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Low output Bitx40

JeffG <jeff.goodley@...>
 

I know this has been around the block a few times, but we need some help.

My BitX40 receives beautifully across the band but we have next to no output power on transmit.  To get anything above 500mW both pots have to be wound fully up.  What can we do to get up to the 5+ Watts we should get out of this rig?

Jeff
G6EXF. 

Guy N7BIR <sgbridge@...>
 

Hi Jeff,
are you seeing this low output in ssb mode or in CW mode?

JeffG <jeff.goodley@...>
 

SSB Guy. With both pots turn right up I barely get a flicker on the needle of the power meter. Measures at 500 to 700 mW on transmit. 

Jeff
G6EXF. 

Allen Woolfrey
 

Just finished wiring my bitx40. 

Draws ~350ma with PTT, but no change at all when yelling or whistling into mic.  (Dummy load present at all times, so don't suspect final blown.)

I see lots of discussion around this issue, but would appreciate advice on how to troubleshoot.

72/3, Al - va3iaw

 

Al, check the electret condenser mic element soldering. Polarity is important.

Raj

At 18-07-18, you wrote:
Just finished wiring my bitx40.

Draws ~350ma with PTT, but no change at all when yelling or whistling into mic. (Dummy load present at all times, so don't suspect final blown.)

I see lots of discussion around this issue, but would appreciate advice on how to troubleshoot.

72/3, Al - va3iaw

Allen Woolfrey
 

Thank you for quick reply.

Re-checked soldered correctly, and confirmed continuity to board connector.

 

1: Power connector all wires connected properly ?

2: RV1 at center ?

At 18-07-18, you wrote:
Thank you for quick reply.

Re-checked soldered correctly, and confirmed continuity to board connector.

 

R136 with Bitx40!

1: Power connector all wires connected properly ?

2: RV1 at center ?


At 18-07-18, you wrote:
Thank you for quick reply.

Re-checked soldered correctly, and confirmed continuity to board connector.

_Dave_ K0MBT
 

Is the low output with you talking into the mic? The carrier is suppressed in SSB as you know and almost no output is expected without modulation.

I had a mic go bad on mine after a month.   Found a replacement in an old portable phone.

Jerry Gaffke
 

You might try Allard's mod to transmit CW, drive the top of C107 from 5v through a 4.7k resistor:
    https://github.com/amunters/bitx40/blob/master/CW-CARRIER%20wiring.png


If that gives power to your dummy load, then the problem is back in the mike amp or mike.

Allen Woolfrey
 

Yes, low output when speaking or whistling into mic.

Power wiring correct AFAIK.

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.

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.