Re: Unexpected Blowing of Fuses on Transmit

Jerry Gaffke

Here's an outline for a step by step procedure to find the fault.
Whether you choose to follow it or not, it should give you some idea of what is going on.

We've all been assuming it's around the IRF510's, since those are the only things that typically pull a few amps.
So disconnect the PA-PWR connector pin in from the power supply, does the fuse still blow when you press PTT?
I assume not.

Now reconnect PA-PWR, with a DVM inline, set to 10 Amps max.  A $2 Harbor Freight DVM will work fine.
Pull up one end of L8,  and also of L9.  So PA-PWR cannot get to the IRF510 drains.
Rotate RV2 and RV3 fully clockwise for 0v at the IRF510 gates (minimum is clockwise, backwards from most pots!)
With 0v on the gates, the IRF510's will not conduct.  They start conducting when there is 3 or 4 volts on the gates.
Apply power and press PTT, does it still blow? 
I assume not.

Should be just a few milliamps going into PA-PWR when you press PTT, I'd guess down around 10ma or so.
A very rough guess, I have not measured it.
But with the DVM at 10 Amps, you should see pretty much zero current on the meter.

Measure the voltage at the IRF 510 gates with PTT pressed, should be zero volts.
(Ideally this would be done with a second DVM, as we will soon need to measure PA-PWR amps again.)
Adjust pots RV2 and RV3, verify that the gate voltage on Q95 and Q94 respectively 
goes from 0v to 5v as you rotate the pot counter clockwise.
Return both pots to 0v, fully clockwise.

Reconnect first L8, press PTT, does it blow?
If not,reconnect L9, press PTT, does it blow?
Since the gate voltages are both still at zero with RV2 and RV3 fully clockwise, the IRF510's are not conducting.
Even though both L8 and L9 are restored, that roughly 10ma of current into PA-PWR should not have risen hardly at all.

Now with PTT pressed, first rotate RV2 counterclockwise very slowly until the PA-PWR current rises to 100ma.
This will happen very suddenly when the gate voltage reaches 3 or 4 volts, so be careful.
If rotating RV2 can bring PA-PWR up to 100ma, then the IRF510 at Q95 is probably ok.

Then rotate RV3 until PA-PWR current increases another 100ma to 200ma total.
If that works, then the IRF510 at Q94 is ok.
Rig is ready to use.

I assume somewhere in that procedure, something went terribly wrong for you.
At exactly what step things when haywire will be a significant clue.

Jerry, KE7ER

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