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CW,T/R pop, modifications

market10gardener
 

Thanks Chris, Arv and Ashhar for comments and advice re the squeak.
I have checked and the relevant diode is in place. I removed the
decoupling electrolytic cap. from pin 6 of the LM386. Interestingly
it had no detectable effect on the function of the amplifier and the
squeak which occurs on changing from transmit to receive was the same
as before. It seems that there is some residual current in the
transmit phase of BITX which causes this pop.
John G0UCP
Arv Evans
 

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John

The decoupling cap on pin-6 of the LM386 would probably not be the culprit causing a short oscillation during the change over from Tx to Rx, as originally described.?

Take a good look at the Microphone Amplifier.? It's output is coupled directly to the input of the Rx audio section via the two 0.1 mfd capacitors.? If there is any residual voltage left on the Mike amp when the Rx audio section is turned on, a feedback path can be established from Mike to Mike Amp to Rx AF preamp to LM386 to Speaker...and back to the mike again.?

Now, how can there be a residual voltage on the mike amp when the transceiver has just switched from Tx to Rx mode?? Look at the Microphone Amplifier decoupling.? This 50 mfd capacitor is isolated from the rest of the Tx circuitry by a 220 ohm resistor.? That may be allowing some residual voltage to remain on the Mike Amp for a few milliseconds after Tx to Rx switchover.

A quick test to see if this is your problem might be to disconnect the speaker and use a set of headphones.? If the problem goes away, then it could have been caused by the scenario outlined above.? Using headphones should remove the acoustic feedback path, thus killing the oscillation.

Arv
_._
On Thu, 2004-08-26 at 15:29, market10gardener wrote:

Thanks Chris, Arv and Ashhar for comments and advice re the squeak.
I have checked and the relevant diode is in place. I removed the
decoupling electrolytic cap. from pin 6 of the LM386. Interestingly
it had no detectable effect on the function of the amplifier and the
squeak which occurs on changing from transmit to receive was the same
as before. It seems that there is some residual current in the
transmit phase of BITX which causes this pop.
John? G0UCP???


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market10gardener
 

--- In BITX20@..., Arv Evans <arvevans@e...> wrote:
a feedback path can be established
from Mike to Mike Amp to Rx AF preamp to LM386 to Speaker...and
back to
the mike again.

A quick test to see if this is your problem might be to disconnect
the
speaker and use a set of headphones. If the problem goes away,
then it
could have been caused by the scenario outlined above. Using
headphones
should remove the acoustic feedback path, thus killing the
oscillation.

Arv
Thanks Arv. I am sure there is a residual current(? RF) in the tx
side.
I have been using headphones all along and there is no mike connected
yet so we need to keep thinking. Enjoy the weekend. John G0UCP
 

and the
squeak which occurs on changing from transmit to receive was the
same
as before. It seems that there is some residual current in the
transmit phase of BITX which causes this pop.
John G0UCP
Now I read it back, I am not 100% sure that in my case the (very
short) beeb occurs while changing from transmit to receive but while
switching from receive to transmit.
At the moment I had not yet a toggle switch mounted, I changed over
by connecting a clamp with 12 volts to the receive part or the
transmit part. At that time, the beeb was not heard, likely because
it took more time then with the toggleswitch.
By the way, I do not find it very irritating, I consider it as a
signal 'on the air now!' ;-)
A pitty I can not check it at this moment because I am busy to put
some shielding (3 cm high circuit board) around the transceiver
board and therefore I desconnected all the wiring ;-(
Then follows the hard part (that several of my build circuits never
reached): build it in a metal box.
Best regards,
Chris.
 

Hi,
now I have a shielding around and also over the PCB including the x-
tals, the 'beeb' that happens while switching to transmit was gone.
It was not the intention to have the shielding for this purpose but
because it is gone now, it is likely that it was caused by
interaction between some RF parts and the LF amplifier...
If it comes back while I build further, I will come back on this
issue.
I really must work now on the IRF part because I hear so many
stations and I do not want to work but I am afraid they do not hear
my 150 mW signal ;-(
Best regards,
Chris.

A pitty I can not check it at this moment because I am busy to put
some shielding (3 cm high circuit board) around the transceiver
board and therefore I desconnected all the wiring ;-(
Then follows the hard part (that several of my build circuits
never
reached): build it in a metal box.
Best regards,
Chris.
Arvid Evans
 

Chris

How are you switching the 12 Volts between Rx and Tx (mechanical
switch, relays, or solid-state switching)?

In developing solid-state switching between Rx & TX, do you think I
should build in a slight delay between TX-Off and RX-On? I suspect
this mioght be one way to minimize the beep on switchover.

Thanks,
Arv - k7hkl
_._

--- In BITX20@..., "vdberghak" <vdberghak@z...> wrote:
Hi,
now I have a shielding around and also over the PCB including the x-
tals, the 'beeb' that happens while switching to transmit was gone.
It was not the intention to have the shielding for this purpose but
because it is gone now, it is likely that it was caused by
interaction between some RF parts and the LF amplifier...
If it comes back while I build further, I will come back on this
issue.
I really must work now on the IRF part because I hear so many
stations and I do not want to work but I am afraid they do not hear
my 150 mW signal ;-(
Best regards,
Chris.

A pitty I can not check it at this moment because I am busy to put
some shielding (3 cm high circuit board) around the transceiver
board and therefore I desconnected all the wiring ;-(
Then follows the hard part (that several of my build circuits
never
reached): build it in a metal box.
Best regards,
Chris.
market10gardener
 

--- In BITX20@..., "vdberghak" <vdberghak@z...> wrote:
Hi,
now I have a shielding around and also over the PCB including the x-
tals, the 'beeb' that happens while switching to transmit was gone.
Hi Chris and others,
I am happy to say that having built and installed the pre-amp stage,
Q14 and put in a little screening around the VFO variable capacitor I
have also lost the squeak which was present on T/R switching. Even
disconnecting the preamp stage doesnt bring it back. I am using
mechanical switching.
The receiver is amazingly clear and sensitive and 20 is good at
present with VP9, KP4, A47 etc all heard yesterday evening and then
an LU station so loud that the volume had to be turned down to
prevent sore ears. Cant wait to get the linear going to join in the
fun.
72, 73 John G0UCP
 

Hi Arv,
I switch by using a toggle switch, which connects the 12 volts to
the TX parts (including antenna relay) or the RX parts.
Because 'the beep' disappeared by the shielding, I do not see why
you should add a delay if you are going to switch solid state.
Will you also switch the antenna solid state? If you are using a
relay and you notice strange effects, you can always use one of the
contacts to switch off the speaker. Or add an extra transistor
to 'mute' an unwanted audio response.
Making delay is not a real solution; it solves the symptom but not
the cause.... What I see now is that shielding (and probably
decoupling of the 12 v lines with feed trough capacitors) as a
solution.
I hope I helped you, I you find out you have somewhere a problem;
you can always add a new circuit ;-)
Best regards,
Chris.

--- In BITX20@..., "Arvid Evans" <arvevans@e...> wrote:
Chris

How are you switching the 12 Volts between Rx and Tx (mechanical
switch, relays, or solid-state switching)?

In developing solid-state switching between Rx & TX, do you think I
should build in a slight delay between TX-Off and RX-On? I suspect
this mioght be one way to minimize the beep on switchover.

Thanks,
Arv - k7hkl
now I have a shielding around and also over the PCB including
the x-
tals, the 'beeb' that happens while switching to transmit was
gone.
Arv Evans
 

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Chris

Thanks for the reply and for your suggestions.

I now have the DC switchover working, without any delay circuit, and all seems OK.
There is a slight click when changing from TX to RX but that may be caused by
the application of power to the RX AF stages.? As part of adding internal CW-mode
capability I will eventually leave the output side of the RX AF amp running all the time
so I can insert CW Sidetone.? That may also solve the slight click when changing
back to Rx mode.

Antenna switching is driven by the TX DC voltage, and uses a pair of forward
biased diodes to block the receiver input during TX.? A 20 pf coupling capacitor
is thus shorted to ground in TX mode, but it does not seem to detune the TX
output circuitry enough to be any problem.

This weekend and the following week are a bit full for me, but I will try to find
time to get the switching circuit drawn up and posted here by late next week.

Yes, I suspect that most of us have the problem of adding yet more circuitry to
solve symptoms, rather than fix the design to eliminate the problem.? I know that
I am guilty of that.? 8-)

Thanks again,

Arv - k7hkl
_._
On Sat, 2004-09-04 at 04:01, vdberghak wrote:

Hi Arv,
I switch by using a toggle switch, which connects the 12 volts to
the TX parts (including antenna relay) or the RX parts.
Because 'the beep' disappeared by the shielding, I do not see why
you should add a delay if you are going to switch solid state.
Will you also switch the antenna solid state? If you are using a
relay and you notice strange effects, you can always use one of the
contacts to switch off the speaker. Or add an extra transistor
to 'mute' an unwanted audio response.
Making delay is not a real solution; it solves the symptom but not
the cause....? What I see now is that shielding (and probably
decoupling of the 12 v lines with feed trough capacitors) as a
solution.
I hope I helped you, I you find out you have somewhere a problem;
you can always add a new circuit ;-)
Best regards,
Chris.

--- In BITX20@..., "Arvid Evans" wrote:
> Chris
>
> How are you switching the 12 Volts between Rx and Tx (mechanical
> switch, relays, or solid-state switching)?
>
> In developing solid-state switching between Rx & TX, do you think I
> should build in a slight delay between TX-Off and RX-On?? I suspect
> this mioght be one way to minimize the beep on switchover.
>
> Thanks,
> Arv - k7hkl

> > now I have a shielding around and also over the PCB including
the x-
> > tals, the 'beeb' that happens while switching to transmit was
gone.



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