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Old 4th May 2011, 02:49 AM   #1
MinesJA is offline MinesJA  United States
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Default New To Multimeters

Hi Guys,

So let me first just say I'm new to electronics and so the answers to some of these questions might seem pretty obvious to some of you. But googling has only gotten me so far on some of these question so here I am. I've been trying to figure out what's wrong with my 7250PE receiver that I picked up at a thrift store for $30. Long story short, a couple people suggested the reason the speaker outputs and headphone out might not work (where as the inputs and tape out does work) is because one or more of the output transistors might have shorted.

I've started testing them with my multimeter and haven't been getting the readings I've expected to.

So I wanted to ask, can I test output transistors while they're still attached to the board and heatsink or does that give me bad readings?

Also, with the diode test of my multimeter, I measured the four 2SD1715 NPN transistors I was told to check first and I only get an open circuit when I connect the positive terminal to the collector and the negative to the base, and when I connect the positive to the collector and the negative to the emitter. All other times I get readings ranging from .559 to 1.223. From doing some light googling, I learned that I should be getting an open circuit reading most of the time. What do you think I'm doing wrong, or what do you think that means?

Thanks!
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Old 4th May 2011, 03:46 AM   #2
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Hi, If you are using a multimeter to test transistors you will need to remove them from the circuit first, other things connected to the transistor (eg capacitors or resistors) can throw out your measurements.

You can get in-circuit transistor testers, but they work because they are a bit more complex than a simple diode test that a multimeter can perform

Tony.
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Old 4th May 2011, 04:21 AM   #3
MinesJA is offline MinesJA  United States
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Aw, man. Alright, I guess I'll start then. Thanks!
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Old 4th May 2011, 04:34 AM   #4
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Originally Posted by wintermute View Post
Hi, If you are using a multimeter to test transistors you will need to remove them from the circuit first, other things connected to the transistor (eg capacitors or resistors) can throw out your measurements.

You can get in-circuit transistor testers, but they work because they are a bit more complex than a simple diode test that a multimeter can perform

Tony.
If they are shorted, you can tell with certainty they are bad while in circuit. To tell if they are good, then they need to be removed but unless you're using a curve tracer, you won't get much info from a meter. The meter gives you one point out of an infinite set. The curve tracer is far more telling.

G
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Old 4th May 2011, 04:39 AM   #5
MinesJA is offline MinesJA  United States
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Wait, how can you tell they're shorted while still in the circuit? What should I be looking for?
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Old 4th May 2011, 04:41 AM   #6
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if they are shorted, then you will see a zero resistance from collector to emitter, or base to collector, or base to emitter.

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Old 4th May 2011, 04:49 AM   #7
MinesJA is offline MinesJA  United States
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Ohhhhh, no kidding. Now, just to be clear, I'll see a zero resistance from collector to emitter while I have the multimeter set to diode test, right? Or should I be using the Ohm setting? I know these are really basic questions, but I really appreciate the patience. This is really helping.
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Old 4th May 2011, 05:00 AM   #8
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ohm setting will read zero (or close, ie less than 1ohm), diode test I think will read 0v drop (normally around 600mv if working), but try simply shorting the multimeter leads together in diode test mode to see what you get

Tony.
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Old 4th May 2011, 05:15 AM   #9
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If the transistor is shorted, u will get zero reading in both (ohm and diode test)settings.
Ohm setting is preferred for shorted transistor test.

Gajanan Phadte
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