First post, and I'm begging for help. I'll contribute something back soon, promise. Anyway, I recently pulled a Sony KV-2900 out of a skip; it's an oldish (12-15 years at a guess) 29" TV. The picture was pretty well out of spec, but using the manual controls inside I've managed to get the picture centred and removed all the pincushion etc., but having adjusted the vertical size I've introduced a serious vertical linearity problem. The centre of the screen is stretched and the top and bottom are compressed. There are two pots (that I know of) that I haven't tweaked yet, mainly out of fear of electrocuting myself. One is H-Phase which as far as I can tell adjusts the horizontal position of the picture- that could do with a slight tweak, but I'm not sure I want to risk electrocuting myself to do it. The other control is a large pot (all the other controls are dinky little ones <1cm across) on the bottom of the neck board; I'm pretty certain it's not focus or overall brightness, so I wonder if it might be the one I need? The trouble is it's very stiff and it's hard to reach under the neck board to tweak it. I could post pics later if that'll help. Thanks in advance, Rich :¬)
Welcome, Rave. Faults arising in old components can also cause non-linearity, may be the reason it was skipped. I fixed an old portable a bit back, people all had short fat legs, by replacing a small capacitor after looking at the fault symptoms guide for that model. Have a look here.
Thanks mate. I followed your link, but couldn't find any mention of the KV2900. When I got it it had a European type 2-pin plug that appeared to have been bent to fit in a UK socket, and it has a lot of funky features like being able to manually switch between PAL/SECAM/NTSC 3.58 and 4.43 etc. so I suspect it might be an obscure model for the middle eastern market or something . I'm sure it was skipped for poor picture quality, I saw the box for a new TV in the same skip, although I also gave it a bit of a battering trying to wrestle it through my front door. The skip was only about 150yds from my house, but by the time I'd got it back my forearms were so pumped up I could hardly open my front door, and I would have dropped it if a passer by hadn't come to my aid! TBH if I can't fix it with tweaking I'll have to put up with it until I can afford to pay someone to do it, I don't havbe the knowledge to start replacing components.... Rich :¬)
Why is nothing ever easy? I've found the Vertical Linearity control, and there's not an adjustment pot in place. It looks like I'm going to have to solder one in. It might help if I bung in a couple of pics here: This is some of the main set of little adjustment pots. This is where the PCB reckons the vertical linearity adjuster would go. If I could get hold of the pot I reckon I'd have no difficulty at all soldering it in place, or I could even solder in a larger pot attached to some fly leads. The trouble is that I have no idea where to obtain such a pot or what resistance I need. Am I likely to damage anything if I solder in something wildly out of spec? It's been a long time since I did any electronics at school so my memory is a bit hazy. Am I right in thinking that if you have (say) a 10Kohm pot with all three pins conncted (like these ones), if the adjuster is in the middle then each side is seeing 5Kohm resistance; and if it's right over at one end then one side sees 10Kohm resistance while the other side sees very little? Cheers, Rich :¬)
Nobody? Looking at the way all these adjusters are set up with all three legs of the pot attached, I reckon that when you adjust the pots it biases an adjustment voltage one way or the other. Now, I'm thinking that if I solder in a very high value (linear) pot, say a 470KOhm, with the adjustment centred then the actual current flowing will be very small and unlikely to kill anything. However, the actual result of tweaking the pot may be very small, or very non-linear, with all the adjustment at the very ends of the ranges (since only there will you start to get any meaningful voltage one way or the other)? Excuse my very tenuous grasp of the principles involved . Will I be safe if I solder in some fly leads and then try attaching pots with progressively smaller values until I get the results that I want? Or is there some hidden danger that I've overlooked in attaching a very high value pot? Please if anyone can help I'd greatly appreciate your input. I'm going to stroll over to Maplin in a minute to check out their range of pots and see if I can snap up a digital multimeter (which I've been meaning to buy anyway). If I could buy some pots tomorrow lunchtime that would be cool, I could work on things over the weekend. Cheers! Rich :¬)
I really doubt you can install a pot and get any results. Notice that there are alot of other parts missing from around the linear pot? The designers have probably started using a vertical drive IC that is internally compensating, and has no external adjustment. Most cases of non-linearity are the result of dried-up electrolytic capacitors in the vertical circuits. This and cold solder joints that become poor connecions once they warm up as the TV operates. The signal that is supposed to be generated in the vertical IC should be a clean, flat ramp, like a 45 degree sawtooth. When you get non-linearity this ramp looks more like a skateboard half-pipe. Bad capacitors will do this. You mentioned that you had to adjust the vertical size to get the screen filled, and in doing so the vertical is no longer linear. Maybe it was linear before you adjusted the size, but now its streached? Perhaps the actual fault with the TV is the vertical size. This can also be caused by bad capacitors, especially the ones that are on the supply line for the vertical IC. If the supply voltage at the IC is allowed to "sag" because of a bad capacitor, it will reduce the size and or shape of the ramp generated by the vertical IC, resulting in insufficient height. There are of course, other things that can cause your problem, but I would start with the IC and its associated parts. It is a common fault found in the repair business. I would also check for bad soldering on the parts that get hot, like power resistors, anything on a heatsinks, etc. Replacing a couple of bad dried up electrolytics may solve your problem. You will have to replace them with parts that have the same value, making sure they are rated for the same voltage also. An example of how they are marked would be 25 volt, 680uF ( microFarad ) , the big black one in the lower picture. Notice the white stripe, this marks the negative lead, which goes in the hole beside the dot on the circle marking the capacitors location. Be careful inside a TV, you don't want a shock. BTW you can't tell by lookiing at an electrolytic capacitor if it has dried up inside or developed a high internal resistance (ESR ). They can be checked with a special ESR meter, but you are not going to get one of those just for a single repair. Very often, since they are fairly cheap, any suspect ones in particular section are often replaced all at once when a service tech does this kind of repair.
I'm pretty sure that the fault is caused by parts getting old and capacitors wearing out.....I was hoping that tweaking it back with the V-Linearity pot would be a reasonably quick fix rather than a permanent solution. At the end of the day this is a TV I got out of a skip and I just want to get it reasonable so I can bung it in the bedroom so that my wife can watch her videos on something better than the 14" that we have in there at the moment. Still, the only components I actually have any experience of replacing up until this point are capacitors (repaired an epox motherboard that way), and as you say they are cheap, so it wouldn't be too much of a problem to swap a few out. Do you think that's the best chance of success? It seems odd that they would have the adjuster positions on the PCB and then do nothing with them- this is an old TV don't forget. I appreciate the help btw even if I might seem to be arguing with you. Rich :¬) Edit- I'm away for the weekend. When I get back I'll follow the traces from the V-Linearity position and see if they actually go anywhere....
As a footnote, the failed cap (after 10 years, so no grumbles) was "Toshiba 1400TBT etc - Field lin - stretched at top, cramped at bottom ... C303 2µ2 field ramp cap" and was a 105°C rated electrolytic. I figure the high temperature rating may be important for another 10 years life. As for spare component sites on the pcb, a lot of manufacturers use a general purpose board that will make a range of models from basic to nicam teletext or to suit international markets, I can't honestly see adding components rather than just replacing helping repair this one.
Right, well caps it is then I guess. I'll try and have a look tonight, although my father-in-law has asked me to come round and help him with a mystery job, so I'm not sure how long that'll take. Is there any easy way to identify which caps are responsible for the vertical IC? Or am I just stuck replacing them at random until I find the ones responsible? On the bright side, I've been meaning to go into Maplin with a large capacitor order for a while now, I have a couple of graphics cards that need their caps replaced and this could be the kick up the backside I've been needing. Rich :¬)
The vertial IC is likely labelled, on the circuit board, if not is is generally a small, flat, vertically mounted ( LOL, seriouosly ) IC on a heatsink that has a part number like TDA1870, or similar. The caps near it are the ones to do first. If you have WinRAR, for unzipping files, you can get the schematics for your TV from here Like I said earlier tho, it may be the cap that is supplying the power to the vertical IC, which may not be close to the IC at all, but near the LOPT transformer ( the one that has the high voltage lead going to the CRT ) Do be careful in there...
Dude! You rule! Thanks, I'll get on it then. I'm not exactly the most organised guy in the world, but I shall report back with my (no doubt slow) progress. Cheers! Rich :¬)
Since this will be the first time I'll actually be touching components in a CRT (my experience has been limited to tweaking up until now) am I right in thinking that if I establish that a capacitor has no residual charge with my multimeter before I start working on it I'm likely to be safe? What sort of voltage is there around the vertical IC? I'll be sure to disconnect if from the mains before doing anything obviously. Rich :¬)
Yes, god idea to verify the caps have discharged before you touch anything. Here is some reading: link about discharging caps. Here is a link to the main site, and if you dig a bit, you can find how to discharge the picture tube ( CRT ) which holds one of the biggest charges in the TV. repairfaq link read the warnings and be careful!
