help req please

Discussion in 'General Electronics Chat' started by mm6rck, Apr 6, 2012.

  1. mm6rck

    Thread Starter New Member

    Apr 6, 2012
    6
    0
    Hi guys,

    looking for a bit help please


    my friend has an rf amp that is a 12vdc input BUT he connected it into his truck that was 24vdc and guess what :D

    so he gave me it to check over i thought it maybe a simple fuse etc but looks like the fuse is fine

    if you test the 2 power leads on the test meter its showing a dead short

    any input on what i should first check

    this is a scematic of the amp in question
    http://www.rmitaly.com/download/manuals/KL203-manual_rel_500.pdf
    thanks for any input
     
  2. #12

    Expert

    Nov 30, 2010
    16,343
    6,828
    Disconnect the wire from S2 to the transformer to see if the MOSFETs are shorted.
     
  3. mm6rck

    Thread Starter New Member

    Apr 6, 2012
    6
    0
    Hi s2 is the on off switch for the amp if disconnected it no longer shorts ... but is this just not like turning it off ?

    thanks



    pics of actual amp/board


    [​IMG]


    [​IMG]
     
  4. #12

    Expert

    Nov 30, 2010
    16,343
    6,828
    Look at the schematic. Find the power line to the transformer. Isolate it and measure to see if the MOSFETs are shorted.
     
  5. mm6rck

    Thread Starter New Member

    Apr 6, 2012
    6
    0


    Thanks for the reply's

    i am in no position to read the schematic lol i am a motorsort engineer so hand me spanners and a welder and i will get it fixed :D but electronics etc i am ok but would not say good lol

    i have all the tools decent test meter power supply etc just not the know how of ciruits


    is there any way i can test the fets on the meter ?

    thanks again

    t
     
  6. mm6rck

    Thread Starter New Member

    Apr 6, 2012
    6
    0
    also diode 9 1n5400 shows contenuity across it ? should it ? i thought a diode only conducted one way


    thanks
     
  7. #12

    Expert

    Nov 30, 2010
    16,343
    6,828
    The measurement of D9 is irrelevant because it is just a measurement of all the possible shorts on the board.

    I already told you how to test the fets (post #2 and post #4). You said you couldn't do it. (post #5)

    No point in continuing if you can't unsolder even one wire.
     
  8. mm6rck

    Thread Starter New Member

    Apr 6, 2012
    6
    0
    Hi sorry BUT i just dont understand reading a diagram of electronics ...i can see 2 lines that come off the switch but when i look at the board i just get lost .. hence why i put a pic up of the actual board itself .... if i remove the blue wire that goes to t2 its still a dead short if thats the one you mean?

    i realy do appreciate you help and input and i am very sorry for sounding like a dummy

    t
     
  9. ssnyde

    New Member

    Apr 4, 2012
    19
    5
    Hi,

    Basically you have a short from VCC to GND, but you can't figure out where it is without isolating parts on the board. The fuse and switch are supposed to be a short, so basically you're looking at parts that are connected to the schematic net (wire) to the left of S2. If you wanted to find out if the MOSFET output stage had a short, you would need to isolate it either by disconnecting T2 center tap, or the MOSFET drains. The coil looks like a short at DC so we don't care which side we isolate. When you say you removed the blue wire, it's not enough information. There are two blue wires, the transformer has two coils. Where did you remove it from (picture would help)?

    T2 center tap is the metal pad to the left of T2 above D9, this is where you would need to remove the transformer connection. I figured this out by the board layout that you linked to and the schematic and the picture. I saw that the blue wire coming out the left goes to C15 and ground (the solid metal pour is usually ground). So this is not what we want to remove. Then I saw that the pads to the right of the transformer are connected to C14, so this is not the center tap. Process of elimination leaves the pad to the left of T2. Hopefully this explanation helps you to read schematics and layouts.

    I think it's more likely that C17 is short, it's a 16V aluminum electrolytic, these can fail short from overvoltage. Again, when you measure with an ohmmeter from VCC to ground your measuring everything that's connected in parallel, so to test if this is the failed part, you need to remove it from the board and measure it to see if it's short, or measure the board to see if the short has been removed.
     
  10. mm6rck

    Thread Starter New Member

    Apr 6, 2012
    6
    0
    Awsome response for mr novice :p here


    i lifted the metal tubes that hold the blue wire win the pic and the short has gone

    [​IMG]

    c17 is mesuring 45uf out of the board so put that back in

    even d9 is normal when the tubes are removed as in the pic


    so dose this mean 4 new fets?


    thanks again guys for the input
     
  11. ssnyde

    New Member

    Apr 4, 2012
    19
    5
    Maybe it means 4 new FET's, but maybe it means two, or one. It's not likely they all failed at the same time, and once one shorted it may have removed the overvoltage condition.

    First confirm the FET's are shorted by measuring resistance between the pad under where you lifted and ground, it should be close to 0. Then you can lift the other side of the transformer, to the left of C14, where there are two pads, and measure resistance between each pad and ground. This should tell you which of the two sets of four FET's are shorted. Then either replace both of those, or pull both off the board, measure them, and replace the failed one.

    But since you likely need to replace FET anyway, might be a good idea to replace them all. Don't know what the replacement part is though, the linked schematic doesn't seem to have a real part number on it.
     
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