557 input boards, but that shouldn't matter because dubbled parts were mostly responsible for rgb synchronisation.so i backtracked a little bit and now i am closer to umm-2001/umi-2001 from m577 than anything else (only adition is the audio output from umm-2002)video synchronisation on white stays the same, go dark and to the left. But only white, all other color signals are displayed correctlyi attach a whole schematic of the tv, most of the modules are the same as those in my tv.http://www.deviantart.com/download/675032120/schemat_m557_by_kokodin-db5wa48.jpg?token=3b831b1811f3370cf0ba6104f5692971ee2ed5d6&ts=1492267783if the upper link does not work just download it from my deviant accounthttp://kokodin.deviantart.com/art/Schemat-M557-675032120edit***********************************i was playing around with the scope, dc coupled and i discovered something. Signal is a little bit compressed on the top. if we belive my scope voltage peak to peak is correct, with a colorbar signal (around 2,5Vpp), and less when i input white signal (around2Vpp), Signal also begin half a volt higher than colorbar signal, so the synchronization pulses are at the black level of the other signal? Like it was more bias than colorbar signal. Synchronisation pulses are also shorter at the synchronisation midule. Anyway i have no idea how to resolve that, because i have 3 modules that don't play along. And i only partialy know how they work:]i will try to simplyfy the schematic to cut out every module that isn't involved with this signal and then i might start to think how to fix itbecause right now when i see this schematic i see too much at once to concentrate.edit***********************************pighededness for the win, or was it leg work?Basically i went trough all revisions of this av board i could find and i tryed out every different bias configuration for t917. Resistors r928 and r929 were often switch for other values in diferent board revisions so it was the easiest thing to notice. In the end i reduced r928 from 18K to 8,2K and left r929 as it was and everything works. I also gain some color saturation in bright colors.So now it works as it should.Thanks you all for help i think you kicked me in the right direction even if i did not understand much about what you were talking about