This involves an old TV which uses a full wave bridge rectifier and filter capacitor
with that voltage going into the STR30130 regulator. It is supposed to regulate to
130vdc. The TV has a protect mode where if the DC rises too high, the set loses
horz. syc and the picture may go to a blue screen which is supposed to encourage
the viewer to turn off the TV.

I'm measuring around 150vdc as the output of this 130volt regulator which is causing
the protect mode to take over. If I recall correctly, the input to the regulator is running
around 160vdc rather than the 140 showing on the schematic. Also the p-p ripple on
input side is maybe 5 volts if I'm reading my scope right but the output of the 130
volt line is clean and flat.... something good going on there to get rid of the ripple but
it is not holding to the 130volts.

If I use a Variac and bring the AC line down to about 115-118 VAC, the symptoms
immediately clear up..... so I can run the line voltage up and down and the symptom
will come and go.

I have put in a brand new regulator but hasn't changed anything... relatively few
related parts and all seem to check OK.

Sooo what is going on here? I have yet another STR30130 regulator on order
but I'm afraid it may not be the solution.

Thanks!

 

This involves an old TV which uses a full wave bridge rectifier and filter capacitor
with that voltage going into the STR30130 regulator. It is supposed to regulate to
130vdc. The TV has a protect mode where if the DC rises too high, the set loses
horz. syc and the picture may go to a blue screen which is supposed to encourage
the viewer to turn off the TV.

I'm measuring around 150vdc as the output of this 130volt regulator which is causing
the protect mode to take over. If I recall correctly, the input to the regulator is running
around 160vdc rather than the 140 showing on the schematic. Also the p-p ripple on
input side is maybe 5 volts if I'm reading my scope right but the output of the 130
volt line is clean and flat.... something good going on there to get rid of the ripple but
it is not holding to the 130volts.

If I use a Variac and bring the AC line down to about 115-118 VAC, the symptoms
immediately clear up..... so I can run the line voltage up and down and the symptom
will come and go.

I have put in a brand new regulator but hasn't changed anything... relatively few
related parts and all seem to check OK.

Sooo what is going on here? I have yet another STR30130 regulator on order
but I'm afraid it may not be the solution.

Thanks!

Go round the PSU area and check any high value resistors - you might get lucky and that chip has external voltage sense divider.

There's probably a lowish electrolytic nearby that holds the voltage sense level - low cap or high ESR can cause trouble.

 

Do you have a schematic of this TV to share?

 

It is this make and model Panasonic TC L55E50
I am not the best at handling files but hope the
file below will be accessable.
The regulator is at lower center and right area
of the diagram.
Hope this helps

 

It is this make and model Panasonic TC L55E50
I am not the best at handling files but hope the
file below will be accessable.
The regulator is at lower center and right area
of the diagram.
Hope this helps

Its a basic integrated linear regulator, there is a voltage sense divider just as I said - check those resistors first.

 

The regulator has a resistor connected between input and output so if the set is drawing too little current then the regulator output voltage may rise. Using the variac to get the lower input voltage (the maximum voltage at which the set will work), does the set work normally, and what is the output voltage of the regulator?

 

Its a basic integrated linear regulator, there is a voltage sense divider just as I said - check those resistors first.

But the voltage sensing circuit is integrated inside the chip - and the chip has been changed.

 

But the voltage sensing circuit is integrated inside the chip - and the chip has been changed.

You might want to take a look what R802, 805 and 813 do.

 

If I run the variac at about 115 - 117VAC the set works normally and the regulator
output is at 130vdc. So since the regular AC line reads closer to 120 vac, there isn't
too much lower at which it will work fine....just is not keeping the regulated voltage
down in the 130 range. I think it rises to about 133-135 when adjusting the variac
at which point it goes into protect mode.

If you just plug right into AC line voltage, the regulator is running nearly 150vdc ouput,

 

If I run the variac at about 115 - 117VAC the set works normally and the regulator
output is at 130vdc. So since the regular AC line reads closer to 120 vac, there isn't
too much lower at which it will work fine....just is not keeping the regulated voltage
down in the 130 range. I think it rises to about 133-135 when adjusting the variac
at which point it goes into protect mode.

If you just plug right into AC line voltage, the regulator is running nearly 150vdc ouput,

R802 is probably open - that external divider influences the internal stabiliser. I suspect that might serve as a fail safe in case R802 fails open - the regulator would simply cease to regulate and pass the full 160V straight through.

 

I checked all resistors with one leg removed from board and all check fine and none
look overheated. The cap ESR is about 0.5 and not bulged. Can't seem to find
anything out of order but still won't regulate as it should.... when I first put the
new regulator in, I had the HOT transistor out of circuit just to see what voltages
would be prior to putting in a new HOT that was found shorted originally.
Could that, with not enough load, cause damage to the STR? Yet there was no
heat and time powered up was very short.

 

Just double checked R802 out of circuit and against a new one (10K) just to be sure
I'm reading meter right and that it isn't giving false readings.

 

You might want to take a look what R802, 805 and 813 do.

I did. They just supply the bias for the series pass transistor. They do not control the output voltage.

 

R802 is probably open - that external divider influences the internal stabiliser. I suspect that might serve as a fail safe in case R802 fails open - the regulator would simply cease to regulate and pass the full 160V straight through.

If R802 was open the series pass element would have no bias and wouldn't conduct at all. The output voltage would be zero.

 

I checked all resistors with one leg removed from board and all check fine

Does that include R804?
If that was lower resistance it might cause the problem.

 

Yes that includes R804.... all the resistors on the board. Darn, just got the mail and
my second new STK didn't arrive so can't sub that.

Golly, this must have worked
right at one time... what could we be missing if every darn part in question seems OK.
I see I did make an error in my first post on saying the schematic shows the first B+ is 140vdc,
it does show 160vdc so that is normal when I check it.

Sure hoping for a solution but seems very elusive at this point.

 

Tried some experimenting..... replaced C806 with no improvement....even tired higher
value. Experimented with sizes of the two high watt resistors, R803,R804 with no
change. I increased as well as decreased their resistance values with no help to
the problem. Also jumped in another filter cap across C805 with no help.

I notice if I run the variac down to a 2-3 volts below line voltage, it will work and that
keeps the 130 at less than 140vdc. As soon as you increase the variac to about
122-123 vac, the problem shows up.
Just don't understand why the regulator is now hold to 130-135 where it works well.
Last hope for now is maybe that new regulator on it's way will work properly.

 

You could usefully measure the voltage on pin 2 and pin 4 of the regulator. If the regulator thinks it is regulating then pin 2 will be 1V - 2V above pin 4. If it is out of regulation then pin 2 voltage will be below that of pin 4.

 

If F802 had blown there woulf be no load on the regulator circuit so the output voltage would be about the same as the input voltage because of R804 (In this situation the regulator chip would have no control of the output voltage.) Here is a link to that datasheet on the STR30130 to help understand how it works.

Les.

 

If F802 had blown there woulf be no load on the regulator circuit so the output voltage would be about the same as the input voltage because of R804 (In this situation the regulator chip would have no control of the output voltage.) Here is a link to that datasheet on the STR30130 to help understand how it works.

Les.

Except that if that fuse had blown then the TV would not work whatever the input voltage was but it does work at lower input voltages.