Anyone help with this ?

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Classic
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Anyone help with this ?

Postby Classic » 14 Jun 2017, 14:11

I have just installed a new irf520 transistor. When i key up the power starts to drop as the transistor heats i mean like with in 20 seconds it goes from 15 watts to under 10 watts is that normal ?

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listner132
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Re: Anyone help with this ?

Postby listner132 » 14 Jun 2017, 14:23

What radio have you fitted the 520 in too ?

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Re: Anyone help with this ?

Postby Classic » 14 Jun 2017, 14:37

listner132 wrote:What radio have you fitted the 520 in too ?

Hi it's in a amp rm 35

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Black Spirit
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Re: Anyone help with this ?

Postby Black Spirit » 14 Jun 2017, 14:56

How much power are you putting into the amp ?, It must be kept below 4w, Also the Mosfet will burn up quickly if the gate voltage is too high [over 4v].

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Re: Anyone help with this ?

Postby Classic » 14 Jun 2017, 15:09

Black Spirit wrote:How much power are you putting into the amp ?, It must be kept below 4w, Also the Mosfet will burn up quickly if the gate voltage is too high [over 4v].

3 watts i've even tried 0.5 watts and it still does the same thing.

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Re: Anyone help with this ?

Postby Classic » 14 Jun 2017, 18:35

Black Spirit wrote: Also the Mosfet will burn up quickly if the gate voltage is too high [over 4v].
In transmit ?

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Re: Anyone help with this ?

Postby Black Spirit » 14 Jun 2017, 19:31

Classic wrote:
Black Spirit wrote: Also the Mosfet will burn up quickly if the gate voltage is too high [over 4v].
In transmit ?


Yes

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Re: Anyone help with this ?

Postby bigal68 » 15 Jun 2017, 19:42

Nick's bang on.... gate voltage is critical on mosfets, as is keeping it stable....
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Re: Anyone help with this ?

Postby Classic » 19 Jun 2017, 10:39

Black Spirit wrote:
Classic wrote:
Black Spirit wrote: Also the Mosfet will burn up quickly if the gate voltage is too high [over 4v].
In transmit ?


Yes

Tested and the meter goes off the scale. Also when i tried the amp on a switch mode psu as soon asi keyed up it blow up. Now that has haqppened 2 times. It don't happen on the old psu which isn't a switch mode.

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Re: Anyone help with this ?

Postby 2E1IIP » 19 Jun 2017, 11:49

Do you have a schematic for the amplifier?
What was the original transistor - is this a "mod"?
Why an IRF520? They are notoriously unstable for RF applications above 7MHz.
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Re: Anyone help with this ?

Postby Classic » 19 Jun 2017, 12:17

2E1IIP wrote:Do you have a schematic for the amplifier?
What was the original transistor - is this a "mod"?
Why an IRF520? They are notoriously unstable for RF applications above 7MHz.

I was told i could use it in this amp no problem.
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Re: Anyone help with this ?

Postby 2E1IIP » 19 Jun 2017, 12:24

Ok thanks I will look over it, but at a glance the gate voltage should never exceed 20V as it clamped with a zenner.
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Re: Anyone help with this ?

Postby Classic » 20 Jun 2017, 16:36

2E1IIP wrote:Ok thanks I will look over it, but at a glance the gate voltage should never exceed 20V as it clamped with a zenner.
So from the diagram is there anything i can change to get the gate voltage to 4 volts as black spirit recommended ?

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Re: Anyone help with this ?

Postby 2E1IIP » 20 Jun 2017, 17:25

Classic wrote:
2E1IIP wrote:Ok thanks I will look over it, but at a glance the gate voltage should never exceed 20V as it clamped with a zenner.
So from the diagram is there anything i can change to get the gate voltage to 4 volts as black spirit recommended ?

The MOSFET Gate is tolerant to +/-20V, the gate is clamped to a maximum of 20V with the zenner diode - I am not sure why it is failing - I would try a different device - is it genuine or a Chinese import?
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Re: Anyone help with this ?

Postby 2E1IIP » 20 Jun 2017, 17:27

Also check the gate voltage with a DVM and an FM carrier not exceeding 5W, check the diodes and the zenner diode to be sure.
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