Mitsubishi TV CS-27301 Roll

[Malnati]

I have a Mitsubishi Receiver Model # CS-27301. Recently the vertical framing began to roll intermittently and stabilized as the set warmed up. I checked and adjusted the AGC level, this was not the problem. I scoped the output of the "TV Signal Processor" IC200. The wavefrom Is normal in the negative direction, but there are 6 pulses going positive preceeding the vertical sync pulse. My guess is that they are serrated sync pulses from the composite video. These 6 pulses should have been combined/integrated by the vertical integrator and eliminated from the output. I then went looking for the integrator components (resistor and capacitor) of IC200, asssuming one of these values was drifting. The vertical sync separator has no obvious components to perform that function. I assume the internal IC design simulates the integrator function, saving a couple of components. "Big Deal".

Am I on the right track? I need help, point me in the right direction!! Is there a method to fix this problem without replacing IC200?

<small>[ February 09, 2006, 12:45 PM: Message edited by: Robert J. Malnati ]</small>

[Robert Reed]

If you had any vertical framing at all during warmup, it sounds like a vertical SYNC problem, The 6 pulses you see are called equalizing pulses (if memmory serves me right) and perform the vertical retrace function.These can be seen under certain fault conditions. I would be looking at incoming sync and sync separator circuits.Also if the vertical oscillator is too far off frequency it wont lock into sync. Haven't worked on any TVs for a while so this just a guess from my experience in the past, but I'll bet Edd Whatley would know.

<small>[ February 09, 2006, 04:09 PM: Message edited by: ROBERT REED ]</small>

[Ken1]

IC200 is probably good. I would check for a leaky electrolytic capacitor as it's by far the most likely component to cause the trouble. If you don't have a capacitor checker, look closely and if you are lucky you might find one with paste oozing out past it's leads. If so, replace it and it should cure the problem.

[Edd]

Well how about that,?.. a model and brand for reference.Veerrry nice.

As per your comment on your scoping and observation of the vert sync signal, I?ll just bet that you are observing the inverted and narrower companion equalizing pulses adjunct to the sides of the vert sync pulse.
NTSC Reference diagram:



Also I am "consuming" that you have already eliminated the possibility of these electrolytic caps in the vert and sync takeoff circuits as either having been changed or their being within acceptable ESR parameters ?
Tech Reference diagram:


And lastly , doesn't that chassis have Pic In Pic incorporation, and do you commonly use that feature or not ?

Standing by for informational feedback.

73's de Edd
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