Small signal NPN transistor 2SC2458GR or 2SC1740S-R - diyAudio
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Old 5th April 2012, 02:13 PM   #1
srinath is offline srinath  United States
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Default Small signal NPN transistor 2SC2458GR or 2SC1740S-R

I have a defective 2SC2458GR or 2SC1740S-R - its the Q921 in the power supply board in the 12v citcuit of an onkyo TX-DS555. I have 12v ac from the transformer, but not 12V at the coil of the electromagnetic reed relay. This is the only component in the middle.

Anyway its a small signal NPN transistor. Can I substitute any other small signal npn transistor in its place ? like this radioshack - 2N3904 NPN Small Signal Transistor ?

Thanks.
Srinath.
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Old 5th April 2012, 02:19 PM   #2
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Can you submit any part of the schematic ?

Generally I would say YES as the PSU should not be too specific, BUT, the PSU can be a High Gain amplifier that may be upset with the wrong choice of component.
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Old 5th April 2012, 02:56 PM   #3
srinath is offline srinath  United States
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Default Thanks for quick reply.

Schematic here.
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Old 5th April 2012, 03:04 PM   #4
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If the issue is to go with something readily available, then yes, you can go
with 2N3904 - nothing bad will happen. The only issue for you will be to twist
3904 legs a little.
Your two original parts have ECB pinout, and 3904 has EBC pinout.
Transistor is used here as a switch, and plenty of TO-92 parts with sufficiently
low saturation resistance, and beta of 180-390 will do.
And, you have to check D925 to make sure it gives you DMM diode reading
(without transistor in place) ~0.6V.
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Old 5th April 2012, 03:13 PM   #5
srinath is offline srinath  United States
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Wonderful. I will test the D925 diode - its part of that diode bridge right - I tested it yesterday they all seemed to have that 1 way thing right, however 2 tested .6 ohm and 2 tested 1.3 if I recall. I will test it without the transistor though, obviously once I saw the transistor could be bad and the diodes are less likely, I pulled it and started celebrating ... yea OK I am clueless.
Thanks.
Srinath.
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Old 5th April 2012, 03:20 PM   #6
srinath is offline srinath  United States
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Sorry I testd d921, 922, 923 and 924, d925 is not part of the diode bridge.
I will test it, but what you mean 0.6v, I thought we measure resistance in 1 direction and its supposed to be open and other direction have connectivity ?
Thanks.
Srinath.
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Old 5th April 2012, 03:23 PM   #7
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Well, this what I am looking at.
You will need that D925 installed in any case. Look at your board.
<<make sure it gives you DMM diode reading (without transistor in place) ~0.6V.>> You may need to unsolder and pull one end of D925 to avoid distorting your diode measurment with the coil resistance in parallel.
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Last edited by StevenOH; 5th April 2012 at 03:31 PM.
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Old 5th April 2012, 04:14 PM   #8
srinath is offline srinath  United States
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Default Schematic ?

What schematic is that, on my amp from q921 it goes onto the power pin of Q702 FL driver. However that d925 is there on my power supply board, so well, looks like your schematic is correct and whatever I pulled up as the TX-DS555 schematic isn't.
Thanks much, I will get that diode out and test it, and I have a few of those diodes I think, my 2325 has those that went. I got a pack of 10 or so.
Thanks.
Srinath.
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Old 5th April 2012, 04:18 PM   #9
srinath is offline srinath  United States
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Oooo would 13V @ that top pin in the "to NAAR-6332" indicate the transistor works and no 12V @RL901 indicate that diode is what is bad.
See I am a bit clueless, I have some solderign and desoldering ability and of course I can read a schematic but I dunno what each component does.
Thanks.

Srinath.
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Old 5th April 2012, 07:45 PM   #10
srinath is offline srinath  United States
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When 1 let is pulled off the board, on my dmm in the diode setting I read 633 on that diode in 1 dir, and open in the other.
When both legs are soldered in, I read 290 in the diode setting on my dmm.
That sounds good ?
Thanks.
Srinath.
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