component testing...please help

mudhens1

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just need coached on the basics....resistors, diodes and transistors....can they be checked while attached to the board? my vectorbeam chassis used to work, now it doesn't....about 1/2 the diodes test bad...i desoldered one leg of the first one i was going to change, then tested it...it tested ok
which components can be tested without removing them first
thanks for the help
 
You can test all parts in the board, but you won't always get a correct reading. It all depends on the circuit they are in. Sometimes a part is in a parallel circuit, so you are reading an average of all the parts in that circuit, or whatever is the lowest-resistance part in the circuit. Other times the part is in a series circuit, and you only read that part.

Sometimes - like you experienced with that diode - you get false readings through transformers or low-value resistors that are in parallel. This is why you should check it while installed. If you get a reading you don't expect from the schematic, pull it and test it again...
 
Most of the time resistors can be checked in circuit. Diodes can alot of times, about half the time you will get false readings like its shorted. I just pop one leg out and retest.
Trasistors are tested pretty much like diodes but you have to know what to expect. If I suspect a bad transistor I usually test it out of circuit.
When in doubt, play around with taking readings from some known good components.
 
thanks for the insight guys...how about a little ohm's law help then...i have a 100uf 2watt resistor...while the board is powered up i get 25v on one leg and about 5v on the other...seems like alot of voltage drop to me...thanks in advance for the calculation :)
 
thanks for the insight guys...how about a little ohm's law help then...i have a 100uf 2watt resistor...while the board is powered up i get 25v on one leg and about 5v on the other...seems like alot of voltage drop to me...thanks in advance for the calculation :)

Well, that's not really possible to do with the information you have given. You'd have to know the TOTAL RESISTANCE of the entire circuit that part is located in.

Check this out:

How to calculate voltage drop across series resistors.

Step 1: Determine Current of total resistors across the series.
(I = Current/V=Voltage/R(t)=Resistance Total)
I = V / R(t)
I = 9 / (2k + 5k + 10K)
I = .53 mA

Step 2: Now that we have the Current across the resistors we can calculate the voltage across EACH resistor.
I = Current
Vx=Voltage (x=Current Value across X Resistor)
Rx=Resistor value(x=resistor#)

V = I x Rx

V1 = .53 mA x 2k
V1 = 1.06V

V2 = .53 mA x 5k
V2 = 2.65

V3 = .53 mA x 10k
V3 = 5.3V

V1 + V2 + V3 = 9.01 (voltage is 9V only 9.01 because i rounded up from .529 to .53) At .529 you can see barely a difference:

V1=1.058V
V2=2.645V
V3=5.290V
V Total= 8.993V

Also If the resistors are all the same value its easier.

So just add them up:

I = V / R(t)
I = 9 / (5k + 5k + 5K)
I = .6 mA

THEN:


V = I x Rx

V1 = .6 mA x 5k
V1 = 3V
so since all Resistors are same value:
V2 = 3V
V3 = 3V

Also To get different values for like switches just use gnd as a start point and the end of each resistor and a end point. This way you get 3 switches from this each having different voltages

SW1 = 3V
SW2 = 6V
SW3 = 9V

Resistors that are in parallel will all have the same voltage drop...
 
wow!!! it's a little too late tonight for me to check it out on the chassis now...guess i'm going to have to take a course in this stuff...learning to read schematics better will help too...everything seems so circular....this resistor is connected to that transistor, that then turns on the other transistor....rectification, voltage drop etc. i'm familiar with all the terms, just not sure how to use them in practical application

i guess i'm fairly intelligent (i'm a doctor) but my brain needs a little rewiring to understand how all these parts inter-relate...i understand them individually, just not all in a circuit together...if i don't have the board powered up, why would it matter if other things were in the circuit if i'm testing an individual component...those are the concepts (laws) that i can't seem to get hold of

thanks again for your wisdom :)
 
i have a 100uf 2watt resistor...

Not possible... Resistors are not measured in uF.

i'm familiar with all the terms, just not sure how to use them in practical application

i guess i'm fairly intelligent (i'm a doctor) but my brain needs a little rewiring to understand how all these parts inter-relate...i understand them individually, just not all in a circuit together...if i don't have the board powered up, why would it matter if other things were in the circuit if i'm testing an individual component...

Can you test the heart all by itself?
 
For resistors, diodes, and transistors... is much easier to just pull them then test them. That way you don't have to worry about tracing the schematic.

For resistors and diodes, just remove a leg from the board, then test.

For resistors, check the value you're seeing on your DMM against what the color code on the resistor says.

0 Black (Bad)
1 Brown (Boys)
2 Red (Rape)
3 Orange (Our)
4 Yellow (Young)
5 Green (Girls)
6 Blue (But)
7 Violet (Violet)
8 Grey (Goes)
9 White (Willingly)

At least thats how I was taught it... when I was 8... ... I've spent the better part of my life knowing Violet was a whore.

For diodes, lift a leg. Run a diode test, then swap the probes and check again. If it doesn't beep (my DMM beeps... most do) for either check, then it's open. If it beeps for both checks, then it's shorted (common cause of "when I turn on the monitor, the fuse blows!" problems)

For transistors... it's a little different. You have to know whether it's a NPN or PNP. A transistor can pretty much be tested as if it were a set of diodes...

For transistors (if you remove them when testing) make sure you put them back in the exact same way they were in when you removed them. For diodes, if they test bad and you have to replace them, make sure they go in the same was as the one you removed (will have a black or white line on one end to remember)... for resistors... they're like Violet, it'll go along with however you put it in... sluts.
 
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