LA7838 Vertical IC and Mitsubishi CS-3535R

T o d d P a t t i s t · 2005-10-11T15:34:16+00:00

I'm at the end of a long repair of this Mitsubishi CS-3535R.I've replaced the switching regulator and related caps toget the primary side working, the 30 volt regulatortransistor and related zener with bias resistors that burnedup, a burned zener and bad cap near the "black levelcorrector IC," a couple of fuses and finally the verticaloutput IC - a Sanyo LA-7838 and all the caps in the verticalsection.The initial failure involved smoke and the repair was apain, as I smoked several components on the main board andsaw smoke once in the vicinity of the CRT neck. The lastbit of repair was the replacement of the LA-7838, which Ireplaced with an NTE sub.The picture currently has a slightly brighter horizontal barabout 20% down from the top. The scan lines are slightlycloser together there, and I suspect that it is the closerscan lines that make it appear brighter there, but it mayactually be brighter. The picture is slightly compressed atthe line, but looks OK above and below. The caps were allreplaced in the vertical section hoping to get rid of thevertical problem (which was not there prior to the powersupply problems.)I've gotten advice from some here whose advice I reallyrespect, and the consensus is that it's probably not thevertical IC, despite the fact that I used a sub. However,having replaced all the electrolytic caps in the verticalsection, I'm not sure where to go now. I'm going to put a scope on the output, and I expect I'llsee a kink in the ramp at the 20% point. I thought I'dscope the power to the vertical IC too, just to see if it'sfluctuating, but it goes through a pair of zeners and alarge cap, and the voltages look OK on the DMM.Anyone want to give some more advice?Thanks.The natural function of the wing is to soar upwards and carry that which isheavy up to the place where dwells the race of gods. More than any other thingthat pertains to the body it partakes of the nature of the divine.Plato, 'Phaedrus.'

Oct 11, 2005 21 Replies

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<eworld 20 years ago

Todd,

If you have not thoroughly checked every polar electrlolytic for leakage and the likely areas for ESR, you will be seeing this set back. We do them al the time and rarely see less than a couple of dozen leaky caps in this series of sets.

Leonard

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T o d d P a t t i s t 20 years ago

Thanks, Leonard. Everything was carefully inspected, electrolyte washed off where found, suspect caps replaced, and the SMPS power, vertical and horizontal sections checked with the ESR. I've got about 15 caps replaced, although 4-5 that were probably OK were pulled from the vertical section and replaced on general principles when I was floundering. I'd say 5 were definitely bad/leaking/bulging, and the rest just questionable/marginal. This sucker weighs in at a couple hundred pounds, so there's no way I want to have to open it again.

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