Mitsubishi 35" TV Vertical Sync

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  #1  
Old 01-20-03, 04:18 PM
Teratum
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Question Mitsubishi 35" TV Vertical Sync

Hello:

I need some tips fixing my Mitsubishi 35" TV, model CS-35301. The vertical does not sync and therfore the picture seems to roll vertically pretty fast. However, I can still see that the horizontal is fine (when I run color bar pattern, I see correct colors on both sides without overlap.) It seems like when the picture scan from left to right, it's oK, but when it scan from top to bottom, it does not quite finish all the horizontal lines then the vertical frame starts again, so I see vertical overlapping picture.

I have purchased a service manual to take a look at. I scoped out the Vout (pin 6) and Hout (pin 13) signals of the IC201 (M51414BSP), they seems to be clean, right size, right shape, no jitters. I also scoped out pin 2 and pin 37 of IC401 (LA7878)vertical amplify, the signals are also clean. But somehow the Vert or the Hor timing are still off.

I remember the older TV has V hold pot for adjustment of the vertical freq to make it match the horizontal frames. How does newer TV sync the V and H timing without this V hold? Are they all done or controlled within the IC201?

I am appreciated any tips or hints you can give me. If I have to replace IC201 (M51414BSP) where do I get replacement part?

Thank you for you help.
Teratum
 
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  #2  
Old 01-26-03, 11:55 PM
Guest
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Teratum,
I had a similar problem with my tv, but it's amazing what an idiot and some spare time can figure out on his own

It may be simpler than you think, depending on how old your tv is. I have the model CS-27301 which should still have the same general layout.

There should be, note, Should be, 4 oscillators on the main board (little silver looking boxes) with flathead setscrews in the tops of them. One of them, most likely the larger one placed by itself is primarily responsible for vertical hold. Another is responsible for hor. hold, another for channel frequency, and to be honest, I'm not quite sure what the other small one is for . But the large one by itself controls vert hold. If you do decide to adjust it, take careful note of it's position, and how many full turns you make so you can take it back to it's default position if necessary.

Also, while it's unsafe, and I'm not telling anyone to do this, (I hold no liability for the following suggestion) it's easiest to monitor your results if you set a large mirror in front of the set while you adjust it. (that includes having the set plugged in and turned on so you can see the result) Adjusting the oscillators poses no threat. The possible injury comes from risk of electrical shock as one would have guessed, however reading your post, you seem to have enough common sense to avoid it.

I successfully fixed the vert. hold on my set, however in the process of finding the solution, I botched my color, brightness, and gain settings, so I've got to tinker w/those again.

Again, if you don't feel comfortable doing this, or are the least bit uneasy about possibly receiving a mild shock, don't do it.

Hopefully this helps you out.
~Lucas
 
  #3  
Old 01-28-03, 06:33 AM
morgan28476
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Very common problem, mentioned here before many times. Change all 10uf and 4.7uf minature aluminum eleectroltic capacitors on the PIP board. No adjustment is necessary. Be careful, some of these caps may have leaked and destroyed the foil beneath.
 
  #4  
Old 01-29-03, 11:41 AM
Teratum
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Thank You Morgan28476 and Webmaster for your comments.

My further investigation found that it's likely from the video circuit on the PIP board. Unfortunately, I lost my HV on the oscilloscope I was using, so I have to detour to fix my scope before I can go on any further. The electrolytic caps on the PIP board make a lot of sense.

Surprisingly that I found the Chip M51414BSP generates Vout and Hout pulses by itself. The Hout freq was 15.75Khz which is correct. The Vout was only 53 Hz, while it should be 60Hz (no V hold adj). I think the Video signal into this chip autamically makes the frequencies synchronize with the picture. I had a very weak Video signal coming into the chip at this moment. While it's supposed to be 1 Vp-p, it was measured less then 0.3V. The signal is from the video amps which consists of 2 transistors which were checked OK and from the matrix chip and the PIP board.

Noticed also that at weak reception, the picture does not roll. Strong signal not only roll the picture but also sometime cause picture to go B/W, which most likely some problem with the video. First I thought it could be the AGC in the IF amp section, but hooking the TV Video out to another known good TV gives perfect pictures which means the AGC and the IF are working correctly. So the problem is narrowed down to the PIP and the matrix chip at this moment. But then my scope blacked out!

There is a small chance that it could still be the M51414 chip. If the Video input of the chip was leaky or short, it can load the signal amplitude and that may be the reason why I did not get 1Vp-p signal and the sync did not lock.

I have just fixed my scope this morning after changing a HV zenor diode and 2 japanese PNP transistors. I'm back in business and the hunt in my Mitsubishi TV will go on.
 
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