Re: Low output Bitx40

Jerry Gaffke

Mucking with RV1 could easily and quickly blow the IRF510.
Whenever adjusting RV1 (the IRF510 gate bias voltage), you should be monitoring 
the drain current into the IRF510 (12v into PA-PWR1) with an ammeter.
RV1 should be set for 100ma and no more (with no audio into the mike), it's very touchy.
At that point (100ma) the IRF510 is just starting to conduct, a little bit more and it will be several amps.
I'm a bit surprised it stopped at 1.6A, perhaps you have a weak power supply, could have been 10A.
Assuming 1.5A going up as heat in the IRF510, that's 12*1.5 = 18 Watts into a very inadequate heatsink.
Will be smoking hot.

This RV1 on the Bitx40 corresponds to RV2 and RV3 on the uBitx (there's two IRF510's on the uBitx).
Same rules apply, don't mindlessly twiddle those pots or you will blow the final.
Should be 100ma into each IRF510.

Twiddling R136 on the Bitx40 and RV1 on the uBitx adjusts the drive level into the transmit pre-amps.
Somewhat less dangerous, as it probably won't cause the IRF510's to blow across the room.
But turning up the gain too high there will cause distortion in your transmitted output
and you will be splattering across the band.
On commercial rigs, this is much like turning the mike gain up too high.

When going for more power by adjusting those pots or increasing the voltage into the IRF510 drain
or when going to long duty cycle transmissions (perhaps digital modes), a good idea to check if
the IRF510 heatsink is getting hot.  If it's too hot to hold your finger on it, that's too hot.

Jerry, KE7ER

On Wed, Jul 18, 2018 at 07:07 AM, Allen Woolfrey wrote:
RV 1 set at about 30% on arrival.  Adjusting to full CCW raises draw to about 1.6 A on PTT, with no change when using mic.  Reset back to about 30%.  Also, adjusting R136 has no effect.

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