STR53041 voltage regulator - how to test?

On 17 Oct 2005 07:05:56 -0700, snipped-for-privacy@cox.net put finger to keyboard and composed:

IME the most common failures in these types of regulators are CE shorts in the output transistors.

Test for a SC between pins 2&4 and 3&4. You should measure two diode drops in one direction, and OC in the other. If you have a zener tester, then do a zener test between pins 5 and 1.

-- Franc Zabkar

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You need to start by posting the complete model number and year of manufacture. In Zenith sets depending on the year and picture tube in use, the picture tube will short and cause catastrophic failure of the power supply components, in other years there are simply two high esr capacitors which will cause a dead set.

Odds are since you did not specify any burnt resistors, blown fuse or transistors, that it is the one with the bad capacitors.

As to testing it, you need to verify that it has the main b+, start up voltage, starts to oscillate, and continues to oscillate and regulate in circuit. If it shorts the fuse will be blown.

David

I'm not at home, so I don't have the model # with me, but the main fuse was found blown. I replaced it, but nothing happened at power up. I'll check again, but I'm pretty sure I found B+ on the regulator, and primary of the switching transformer as said. I'm assuming it isn't oscillating since there's nothing on the secondary. Am I thinking about this logically? This may be a no-brainer question for most of you, but I have a new Fluke meter with frequency measurement that I haven't used yet, just got it. Is this how I would check to see if the regulator is oscillating? Thanks to all that have replied. Great info.

On 18 Oct 2005 03:13:58 -0700, snipped-for-privacy@cox.net put finger to keyboard and composed:

Check whether the startup resistor, Rs, (page 3 of the datasheet) is OC:

The datasheet also shows that a winding of the flyback transformer is used to synchronise the PSU's oscillator to the horizontal frequency. Is the FBT OK? Have you checked the HOT?

-- Franc Zabkar

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I checked page 3 of data sheet, but not sure what the startup resistor, R's is. Page 3 was a test circuit. I have checked pin 1 and 5 and have resistance. I also checked between pins 2/4, 3/4 as you suggested. Pin

2/4 seems to be OK, Pin 3/4 is infinite resistance both directions. Since pin 3 is the input, I am again assuming this IC is bad. I checked the HOT when I first opened the set and it's good. I haven't checked the flyback yet. The model # of the set is C25A24T. Would you agree with my findings? Thanks alot.

Checking the resistance of pins on the IC won't do you a whole lot of good, check for voltage at them and compare it to what should be there.

On 18 Oct 2005 15:20:36 -0700, "Golf" put finger to keyboard and composed:

One end is connected to the + terminal of the main filter capacitor (200V or 400V), the other end connects to pin 2 of the regulator. This resistor kickstarts the oscillator. Take care to discharge the main filter cap when probing in this area.

I'm a little confused by the equivalent circuit on page 1. It looks like the emitter of the PNP transistor should be connected to the base of the NPN transistor, but the drawing is unclear. My initial post assumed there was no connection. If the two *are* connected, as is suggested by the application circuit, then pins 2,3,4 would the BCE of an NPN transistor, and you would then test it in the normal way.

These sorts of faults are usually diagnosed by disconnecting the line stage and substituting a dummy load. I use a 240V 100W lamp. Unfortunately your test circuit gets its feedback from the FBT, so I don't know how it will run without it. The supply may just squeal, or it may not oscillate. Hopefully nothing will break.

-- Franc Zabkar

Please remove one 'i' from my address when replying by email.