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Thread: FT-450D finals test

  1. #1

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    Default FT-450D finals test

    My uncle gave me his non working FT450D.
    He told me the finals are bad. He stated one of his HAM friends looked at it and said that was the case.
    It supposedly turns on.
    I see the finals only for about 40 before shipping.
    I am hoping to test the finals before blindly purchasing.
    Hoping to not have to take to a repair shop.
    I pulled the finals.
    RD100HHF1
    Is it possible to test with a multi-meter?
    With positive on Source and Negative on Drain using Diode/Ohm I get a reading on both finals of around .710
    No other combinations give a reading.

    Please note I am also looking for an Elmer also. Have my license for about 5 years now but only have a couple handsets in case SHTF: KD9FDW
    Last edited by pctech101; Fri 16th Oct 2020 at 12:08. Reason: Corrected the time I have been licensed

  2. #2

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    Mosfets can usually be tested with a multimeter by using the diode setting. You will first need to take a look at the datasheet https://www.mitsubishielectric.com/s.../rd100hhf1.pdf and identify the gate, drain and source pins ~ looks like you have done that already...

    Then, with the mosfet sitting on a nonconductive surface (and the meter in diode mode - not ohms), touch the negative probe to the source and touch the positive probe to the gate. This charges up the gate capacitance. Now, while continuing to hold the negative to the source, move the positive probe to the drain. You should see the meter show a low voltage reading that slowly climbs as the gate loses its charge. Then, to turn the device back off, leave the positive probe on the drain and take the negative off the source and touch it to the gate. Now the meter should read overload (no voltage) when the negative probe is returned to the source.

    Another test you can do to, and often easier when testing final mosfets is to see if there is continuity (the ohms setting) between the source and drain with, or after, the gate is shorted to the source. If there is no voltage stored on the gate and the device conducts, the mosfet is junk. When mosfets fry, they turn into resistors.
    Last edited by brandon lind; Wed 14th Oct 2020 at 20:20.

  3. #3

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    Quote Originally Posted by brandon lind View Post
    Then, with the device sitting on a nonconductive surface (and the meter in diode mode - not ohms), touch the negative probe to the source and touch the positive probe to the gate. This charges up the gate capacitance. Now, while continuing to hold the negative to the source, move the positive probe to the drain. You should see the meter show a low voltage reading that slowly climbs as the gate loses its charge.
    When I follow this exactly nothing show, just .0F
    But if I swap the black and red leads I do get a reading of of .550

    Quote Originally Posted by brandon lind View Post
    Then, to turn the device back off, leave the positive probe on the drain and take the negative off the source and touch it to the gate. Now the meter should read overload (no voltage) when the negative probe is returned to the source.
    I attempt the above and I get nothing to happen when the Black lead is touching Source. Only time any readings happen is if I put Red lead on Source.


    Quote Originally Posted by brandon lind View Post
    Another test you can do to, and often easier when testing final mosfets is to see if there is continuity (the ohms setting) between the source and drain with, or after, the gate is shorted to the source. If there is no voltage stored on the gate and the device conducts, the mosfet is junk. When mosfets fry, they turn into resistors.
    Am I to understand that shorting Gate to Source should remove the voltage on the Gate? And that if their is resistance (by getting a reading of OHMs) that it is junk?
    So I shorted Gate to Source and remove short.
    I have multi-meter set to OHMs.
    I touch leads to Drain & Source and I get a reading of 12.70 and it slowly declines

    I'm a little confused. Apologies but thanks for your words.

  4. #4

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    Quote Originally Posted by pctech101 View Post
    When I follow this exactly nothing show, just .0F
    My multimeter has 2 functions on the diode setting. By default it is measuring capacitance until I press the select button and change it to diode. Because I am unfamiliar with your particular meter (you should tell me the model), I am unsure if the F in .0F means Farad (telling me you have it in capacitor mode) or if it is some foreign abbreviation for overload.

    If your probes are plugged into the meter wrong (red in negative, black in positive) you could see this result. Check that.

    Quote Originally Posted by pctech101 View Post
    Am I to understand that shorting Gate to Source should remove the voltage on the Gate? And that if their is resistance (by getting a reading of OHMs) that it is junk?
    Yes. At 12ohm, that bugger is toast.

    Crash course in mosfets... MOSFET stands for Metal Oxide Semiconductor Field Effect Transistor. The metal oxide part means there is a microscopic layer of oxide insulating the gate from the rest of the device. There is no conductive path between the gate and the rest of the pins. Field Effect tells us that it is the electric field on this gate surface that influences the conductivity of the junction between the source and drain, but without directly connecting to it. It is similar to static electricity on a comb pushing or pulling on nearby pieces of paper, but in this case, the objects are charge carriers that affect the resistance of the path.

    What you have there is an enhancement mode mosfet, which means a + voltage on the gate "enhances" the conductivity of the source drain channel. In other words, think of the source drain path as having a switch between them that is open (non-conducting) with 0 gate voltage and closed (conducting) when the gate has a voltage higher than the source pin (there is a threshold value, im not talking about half a volt here, more like 2 to 4 volts). If the gate voltage is 0 with respect to the source, the source-drain channel should not conduct.
    Last edited by brandon lind; Thu 15th Oct 2020 at 18:09.

  5. #5

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    ...and be sure to wear a grounded wrist strap when putting the new mosfet in. That thin layer of oxide can be damaged by anything over (for that model) 50v. That means a static discharge from your finger so small you don't even notice will destroy the mosfet. And that isnt the only mosfet in that radio, there are much smaller ones! if you dont have one, touch something that is earth grounded nearby each time you handle the part or radio until the cover is back on. If your local humidity is really high, its not such a big deal, but if it is dry there, be very very careful!!!

  6. #6

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    I was testing it with a small digital radioshack multimeter (catalog# 22-820).
    It has a button to select diode, continuity w/sound and ohms.
    The first tests I was doing it in the diode mode.
    THe leads are soldered to the board. I'm confident they are correct.

    Quote Originally Posted by brandon lind View Post
    My multimeter has 2 functions on the diode setting. By default it is measuring capacitance until I press the select button and change it to diode. Because I am unfamiliar with your particular meter (you should tell me the model), I am unsure if the F in .0F means Farad (telling me you have it in capacitor mode) or if it is some foreign abbreviation for overload.

    If your probes are plugged into the meter wrong (red in negative, black in positive) you could see this result. Check that.



    Yes. At 12ohm, that bugger is toast.
    Thanks for the explanation ^^^^

    Quote Originally Posted by brandon lind View Post
    ...and be sure to wear a grounded wrist strap when putting the new mosfet in. That thin layer of oxide can be damaged by anything over (for that model) 50v. That means a static discharge from your finger so small you don't even notice will destroy the mosfet. And that isnt the only mosfet in that radio, there are much smaller ones! if you dont have one, touch something that is earth grounded nearby each time you handle the part or radio until the cover is back on. If your local humidity is really high, its not such a big deal, but if it is dry there, be very very careful!!!
    Thanks for the important tip.
    I'm very grateful for your time and words.

    I will order the finals after I get the money together.

    Many blessings upon you and yours!

  7. #7

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    The particular model meter you have can only check diodes up to 2v. Therefore, I doubt it would provide enough voltage to charge the gate capacitance enough to turn on the mosfet. Meters that run on a 9v battery are much better suited for testing mosfets using the diode setting. The results you got with the ohms setting having the gate shorted to source are just as informative. Hopefully you get that new mosfet in and the radio running. Just wanted to add this to explain the strange results you got using the diode setting.

    73! Brandon

  8. #8

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    Quote Originally Posted by brandon lind View Post
    The particular model meter you have can only check diodes up to 2v. Therefore, I doubt it would provide enough voltage to charge the gate capacitance enough to turn on the mosfet. Meters that run on a 9v battery are much better suited for testing mosfets using the diode setting. The results you got with the ohms setting having the gate shorted to source are just as informative. Hopefully you get that new mosfet in and the radio running. Just wanted to add this to explain the strange results you got using the diode setting.

    73! Brandon
    I'm grateful for your words! Have blessed days ahead.!

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