UPDATE: Hitachi 50EX20B SMPS shutdowns on brights

[Jason D.]

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

 

[[email protected]]

Which shutdown circuit is turning the set off?

 

[Jason D.]

Which shutdown circuit is turning the set off?

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.

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

 

[Jason D.]

Which shutdown circuit is turning the set off?

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

 

[[email protected]]

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.