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UPDATE: Hitachi 50EX20B SMPS shutdowns on brights

Just finished checking the shutdown circuits, this is not the problem I'm looking at, voltages and overcurrent trip circuits are not tripping.

Shutdown circuit is kicked every time user menu contrast control is increased while displaying white raster or any bright pic. Also the picture starts "inflates" around 2 clicks past midrange on contrast diaplay. In other word; pic inflates in step with each increment on contrast control, below, steady pic, no inflating or uninflating.

By default for contrast is 100%, had to decrease it to 60% or less.

The clue was that last page in PDF document for adding CRT protection trip circuit made me look at this problem differently.

I need schematics for the CRT over-load beam circuits and setting up beam current aka screen adjustments instructions. (AKB) This is via DN10 diode that hooks into shutdown circuit.

What voltage I need to measure on what test point when I adjust screen for all 3 CRTs to get them aligned and prevent tripping?

Oh, by the way, what's the message for error code 31?

Thanks, Happy New Year! Cheers, Wizard

Reply to
Jason D.
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Which shutdown circuit is turning the set off?

Reply to
dkuhajda

To ones who are not famillar with this Hitachi's design, I find this nice that this doesn't blow up SMPS and easier to track problem down, just with scope and voltage meter plus 4 green LEDs on 4 of voltage outputs.

The core shut down circuit is frightfully simple: small SCR hooked to the low voltage via resistor to limit current via SCR to ground feeding to the protection shutdown optisolator via this on hot side to the oscillator pin on STR6511, red LED is hooked to this IC's osc pin also for visual status: dark: no power or no oscillation, half brightness is normal (slight brightness changes is OK), full brightness is tripped condition via that SCR.

Ok, the trip circuits are monitoring the voltage outputs for loss using diodes, over current circuits are done with transistors across low value resistors in series, voltage rises with current increase, if high enough breaks through one of those zeners and SCR latches. Oh, Scan loss, vertical problems is included. Short and voltage outputs loss via diodes.

All this goes right to the one SCR.

IMHO, this is what should be this way in other makes but thanks to the young MDA in business people, left this features out. :P

Even I wish RCA used this concept to stop the SMPS oscillation instead of trying to driving into shorted load and blow up like CTC203 does.

Cheers, Wizard

Reply to
Jason D.

Whoops, to answer your question, not sure, this is reason I need schematic from the CRT that are hooked into this shutdown circuit. This is what I was asking about in first place when I posted update article.

Cheers, Wizard

Reply to
Jason D.

Log into Hitachi and download the service manual for this model. Any servicer can get Hitachi service information online.

To answer how to find out which circuit to look at. Since the problem is not a scan loss situation, you can temporarily disable each shutdown circuit with a jumper wire to ground before the final diode that ties into the scr circuit. When you have stopped the shutdown condition when turning up the contrast, then you know to work backwards from there to the offending circuit.

In cases of not knowing 100% for sure that it is not a scan loss causing shutdown, then turning down the G2 voltages (being careful to mark the exact position first) can prevent phosphor burn, or disconnecting the crt sockets and connecting the test sockets to ground on all three crts.

Reply to
dkuhajda

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