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1994 Panasonic CT 27SF31S - Image suddenly shrunk to about a third of the TV screen's height

CT27SF31S

New Member
Hello,

I'm brand new here, so please let me know if I'm posting incorrectly or in the wrong section.

I need help diagnosing a sudden picture issue with my 1994 Panasonic CT 27SF31S. I've used it almost everyday for watching TV, watching videos, and gaming since I bought it, and I've never had a problem with it. The other day while I was watching TV, the screen shrunk vertically from both the top and bottom in slow motion and came to a stop when it was about the height of the middle third of the TV. Further, the edges of the image were 'rounded', meaning that the image appeared slightly tubular at the top and bottom. The image shrunk equally and none of it was cut off. I know CRTs can be dangerous to mess with, so I let it sit unplugged for a few days in order to take a look inside (although I know that it still might have a built-up charge), and, unfortunately, I hadn't gotten a good photo of the issue before I did. I know that makes things difficult, but it looked a bit less dramatic then if someone had wrapped the image around the visible side of the codex from the Da Vinci Code, if that helps paint a picture of what I mean.
Also, I wouldn't describe the issue as the black bars on the top and bottom growing larger, because I believe the original black widescreen bars have shrunk in proportion with the rest of the image. I believe this because I can see them as a slightly grayer color than the true black now occupying the top and bottom thirds of the screen. I know for a fact it's the TV and not the Verizon box, because the TV controls (volume and the menu, etc.) also show up within this distortion, as does the independently attached video player.

One additional piece of information - I don't know if this is related, but before all this I had noticed a few times when the image was sort of "bouncing" or "breathing" to life after I turned the TV on, instead of just - turning on. When I say the image "bounces" or "breathes", I don't mean that the entire picture bounces around within the frame. The image filled up the screen normally (prior to this sudden height issue) and remained centered throughout, but it's size would be rapidly stretched and then shrunk over and over around the edges until finally settling down to it's normal size. This "bouncing" effect would last maybe 10 to 20 seconds. I had thought this was actually an issue with the Verizon box or the signal to it (I've had reception issues with Verizon before, which had caused other glitches with the display), but now that the TV is having this issue with image height, I'm not so sure anymore.

Any ideas on what specifically is causing the problem?
 

ramondo

Member
Most likely electrolytic capacitors around VERT OUT IC,, If you are comfortable inside with power on you might try using coolant spray on selected caps and observe the resultant display. Spray one cap at a time and limit overspray. Could also be the IC itself. Look online for service manual for your chassis or one using the same circuits. If you have Vert size and/or Vert lin adjustment pots they may need cleaned with contact cleaner.
 

unclejed613

Well-Known Member
Most Helpful Member
the screen shrunk vertically from both the top and bottom in slow motion and came to a stop when it was about the height of the middle third of the TV. Further, the edges of the image were 'rounded', meaning that the image appeared slightly tubular at the top and bottom.
you had one of the electrolytic caps associated with the vertical output IC fail. since that part of the circuit board on most TVs tends to run a bit hot, i would replace all of the electrolytic caps in that area of the board. since you saw the failure happen in real time, i suspect you will find an electrolytic cap that's bulged out or the top bulged with little wisps of paper fiber sticking out the top.

The image filled up the screen normally (prior to this sudden height issue) and remained centered throughout, but it's size would be rapidly stretched and then shrunk over and over around the edges until finally settling down to it's normal size. This "bouncing" effect would last maybe 10 to 20 seconds.
this sounds like the high voltage regulation starting to act up. most TVs after about the year 1985 derive their power supply voltages from the flyback transformer. the method of regulating the high voltage is by controlling the pulse width going to the horizontal output transistor. a winding on the flyback feeds a diode and capacitor that generates a low voltage (usually around 12V) that is proportional to the high voltage going to the CRT. this low volltage is sent back to the horizontal oscillator as a feedback signal. the horizontal oscillator modulates the pulse width going to the horizontal driver, which then drives the horizontal output transistor. at least that's how it usually works. i've seen other methods used. if anything in the feedback loop is failing (like getting out of spec at room temperature but it's ok after it heats up) it will not provide proper feedback.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
you had one of the electrolytic caps associated with the vertical output IC fail. since that part of the circuit board on most TVs tends to run a bit hot, i would replace all of the electrolytic caps in that area of the board. since you saw the failure happen in real time, i suspect you will find an electrolytic cap that's bulged out or the top bulged with little wisps of paper fiber sticking out the top.


this sounds like the high voltage regulation starting to act up. most TVs after about the year 1985 derive their power supply voltages from the flyback transformer. the method of regulating the high voltage is by controlling the pulse width going to the horizontal output transistor. a winding on the flyback feeds a diode and capacitor that generates a low voltage (usually around 12V) that is proportional to the high voltage going to the CRT. this low volltage is sent back to the horizontal oscillator as a feedback signal. the horizontal oscillator modulates the pulse width going to the horizontal driver, which then drives the horizontal output transistor. at least that's how it usually works. i've seen other methods used. if anything in the feedback loop is failing (like getting out of spec at room temperature but it's ok after it heats up) it will not provide proper feedback.
I would suggest well before 1985 - however, I've never heard any suggestion that the supplies off the LOPTX are regulated in any way, are you perhaps confused by the flywheel sync circuit where AC is fed back from LOPTX to line oscillator?. Another source of potential confusion is the over-voltage protection (called x-ray protection in Japanese circuits), which is a monitored DC feedback that shuts everything down if it gets too high.
 

unclejed613

Well-Known Member
Most Helpful Member
I've never heard any suggestion that the supplies off the LOPTX are regulated in any way,
maybe i'm thinking of the HVREG circuit i saw in NEC Multisync monitors, where there was a series regulator transistor used in the +200V rail feeding the primary of the FBT. i do remember a test pattern used to detect HV regulation problems, where the center of the screen (about 50% of the screen area) would alternate white and black, then the rest of the screen was black, except for a white border. if the border moved in and out with the flashing of the center, it indicated a problem with the HVREG circuit. some cheap monitors did that anyway to some extent, but for more expensive monitors like NEC and IBM and Mitsubishi, it was something that shouldn't happen.
 
Last edited:

augustinetez

Active Member
Been a long time since I was poking my head inside TV's, but from memory, the vertical output circuit derives it's supply line from the flyback transformer and one of the electrolytic caps on that line is probably faulty.
 

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