Re: No RF Power Output From Bitx40
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Vince
Thanks.? I will try it.
Arv
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On Sat, Jul 8, 2017 at 1:01 PM, Vince Vielhaber <vev@...> wrote:
Low power tuning?? No problem.? I used the CW trick (part of the
recent Raduino mods) with the 10K resistor but instead of hooking up the other end of the resistor to the raduino, I hooked it up to a 100K pot to 5v thru a switch.? Switch off, the BitX operates normally.? Switch on, the BitX sends a carrier, of whatever power you want based on where the 100K pot is set, when you hit the PTT.? With the 100K you can take it down to nothing.? It's very possible a 25K or 50k would also work satisfactorily but I didn't check, the 100K was handy.
Vince.
On 07/08/2017 12:51 PM, Arv Evans wrote:
Rich? KC8MWG
MOSFETs will blow from over-current faster than I can reach the
controls, but
I am old and slow so your mileage may vary.
Tune up has always been one of my complaints about the BITX design.? We
need a low-power output setting for adjusting the ATU, and then ability
to switch
to higher power for normal use.? Back in the BITX20A and BITX17A days I
used
an audio oscillator with two switch selected output levels.? Low level
was used
to generate less than 0.1 watts output for tuning the ATU and high level
AF out
for sending CW.? This works if the antenna is not too far from 50 ohms,
but does
not protect the RF PA if no antenna is connected.
Some have experimented with current-limited power supplies for the PA stage.
I'm not sure how effective that might have been.
I wonder what might happen if a small resistance and bypass capacitor were
placed in series with the IRF510 source lead?
Back in the bad old days of vacuum tubes and thermionic emission we used
small inductances and parallel capacitors? in plate circuits to minimize
parasitic
oscillations.? ? The oscillation mode is different with MOSFETs? but
this might
warrant? further exploration.
ALC circuits for the RF PA stage might be useful for detecting and
taming some
oscillations, but might be tricky to adjust for SSB use.
I wonder if the RF PA voltage might be reduced to some low level for tune-up
and then switched to higher voltage for normal operation?? Using low
voltage
on the drain lead during tune-up could cause non-linearity and spurious
outputs but for short-term tune-up action this might not be too
objectionable.
Part of the attraction of BITX designs is that is always potential for
modification
and hopefully improvement.
Arv? K7HKL
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On Sat, Jul 8, 2017 at 8:36 AM, Richard Andrew Knack via Groups.Io
<ihc73scout2=yahoo.com@groups.io
<mailto:ihc73scout2=yahoo.com@groups.io>> wrote:
? ? How quickly will IRF-510's blow? And how much power will it take? In
? ? other words, if you have a transceiver or an amp that has them for
? ? finals, do you have time to use an automatic antenna tuner to get an
? ? impedance match? Or will the finals blow while the tuner is running
? ? through relays and inductances trying to find a match?
? ? Rich
? ? KC8MWG
? ? On Saturday, July 8, 2017, 10:31:21 AM EDT, Arv Evans
? ? <arvid.evans@... <mailto:arvid.evans@...>> wrote:
? ? K3HJP
? ? Like Raj said...the IRF510 is probably blown, usually due to
? ? transmitting into something other
? ? than a 50 ohm load.? If the output impedance is too high the final
? ? will go into oscillation and
? ? self-destruct.
? ? Arv? K7HKL
? ? _._
? ? On Sat, Jul 8, 2017 at 7:32 AM, <Kc3hjp@...
? ? <mailto:Kc3hjp@...>> wrote:
? ? ? ? Hi guys, I finally got around to finishing my Bitx40 project.
? ? ? ? Everything works but it will not transmit. This has me
? ? ? ? scratching my head. Here are some details.
? ? ? ? - when I press the PTT on the mic I can hear the relay pull in
? ? ? ? and receive audio goes away as it should.
? ? ? ? - I've verified that I have 12vdc on the PA connection and it's
? ? ? ? wired in the right direction.
? ? ? ? - mic element ohms out properly and it's wired in the right
? ? ? ? direction according to instructions
? ? ? ? - 152mA on receive
? ? ? ? - 260mA on transmit with no audio
? ? ? ? - current may change by .001mA when yelling into the mic
? ? ? ? - set PA bios pot makes no difference in current reading with
? ? ? ? mic keyed (audio/no audio)
? ? ? ? - mic element 0.877k ohms in one direction and 1.101k ohms in
? ? ? ? other direction
? ? ? ? - same mic ohm values on the mic connector and board connector
? ? ? ? - 12vdc between the Gate and Drain of the PA. No voltage between
? ? ? ? the Gate and Source when mic is keyed
? ? ? ? Any ideas?
--
? Michigan VHF Corp.? ?? ?
? ? ? ? ? ? ? ? ? ? ? ? ?
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