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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets. |
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10th Mar 2019, 12:15 am | #1 |
Octode
Join Date: Jul 2017
Location: Newcastle Upon Tyne, Tyne & Wear, UK.
Posts: 1,177
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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 |
10th Mar 2019, 12:29 am | #2 |
Nonode
Join Date: May 2007
Location: Redruth, Cornwall, UK.
Posts: 2,562
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Re: Transistor Identification
Hi
I think it could be a voltage reference IC, possibly a TL431CZ. Regards, Symon. |
10th Mar 2019, 12:52 am | #3 |
Dekatron
Join Date: Apr 2012
Location: Worksop, Nottinghamshire, UK.
Posts: 5,549
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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. |
10th Mar 2019, 12:58 am | #4 |
Octode
Join Date: Feb 2018
Location: Southampton, Hampshire, UK.
Posts: 1,049
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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 Last edited by Jolly 7; 10th Mar 2019 at 1:23 am. |
10th Mar 2019, 2:27 am | #5 |
Nonode
Join Date: Oct 2010
Location: Tintinara, South Australia, Australia
Posts: 2,324
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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 |
10th Mar 2019, 6:17 am | #6 |
Dekatron
Join Date: Jun 2015
Location: Biggin Hill, London, UK.
Posts: 5,190
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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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10th Mar 2019, 2:31 pm | #7 |
Dekatron
Join Date: Dec 2007
Location: Haarlem, Netherlands
Posts: 4,184
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Re: Transistor Identification
431CZ seems pretty clear to me.
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10th Mar 2019, 5:41 pm | #8 |
Octode
Join Date: Jul 2017
Location: Newcastle Upon Tyne, Tyne & Wear, UK.
Posts: 1,177
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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. |
11th Mar 2019, 2:15 pm | #9 |
Heptode
Join Date: May 2008
Location: Selby, North Yorkshire, UK.
Posts: 949
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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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11th Mar 2019, 9:42 pm | #10 |
Dekatron
Join Date: Dec 2007
Location: Haarlem, Netherlands
Posts: 4,184
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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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11th Mar 2019, 10:25 pm | #11 |
Octode
Join Date: Jul 2017
Location: Newcastle Upon Tyne, Tyne & Wear, UK.
Posts: 1,177
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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. |
13th Mar 2019, 12:36 pm | #12 |
Rest in Peace
Join Date: Sep 2010
Location: Ripley, Derbyshire, UK.
Posts: 785
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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. |
13th Mar 2019, 1:11 pm | #13 |
Nonode
Join Date: Oct 2010
Location: Tintinara, South Australia, Australia
Posts: 2,324
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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 |
13th Mar 2019, 11:20 pm | #14 |
Octode
Join Date: Jul 2017
Location: Newcastle Upon Tyne, Tyne & Wear, UK.
Posts: 1,177
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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 |
14th Mar 2019, 1:30 am | #15 |
Nonode
Join Date: Oct 2010
Location: Tintinara, South Australia, Australia
Posts: 2,324
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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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14th Mar 2019, 10:39 am | #16 |
Octode
Join Date: Jul 2017
Location: Newcastle Upon Tyne, Tyne & Wear, UK.
Posts: 1,177
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Re: Transistor Identification
Surprisingly, they all test nearly bang on for capacitance and ESR is really low.
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14th Mar 2019, 12:52 pm | #17 |
Nonode
Join Date: Oct 2010
Location: Tintinara, South Australia, Australia
Posts: 2,324
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Re: Transistor Identification
That's good to hear and surprising considering the age of the things now.
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16th Mar 2019, 8:47 pm | #18 |
Octode
Join Date: Jul 2017
Location: Newcastle Upon Tyne, Tyne & Wear, UK.
Posts: 1,177
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Re: Transistor Identification
I replaced the thyristor and the PSU is now working.
Thanks for the help! |