sylvania tv lost video (have raster)
 

Sylvania model ca8112wh
chassis ident number- e210002-1464351
mfgd- april '78 (<-figures....just hit 30)


a while back i posted about a sylvania TV i have, at the time it had some issues making noises and came to find out that the supposed 'repairman' actually did fix something (tripler). Ive been using it since then without incident till tonite. I was about to go to bed, i was watching Mythbusters when at the very end of the episode (closing credits) and video went out. at first i figured it was on the cable company's end but kinda thought it was awful clear blank raster (no static or anything) to be that. I got up and switched channel and it was still blank. I have sound, i have raster, just blank like the tuner is not workng. No magic smoke or any noises, just stopped displaying video.
Now be gentle, im a mechanic. i concider myself a decent mechanic but im still a just a mechanic, and while i enjoy electronics and the like ive never done much other then fixing a few old radios (recaps), and the regular stuff at work dealing with electrical issues (i work at a dealer so we see all the "bring it to a dealer" issues).
can you help? does anything commonly go bad on these to kill video? No matter what im not getting rid of it, just wanna save some money and get satisfaction out of trying to fix this myself. Any ideas would be appreciated.

Hi, I don;t know about that specific model, but I'd resolder the entire crt neckboard (video-output board) just to rule that out, and then look for bad caps and connections on your power supply board.

yeah i guess ill open it up tomorrow or over the weekend, just hopin someone may have seen a bunch of these have issues and be able to shoot off some parts to check. Im still accepting ideas.

thanks again everyone. Anyone have a schematic for this?

k, most likely be calling this place "dragon tv repair" monday but i figured maybe itd roust some more brainstorming if i give a few more details:
i played around with stuff today to try to give you an idea of what does and doesnt work.

Tint controll seems to make the black go from greenish black to purpleish black-so that seems ok. Horizontal hold makes the screen flicker (i assume if there was a picture it would be rolling). brightness and contrast react normally....on the blank raster. The tuner audio works fine, no video on VHF or UHF. I dunno...maybe this helps. i opened it up and didnt see anything obvious (bulging caps or bad solder joints). the crt neckboard doesnt have much on it, a few resisters and thats about it. looks like individual color drive is on the main board...side by side red, green, and blue (judging by similar layout of components), but i assume thats all working as i can change the tint of the 'black'.

Maybe the digital switchover has already happened at your house :)

Lol. Keefla I recommended resoldering the neckboard because on most sets this is where your video out transistors are located. If they aren't, then I would go ahead and find those since they're high heat devices....Also check that the brightness/contrast pots aren't losing contact with the board.

That should be an E21 chassis. Check the service switch. Switch it back and forth a few times. All video passes through this switch, and dirty contacts can cause you to loose the video. Back from the switch is the video amp transistor Q904 (I think), it will cause no video as would the failure of the video buffer transistor Q902 (I think). If you have sound then everything up to the sound take off should be ok. This leaves the switch, about 3 transistors, and the low level IC for processing the video. As always 30 year old caps and out of tolerance resistors can kill a transistor, or IC, and bad solder joints in this area could be a problem. There isn't much on the neck board as you noticed other than some biasing resistors.

Bill R

Bill, thanks for the suggeations. it is an e21 chasses, and according to the transistor layout sheet inside the set it lists Q904 as the video driver and Q902 as the video amp (so you were on target just backwards....i say thats close enough for probably not having seen one for 20 years!). So i will remove and test them. I did try the service switch and it seems fine, i saw no hint of video when switching it back and forth, even hit it with some contact spray just to be sure. ill post back as soon as i test the transistors and let you know.

Bill,

i checked Q902, Q904 and q202 (video buffer). all seem fine, 800 or so ohms b-c and b-e, even collector to emmitter didnt show any continuity on my meter on the 2 meg scale. so i figure theyre all ok. any ideas what voltages i should be getting on the pins on the low level video ic? no bad solder joints were seen and no obviously bad caps.

Here's what I have on the Sylvania E21 chassis. Sorry for the mess - it's an old DBIII+ database, converted to text, then to Word, then to PDF for posting.

Disclaimer - I've not worked one of these sets in more than 5 years and haven't even looked at the Sams for it.

Hope the tips are useful.

Cheers

thank you for that file, no need to appologise for the layout, it was perfectly readable. i found a couple that kinda match the symptoms...namely dark picture...but i have no picture at all....maybe ill start by checking those components (caps it loosk like)...see where i get with that. as always still open to suggestions and thanks you all for your help!

Good luck in getting her going again. I'm no Sylvania buff but they were always very interesting sets to me. :thmbsp:

Sorry about the number mix up. I pulled the schematic and looked again. Q904 is the video driver, Q902 is the video amp, and the video buffer is Q202. The video comes from IC202 which is called the low level detector. It contains the video dector and first video amp. From there signal passes through Q202, Q902, then Q904. If you have a scope you can look for video on pin 4 of IC202, if it is there check the emitter of Q202. If you have no video there then look at the base of Q202. If video is ok at emitter of Q202, move on to Q902 base. There is a 5mfd 25 volt capacitor (c247) for coupling Q202 to the base of Q902. From the collector of Q902 video passes through the delay line and to the base of Q904. It then goes from the emitter of Q904 through the service switch then to the red,green, and blue output transistors and on to the crt. I do not have any voltages listed, the scope though would be the easiest check since you could just follow the signal path and look for the video signal. Prime suspects are the service switch, Q904,Q902,Q202,IC202,c247, and I have seen open delay lines. The 24 volt power supply is used here, but also in the audio section, so since you have sound still the 24 volt supply is probably ok.

Bill R

bill,

that info will be extremely helpful, i do have a scope thow its on lend to a friend right now. Ill go grab it back for a few days thow. ill be able to do that tomorrow after work and should have a better idea of what part of the chain is broken for you by friday. thanks again for everything ill keep you posted.

update time,
grabbed the scope today and checked at pin 4 of IC202, had nothing. I tried injecting video at pin 4 (to check the rest of the chain and found a scrabmled picture...however i found that if i inject video into pin 2 i get a deciferable picture on the screen (vcr setup screen). so i guess its before that IC? is pin to the input or am i just bypassing something ? what would you suggest i check next? I tried to find a pinout but to no avail.