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Old 10th Mar 2019, 12:15 am   #1
OldTechFan96
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Default Transistor Identification

I am having difficulty finding out what this transistor is. I think it is defective. It is from an Amiga SMPSU. It has the following markings:

S431
CZ
926
I think that the 'S' is the manufacturers logo.

When tested with my component tester it gives an odd result and shows that it is an NPN BJT. It shows a diode between C and E. I'm not sure if this is a sign of the transistor being defective.

Testing the transistor with diode check on my DMM also gives wrong looking results, as if it is leaky?

Does anybody recognise this transistor or know if it is faulty?

Thank you
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Old 10th Mar 2019, 12:29 am   #2
Philips210
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Default Re: Transistor Identification

Hi

I think it could be a voltage reference IC, possibly a TL431CZ.

Regards,
Symon.
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Old 10th Mar 2019, 12:52 am   #3
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Default Re: Transistor Identification

I would go with The voltage reference IC too.
In circuit it should be a component or two away from the opto coupler on the secondary side.
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Old 10th Mar 2019, 12:58 am   #4
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Default Re: Transistor Identification

I have a couple of TL431A voltage regulators and here is one on my component tester. My readings are quite different from yours.

The closest match I could find for a 926 transistor is below, but it is a PNP device.
https://www.radiomuseum.org/tubes/tube_2sb926.html
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Old 10th Mar 2019, 2:27 am   #5
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Default Re: Transistor Identification

I second the TL431 theory.

Can't think of a BJT failure mode that's going to show a reversed bias diode across C-E
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Old 10th Mar 2019, 6:17 am   #6
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Default Re: Transistor Identification

I'll go with 'TL431' too. It's a very common device to find in the voltage regulator section of small SMPSUs.
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Old 10th Mar 2019, 2:31 pm   #7
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Default Re: Transistor Identification

431CZ seems pretty clear to me.
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Old 10th Mar 2019, 5:41 pm   #8
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Default Re: Transistor Identification

Thanks for the information! It is near the opto coupler (which I think is suspect).

Since it is not a conventional transistor I won't expect it to test as one.
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Old 11th Mar 2019, 2:15 pm   #9
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Default Re: Transistor Identification

A little trick when using Google to try and find these obscure parts - add in to the search a feature of the device, as well as the numbers. For instance, in this case, the numbers just brought up jewelry! But S431 with the addition of TO92 for its package, instantly brought up the TL431 datasheet!
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Old 11th Mar 2019, 9:42 pm   #10
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Default Re: Transistor Identification

In case of doubt, it's good practice to replace both the 431 and the optocoupler as any fault therein could blow up the primary side of the supply or at least result in a severe overvoltage condition. Also measure the secundary diodes for shorts and opens, and the electrolytics for capacity and impedance.
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Old 11th Mar 2019, 10:25 pm   #11
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Default Re: Transistor Identification

Martin, thanks for the tip!

Maarten, I have tested nearly everything I can with my DMM and component tester. I found that the mains input filter choke had an open winding which was repaired with fine copper wire. I replaced the opto coupler and I am now getting 320Vdc across the filter capacitor on the primary side. Still nothing at the secondary side.

I think the switching transistor (BU508AF) could be suspect as it does get very hot. It tests good with my DMM so I suspect it might fail when high voltages are present.

I don't think anything is shorted as the PSU does not make the bulb in my lamp limiter to glow.
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Old 13th Mar 2019, 12:36 pm   #12
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Default Re: Transistor Identification

If the BU508AF has failed, I would suggest that you check all electrolytics for a satisfactorily low E.S.R., especially the ones on te primary side of the circuit.
In my experience, poor or dry P.C.B. joints on the chopper transformer pins are often the cause of tragic failures in S.M.P.S. Tony.
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Old 13th Mar 2019, 1:11 pm   #13
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Default Re: Transistor Identification

Assuming this is the same supply as yours, look for a couple of burnt out resistors as in the video (just don't get seasick watching it).

https://www.youtube.com/watch?v=poI-UAIgFwA
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Old 13th Mar 2019, 11:20 pm   #14
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Default Re: Transistor Identification

Thanks for the video Terry. The same thing happened to my PSU. The suspect part in the video was a BRX45 thyristor. I'll check it tomorrow. I'll order one since they are still available. It apparently is a common failure.

EDIT: Just checked the thyristor. It is open gate to anode!

Last edited by OldTechFan96; 13th Mar 2019 at 11:22 pm. Reason: More info
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Old 14th Mar 2019, 1:30 am   #15
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Default Re: Transistor Identification

As mentioned above, I would also do a check on all the capacitors, seeing as you have it in pieces.
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Old 14th Mar 2019, 10:39 am   #16
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Default Re: Transistor Identification

Surprisingly, they all test nearly bang on for capacitance and ESR is really low.
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Old 14th Mar 2019, 12:52 pm   #17
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Default Re: Transistor Identification

That's good to hear and surprising considering the age of the things now.
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Old 16th Mar 2019, 8:47 pm   #18
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Default Re: Transistor Identification

I replaced the thyristor and the PSU is now working.

Thanks for the help!
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