Date   

VFO P to P info

kwroberson
 

Jim and Levent , tu fer fb info , just what i needed.
Im hooking up a digi vfo , i hve abt 1.2 volts P-P @ Q6
collector , but seems to work ok. very stable now, with
10 digi LCD read out down to 1 HZ. 73 Ken K5DNL


Re: vfo trouble - bitx20a schematic.

Greg W:-) <onegammyleg@...>
 

Hi Arv

Yeup , a simple pi was the ticket , I set it at 6dB , but perhaps I
need a little more attenuation for my purpose.
Currently it is 1.75V P-P and I would like 0.9V P-P -> 1.1V P-P , my
frequency counter didn't load it down any as I had expected , so I may
have to recalculate the pi.

The waveform actually looks very nice now .- better than any other VFO
I have made.

Thanks.

gregW:) OH2FFY

=====================================================================

--- In BITX20@..., Arv Evans <arvid.evans@...> wrote:
SNIP
As to your question about reducing VFO output for your non-BITX20
application, I would probably look into designing a resistive Pi type
attenuator for that purpose. This lets you match impedances as needed,
does not change the waveform, and helps maintain a fixed load on the
VFO
output.

Arv
_._


Re: BITX20A: How to connect a Huff&Puff stabilisation

Arv Evans
 

Uli

VFO output for an H&P stabilizer could be taken from the top of R-27, or
maybe the signal end of R-31. Since this is an analog output you may
want to square it up before it gets to the sampling part of the H&P circuit.
It may be possible to use the BITX20A internal varicap (D-9) as part of
the H&P circuit, or you could capacitively connect an external varicap
to the signal side of the main tuning varicap (C-38).
When you get it working properly you might want to post the result here
and on the Huff & Puff group so that others can benefit from your
experience.

Arv
_._


Uli wrote:


Hello,
how can I connect an Huff&Puff from Hans Summer to th VFO ?
Many thanks for any idea.

--
======================================
Web: http://K7HKL.homelinux.net
http://bitx20a.livejournal.com
http://home.earthlink.net/~arvevans/
http://www.geocities.com/k7hkl_arv/
======================================


Re: VFO P-P Vlotage Reading

Jim Kortge <jokortge@...>
 

kwroberson wrote:
Hello All , Cud someone pse give me an O-Scope P TO P voltage reading at Q6 , R27 , tnx 73 Ken K5dnl
Ken,

1.9 volts P-P on the collector end of R27 and 1 volt P-P on the base end of R27. The static bias level on Q6 is 4 volts on the base, the collector is at 8 volts, with the emitter at 3.3 volts. For those who are curious, the base of Q8 has a 9 volt P-P signal signal level on it. The combination of R32 and C91 form a resistive-capacitive voltage divider and low pass filter to deliver 1 volt P-P to the base of Q7. Transistor Q6 runs at 85 degrees F and its emitter resistor, R29 is at a temperature of 29 degrees F. Nothing in my prototype rig's VFO is hot to the touch, and barely warm, per the above measurements.

72 and kind regards,

Jim, K8IQY


Re: VFO P-P Vlotage Reading

lsasmazel
 

I have 0.8V PTP at the Q7's output. I hope this will help.
Levent - WW2L


--- In BITX20@..., "kwroberson" <kwroberson@...> wrote:

Hello All , Cud someone pse give me an O-Scope P TO P voltage
reading
at Q6 , R27 , tnx 73 Ken K5dnl


BITX20A: How to connect a Huff&Puff stabilisation

Uli <uli@...>
 

Hello,
how can I connect an Huff&Puff from Hans Summer to th VFO ?
Many thanks for any idea.


VFO P-P Vlotage Reading

kwroberson
 

Hello All , Cud someone pse give me an O-Scope P TO P voltage reading
at Q6 , R27 , tnx 73 Ken K5dnl


Re: vfo trouble - bitx20a schematic.

Jim Kortge <jokortge@...>
 

cornhusker wrote:
HI!
I am having a similar problem. I am not getting any VFO output at all and R29 is warm along with Q8 and Q6. I redid T4, the trifilar toroid and checked C30 and C31 for shorts, but none there. The voltage on the base of Q8 is about .41V and the supply voltage to the VFO at the Zener is 5.6V . Where do I go next?
Joe,

I'm guessing that you have installed the 5.6 volt Zener who will go in the D1 spot on the PCB into the location meant for the 9.1 Zener which should be installed at the D8 location. As for the 0.41 volts on the base of Q8, it should be much higher than that, nearly 1/2 the supply voltage of 9.1 volts unless it is being loaded down by something downstream or the VFO is really oscillating and the reading device is getting confused by the presence of RF since both are present on the base of Q8. I guess that would go back and make sure that all of the other parts used in the VFO and its buffers are indeed the correct value. Some of the colors on resistors are a bit misleading. A "brown" for instance may well be "violet".

72 and kind regards,

Jim, K8IQY


Re: Low output level

david.g3ryp <david.g3ryp@...>
 

Alan

You don't have a problem! But you may have if the drive is excessive.

When you whistle into a microphone it only approximates a sine wave and you'll need, as you've found, to whistle hard to get full output. Unless you have a level sinewave to the mic input then your power meter cannot respond fast enough and hold long enough to show the full level being produced. When you talk you'll see even less power. But don't worry it's there!

To see what's really happening you'll need a peak reading power meter - I guess you'll find schematics on the web somewhere.

My BITX20A is working fine and I've made several contacts but no DX yet! Funny how it always works better when there's a contest on!

David


Re: Low output level

bugs5504
 

Hi

It may be worth connecting an AF signal generator (or the earpiece
output of a radio) to the Mic connections and see what output that
gives you. My BITX20a gives the full 10W out ,but looking at my cheap
wattmeter you have to whistle hard to get anywhere near that reading.
Using the signal generator I can get 10W out. I'm building an extra
stage of Mic. amplification plus a diode clipper circuit to boost the
average power out.

Alan


Low output level

lsasmazel
 

Can any one tell what is minimum driving signal for Q3 to achive 10
Watts? I did all DC adjustments but I am not seeing even 1/2 watts of
output from BITX20 (actually I am seeing 1V PEP to the 50 ohm load 20
mW) . I have no problem with receive. I am using dynamic microphone
and R92 is not installed it this time. Whistling to the mike is
generating 10mV pep at the gate of Q20. If any body have signal
information at the major connection points?

Thanks,
Levent – WW2L


Re: vfo trouble - bitx20a schematic.

cornhusker
 

HI!
I am having a similar problem. I am not getting any VFO output at
all and R29 is warm along with Q8 and Q6. I redid T4, the trifilar
toroid and checked C30 and C31 for shorts, but none there. The
voltage on the base of Q8 is about .41V and the supply voltage to the
VFO at the Zener is 5.6V . Where do I go next?
73, Joe K0NEB



-- In BITX20@..., Arv Evans <arvid.evans@...> wrote:

Greg

Good information that you found and fixed the VFO problems.
Most of the BITX20 builders have found both the original VFO
version and
the later one in the BITX20A to be quite stable.
As to your question about reducing VFO output for your non-BITX20
application, I would probably look into designing a resistive Pi
type
attenuator for that purpose. This lets you match impedances as
needed,
does not change the waveform, and helps maintain a fixed load on
the VFO
output.

Arv
_._


Greg W:-) wrote:

Hi Arv .

I found out what my problem was , it was a short circuit --- in My
BRAIN.! hihihi

Actually there was a number of separate issues and I had mislead
myself a bit.

This is my explanation for what occurred with that VFO (weak
attempt
at self justification)

The time that I get to do my home-brewing is between 9 to 12 at
night.
So by the time I had finished the VFO it was about 11:30.(thats my
first problem)
My 100mhz CRO is too large to leave on my bench so I must lift it
up
when needed and let it stand on its back feet., meaning I must
peer
over the top (when sitting) to see the display tube.
So I started the VFO up and connected the CRO probe and I saw that
waveform that I described yesterday.
From where I was sitting the CRO settings looked right but they
were not.
The timebase knob was right around the other side.
Due to tiredness (or dumbness) and my sitting position I missread
the
knobs positions on my Philips CRO.
What I thought was a bombed out waveform that wasn't oscillating
was
actually only a portion of a cycle being shown.
In other words , the VFO first stage WAS working.(somewhat)

So thinking that there was no oscillation , now accompanied by the
smell of burning parts you can understand what I was guessing.

Before rebuilding the VFO last night I set out to try and find the
causes for this burn up.

Remember that I said that R26 was very hot , looking around the
area I
found that my toroid (twisted winding bifilar)* had shorted out.
This is unusual, and I can't be sure if it was shorted before or
after
I applied power.(from heat)
Perhaps exposed conductors of the windings were made when
twisting ?
(not likely)
A fair bit of current was going to ground through the toroid
heating
the R26., but still it was providing power to Q6 because it was
getting very warm too.

I can't work out why Q6 would have got so hot.(maybe my Q6 was
faulty
to begin with?)
I think other things were going on too , on a component level ,
but as
I had previously snipped them out and cant test them now.

In the rebuild I checked my remaining components and they looked
fine
, as did the newly installed ones.
The board art was fine with no solder bridges or other faults.

Anyway , I powered it up and the new one worked first time , and
at
about 3mhz it has 5V P-P output.(probably rite for diode ring
mixer app)

The output on the scope is better than all of my other VFO's at
3Mhz ,
but it still is not perfectly clean.(its probably good enough)

I will need to reduce this output to 1V P-P for my application.
Would you recommend achieving that with decoupling the stage
further
of by bias adjustments of the buffers.??

Oh yeah , and I need to get a tuning cap with more pF swing , the
150khz tuning range that I have now isn't enough for full phone
band
on 20 meters in region 1., OR I might have to revise some of the
other
caps in the circuit.

One thing that I did notice , and something I will have to
investigate
further is the LACK of frequency shift from hand capacitance.
I am using a inductor wound on a small plastic (nylon?) rod with
windings running up and then back on itself - 2 layers.
Only when my fingers are almost touching it do i see my frequency
counter change.,, and then , not a lot , (a few kc's only)

I will have to look into this further.

Anyway , Thanks for the reply.

Regards

gregW:-) OH2FFY

*this VFO is not for a bitx 20
======================================================================

--- In BITX20@... <mailto:BITX20%40yahoogroups.com>,
Arv
Evans <arvid.evans@> wrote:

Greg

Good. It sounds like you are well on the way to figuring out
what went
wrong. The fact that R-26 became hot is possibly key to
understanding
what happened. This would make a shorted C-30 as the first
suspect.
Following the circuit would also indicate that a short to
ground or
wiring error in the T-4 and T-5 area could also be a problem. A
shorted
C-31 could put Q-6 near it's maximum power rating and eventually
destroy
that device. If C-31 were OK but the transistor Q-6 was bad, I
would
expect R-29 to be destroyed also. C-32 isolates the 2nd VFO
buffer
from
the 1st VFO buffer so I would not expect something that burned
up R-26
to cause problems for circuitry prior to C-32.

The BITX20A VFO was designed the way it is in order to insure
that the
output was as clean as possible, while still maintaining good
stability. There are a number of small changes that were made
to the
BITX20A design in order to minimize spurious content in the
output
signal. Some were made necessary by close component spacing
involved in
the PCB layout, while others were made possible by having a
standardized
kit of components available for the build. C-25 and L-10 were
necessary
to minimize VFO energy, unwanted mixer products, and possible
harmonics
that might get into the transmitted signal. The largest change
of
course is in the kit having push-pull power output and driver
stages.
This specifically reduces 2nd harmonic content in the output.

Good luck on your VFO debugging, and please let us know what
you find,
as this information may help some other BITX20 builder with
similar
problems.

Arv
_._
--
======================================
Web: http://K7HKL.homelinux.net
http://bitx20a.livejournal.com
http://home.earthlink.net/~arvevans/
http://www.geocities.com/k7hkl_arv/
======================================






Re: vfo trouble - bitx20a schematic.

Allison Parent <kb1gmx@...>
 

--- In BITX20@..., "sg2112" <SG2112@...> wrote:

I think you missed the point

The point was that Jim was able to get a stable, repeatable VFO using
a varactor instead of an air variable which is hard to do without a
PLL or huff puff circuit stablizing it

What is similar between the two VFOs is using a bipolar transistor
instead of the FETs we usually see in VFOs and taking the output off
the tank instead of the transistor

JR
Actually the topology and device used (assuming it's used and biased
correctly) makes little difference. It's contruction, attention to
parts quality (getting COG caps that really are) and if you take the
time a bit of compensation. The result can be a stellar quality VFO.
Varactor over air capacitor does make for some drift but it's
predictable and correctable. Crappy parts, long floppy leads,
parts that aren't well secured, transistors or fets incorrectly
biased and unregulated or poorly regulated DC and control votages
means drifty VFO.

Finally stage isolation or better a consistant and unchanging load
will enhance stability.

The basic Farhan design was pretty decent and I found it
acceptably stable as I assembled it using the FAR circuits
board. Mine ran at 4.6mhz (9.6mhhz IF) for 20M.

Allison







--- In BITX20@..., "Greg W:-)" <onegammyleg@> wrote:

Hi JR ..I looked at the schematics for the 2N2 radios and the VFO in
them look like conventional Colpitts designs , not the same as used in
the Bitx20A.

Drift shouldn't be a huge problem if you use an air spaced variable
capacitor for tuning (silver plated not alluminium) , an air core
inductor (not ferrite core) NP0 or C0G capacitors throughout and + or
- caps used for temperature correction.
Any trim caps used also should be of the highest quality.
Mechanical construction should also be very good.
Single sided boards only.
Fully shielded (including tune cap) with feedthough caps for supply
voltage.
No varactor tuning.
Use the lowest voltage possible to power the VFO to limit heating of
the parts internally.
Use at least 2 buffer stages.

Theses are the rules that I follow and my vfo's don't drift apart from
the initial drift which cant be helped.

Regards

gregW:-) OH2FFY

=====================================================================


--- In BITX20@..., "sg2112" <SG2112@> wrote:

Is the VFO a K8IQY design? It looks similar to some of Jim's 2N2-xx
transceivers VFO's .... They are without a doubt the most stable
varactor tuned VFO's I've ever built .... every other one I built
before that drifted like an old cowboy ....

JR


RX Voltages

Charlie <charlie@...>
 

Here are some volatge measurements taken on recieve using my DVM.
There is a 50 ohm dummy load on the input, the supply voltage is
12.7 and the current with no signal is 127 milliamps. I'm using a 4
ohm loadspeaker. I will put these and other measurements onto a
circuit diagram.

Measurement Point Voltage

Cathode D18 12.40

Base Q25 0.63
Collector Q25 5.50
Junction R79/C80 11.80
Emitter Q16 4.85

Emitter Q13 5.85
Emitter Q14 6.00
Collector Q13/14 11.20

Base Q8 4.40
Emitter Q8 5.60
Base Q7 4.30
Emitter Q7 3.65
Collector Q6 8.50
Base Q6 4.10
Emitter Q6 3.45

Collector Q11 4.60
Base Q11 6.65
Collector Q17 10.65

Collector Q9 8.15
Base Q9 2.50
Emitter Q9 1.85

Collector Q5 8.15
Base Q5 2.50
Emitter 1.85

I have rounded to the nearest 50 millivolts.

I have awful plasma TVI here so will have to get up early on the
weekend to set up the but it's certainly picking up that TVI!


Charlie.


Re: vfo trouble - bitx20a schematic.

Arv Evans
 

Greg

Good information that you found and fixed the VFO problems.
Most of the BITX20 builders have found both the original VFO version and
the later one in the BITX20A to be quite stable.
As to your question about reducing VFO output for your non-BITX20
application, I would probably look into designing a resistive Pi type
attenuator for that purpose. This lets you match impedances as needed,
does not change the waveform, and helps maintain a fixed load on the VFO
output.

Arv
_._


Greg W:-) wrote:


Hi Arv .

I found out what my problem was , it was a short circuit --- in My
BRAIN.! hihihi

Actually there was a number of separate issues and I had mislead
myself a bit.

This is my explanation for what occurred with that VFO (weak attempt
at self justification)

The time that I get to do my home-brewing is between 9 to 12 at night.
So by the time I had finished the VFO it was about 11:30.(thats my
first problem)
My 100mhz CRO is too large to leave on my bench so I must lift it up
when needed and let it stand on its back feet., meaning I must peer
over the top (when sitting) to see the display tube.
So I started the VFO up and connected the CRO probe and I saw that
waveform that I described yesterday.
From where I was sitting the CRO settings looked right but they were not.
The timebase knob was right around the other side.
Due to tiredness (or dumbness) and my sitting position I missread the
knobs positions on my Philips CRO.
What I thought was a bombed out waveform that wasn't oscillating was
actually only a portion of a cycle being shown.
In other words , the VFO first stage WAS working.(somewhat)

So thinking that there was no oscillation , now accompanied by the
smell of burning parts you can understand what I was guessing.

Before rebuilding the VFO last night I set out to try and find the
causes for this burn up.

Remember that I said that R26 was very hot , looking around the area I
found that my toroid (twisted winding bifilar)* had shorted out.
This is unusual, and I can't be sure if it was shorted before or after
I applied power.(from heat)
Perhaps exposed conductors of the windings were made when twisting ?
(not likely)
A fair bit of current was going to ground through the toroid heating
the R26., but still it was providing power to Q6 because it was
getting very warm too.

I can't work out why Q6 would have got so hot.(maybe my Q6 was faulty
to begin with?)
I think other things were going on too , on a component level , but as
I had previously snipped them out and cant test them now.

In the rebuild I checked my remaining components and they looked fine
, as did the newly installed ones.
The board art was fine with no solder bridges or other faults.

Anyway , I powered it up and the new one worked first time , and at
about 3mhz it has 5V P-P output.(probably rite for diode ring mixer app)

The output on the scope is better than all of my other VFO's at 3Mhz ,
but it still is not perfectly clean.(its probably good enough)

I will need to reduce this output to 1V P-P for my application.
Would you recommend achieving that with decoupling the stage further
of by bias adjustments of the buffers.??

Oh yeah , and I need to get a tuning cap with more pF swing , the
150khz tuning range that I have now isn't enough for full phone band
on 20 meters in region 1., OR I might have to revise some of the other
caps in the circuit.

One thing that I did notice , and something I will have to investigate
further is the LACK of frequency shift from hand capacitance.
I am using a inductor wound on a small plastic (nylon?) rod with
windings running up and then back on itself - 2 layers.
Only when my fingers are almost touching it do i see my frequency
counter change.,, and then , not a lot , (a few kc's only)

I will have to look into this further.

Anyway , Thanks for the reply.

Regards

gregW:-) OH2FFY

*this VFO is not for a bitx 20
======================================================================

--- In BITX20@... <mailto:BITX20%40yahoogroups.com>, Arv
Evans <arvid.evans@...> wrote:

Greg

Good. It sounds like you are well on the way to figuring out what went
wrong. The fact that R-26 became hot is possibly key to understanding
what happened. This would make a shorted C-30 as the first suspect.
Following the circuit would also indicate that a short to ground or
wiring error in the T-4 and T-5 area could also be a problem. A
shorted
C-31 could put Q-6 near it's maximum power rating and eventually
destroy
that device. If C-31 were OK but the transistor Q-6 was bad, I would
expect R-29 to be destroyed also. C-32 isolates the 2nd VFO buffer
from
the 1st VFO buffer so I would not expect something that burned up R-26
to cause problems for circuitry prior to C-32.

The BITX20A VFO was designed the way it is in order to insure that the
output was as clean as possible, while still maintaining good
stability. There are a number of small changes that were made to the
BITX20A design in order to minimize spurious content in the output
signal. Some were made necessary by close component spacing involved in
the PCB layout, while others were made possible by having a
standardized
kit of components available for the build. C-25 and L-10 were
necessary
to minimize VFO energy, unwanted mixer products, and possible
harmonics
that might get into the transmitted signal. The largest change of
course is in the kit having push-pull power output and driver stages.
This specifically reduces 2nd harmonic content in the output.

Good luck on your VFO debugging, and please let us know what you find,
as this information may help some other BITX20 builder with similar
problems.

Arv
_._
--
======================================
Web: http://K7HKL.homelinux.net
http://bitx20a.livejournal.com
http://home.earthlink.net/~arvevans/
http://www.geocities.com/k7hkl_arv/
======================================


Re: vfo trouble - bitx20a schematic.

sg2112 <SG2112@...>
 

I think you missed the point

The point was that Jim was able to get a stable, repeatable VFO using
a varactor instead of an air variable which is hard to do without a
PLL or huff puff circuit stablizing it

What is similar between the two VFOs is using a bipolar transistor
instead of the FETs we usually see in VFOs and taking the output off
the tank instead of the transistor

JR

--- In BITX20@..., "Greg W:-)" <onegammyleg@...> wrote:

Hi JR ..I looked at the schematics for the 2N2 radios and the VFO in
them look like conventional Colpitts designs , not the same as used in
the Bitx20A.

Drift shouldn't be a huge problem if you use an air spaced variable
capacitor for tuning (silver plated not alluminium) , an air core
inductor (not ferrite core) NP0 or C0G capacitors throughout and + or
- caps used for temperature correction.
Any trim caps used also should be of the highest quality.
Mechanical construction should also be very good.
Single sided boards only.
Fully shielded (including tune cap) with feedthough caps for supply
voltage.
No varactor tuning.
Use the lowest voltage possible to power the VFO to limit heating of
the parts internally.
Use at least 2 buffer stages.

Theses are the rules that I follow and my vfo's don't drift apart from
the initial drift which cant be helped.

Regards

gregW:-) OH2FFY

=====================================================================


--- In BITX20@..., "sg2112" <SG2112@> wrote:

Is the VFO a K8IQY design? It looks similar to some of Jim's 2N2-xx
transceivers VFO's .... They are without a doubt the most stable
varactor tuned VFO's I've ever built .... every other one I built
before that drifted like an old cowboy ....

JR


Re: vfo trouble - bitx20a schematic.

Greg W:-) <onegammyleg@...>
 

Hi Arv .

I found out what my problem was , it was a short circuit --- in My
BRAIN.! hihihi

Actually there was a number of separate issues and I had mislead
myself a bit.

This is my explanation for what occurred with that VFO (weak attempt
at self justification)

The time that I get to do my home-brewing is between 9 to 12 at night.
So by the time I had finished the VFO it was about 11:30.(thats my
first problem)
My 100mhz CRO is too large to leave on my bench so I must lift it up
when needed and let it stand on its back feet., meaning I must peer
over the top (when sitting) to see the display tube.
So I started the VFO up and connected the CRO probe and I saw that
waveform that I described yesterday.
From where I was sitting the CRO settings looked right but they were not.
The timebase knob was right around the other side.
Due to tiredness (or dumbness) and my sitting position I missread the
knobs positions on my Philips CRO.
What I thought was a bombed out waveform that wasn't oscillating was
actually only a portion of a cycle being shown.
In other words , the VFO first stage WAS working.(somewhat)

So thinking that there was no oscillation , now accompanied by the
smell of burning parts you can understand what I was guessing.


Before rebuilding the VFO last night I set out to try and find the
causes for this burn up.

Remember that I said that R26 was very hot , looking around the area I
found that my toroid (twisted winding bifilar)* had shorted out.
This is unusual, and I can't be sure if it was shorted before or after
I applied power.(from heat)
Perhaps exposed conductors of the windings were made when twisting ?
(not likely)
A fair bit of current was going to ground through the toroid heating
the R26., but still it was providing power to Q6 because it was
getting very warm too.

I can't work out why Q6 would have got so hot.(maybe my Q6 was faulty
to begin with?)
I think other things were going on too , on a component level , but as
I had previously snipped them out and cant test them now.

In the rebuild I checked my remaining components and they looked fine
, as did the newly installed ones.
The board art was fine with no solder bridges or other faults.

Anyway , I powered it up and the new one worked first time , and at
about 3mhz it has 5V P-P output.(probably rite for diode ring mixer app)

The output on the scope is better than all of my other VFO's at 3Mhz ,
but it still is not perfectly clean.(its probably good enough)

I will need to reduce this output to 1V P-P for my application.
Would you recommend achieving that with decoupling the stage further
of by bias adjustments of the buffers.??

Oh yeah , and I need to get a tuning cap with more pF swing , the
150khz tuning range that I have now isn't enough for full phone band
on 20 meters in region 1., OR I might have to revise some of the other
caps in the circuit.

One thing that I did notice , and something I will have to investigate
further is the LACK of frequency shift from hand capacitance.
I am using a inductor wound on a small plastic (nylon?) rod with
windings running up and then back on itself - 2 layers.
Only when my fingers are almost touching it do i see my frequency
counter change.,, and then , not a lot , (a few kc's only)

I will have to look into this further.

Anyway , Thanks for the reply.

Regards

gregW:-) OH2FFY



*this VFO is not for a bitx 20
======================================================================

--- In BITX20@..., Arv Evans <arvid.evans@...> wrote:

Greg

Good. It sounds like you are well on the way to figuring out what went
wrong. The fact that R-26 became hot is possibly key to understanding
what happened. This would make a shorted C-30 as the first suspect.
Following the circuit would also indicate that a short to ground or
wiring error in the T-4 and T-5 area could also be a problem. A
shorted
C-31 could put Q-6 near it's maximum power rating and eventually
destroy
that device. If C-31 were OK but the transistor Q-6 was bad, I would
expect R-29 to be destroyed also. C-32 isolates the 2nd VFO buffer
from
the 1st VFO buffer so I would not expect something that burned up R-26
to cause problems for circuitry prior to C-32.

The BITX20A VFO was designed the way it is in order to insure that the
output was as clean as possible, while still maintaining good
stability. There are a number of small changes that were made to the
BITX20A design in order to minimize spurious content in the output
signal. Some were made necessary by close component spacing involved in
the PCB layout, while others were made possible by having a
standardized
kit of components available for the build. C-25 and L-10 were
necessary
to minimize VFO energy, unwanted mixer products, and possible
harmonics
that might get into the transmitted signal. The largest change of
course is in the kit having push-pull power output and driver stages.
This specifically reduces 2nd harmonic content in the output.

Good luck on your VFO debugging, and please let us know what you find,
as this information may help some other BITX20 builder with similar
problems.

Arv
_._


Re: vfo trouble - bitx20a schematic.

Greg W:-) <onegammyleg@...>
 

Hi JR ..I looked at the schematics for the 2N2 radios and the VFO in
them look like conventional Colpitts designs , not the same as used in
the Bitx20A.

Drift shouldn't be a huge problem if you use an air spaced variable
capacitor for tuning (silver plated not alluminium) , an air core
inductor (not ferrite core) NP0 or C0G capacitors throughout and + or
- caps used for temperature correction.
Any trim caps used also should be of the highest quality.
Mechanical construction should also be very good.
Single sided boards only.
Fully shielded (including tune cap) with feedthough caps for supply
voltage.
No varactor tuning.
Use the lowest voltage possible to power the VFO to limit heating of
the parts internally.
Use at least 2 buffer stages.

Theses are the rules that I follow and my vfo's don't drift apart from
the initial drift which cant be helped.

Regards

gregW:-) OH2FFY

=====================================================================

--- In BITX20@..., "sg2112" <SG2112@...> wrote:

Is the VFO a K8IQY design? It looks similar to some of Jim's 2N2-xx
transceivers VFO's .... They are without a doubt the most stable
varactor tuned VFO's I've ever built .... every other one I built
before that drifted like an old cowboy ....

JR


Re: vfo trouble - bitx20a schematic.

sg2112 <SG2112@...>
 

Is the VFO a K8IQY design? It looks similar to some of Jim's 2N2-xx
transceivers VFO's .... They are without a doubt the most stable
varactor tuned VFO's I've ever built .... every other one I built
before that drifted like an old cowboy ....

JR


--- In BITX20@..., Arv Evans <arvid.evans@...> wrote:

Greg

Interesting observation, but not quite correct. The first buffer
shares
the bias resistors with the oscillator transistor itself. This holds
the emitter on that buffer at approximately 1/2 Vcc from that pair of
10K dividers (R-33 and R-34). The DC path for this bias is off the
junction of these two resistors, through L7 and R-32 to the base of
Q-7. Drive for the 1st buffer is taken from the VFO tank circuit, not
from the VFO emitter. This is a little different from some popular VFO
designs, but it does insure that output from this buffer is clean and
does not get driven into saturation or shut-off at any point in the
cycle.

<snip>


Re: VFO Drift

kwroberson
 

Charlie : OK , Tnx fer info , however the vfo is not stable
enough for the digi modes , so i am working on DDS fer vfo.
tnx agn 73 Ken K5DNL

-- In BITX20@..., Management <charlie@...> wrote:

kwroberson wrote:
Hello All,
Im at the point where Book said i cud test fer
signal with freq counter or rx , I did both and
the VFO has a lot of drift , lef>t it on fer abt
1 Hour , still drift.
I waved my hand over osc and freq went down abt
400 HZ and slowly drift back up , its all over the
place , is this the best it gets or is something
wrong with mine ???

Tnx agn Ken K5DNL


Ken,

It's hardly surprising that the oscillator drifts if you wave your
hand over it! Mine does too but if I leave it sitting there on the
bench and tune it in on my IC703 then it is incredibly stable
considering it's open to the air moving around the coil, etc.

I have to take some turns off the coil as I'm tuning below 14 MHz
but when I have done that I will do some checks on the stability.
(Just working on the RX not txing!)



Charlie.

--
M0WYM
www.radiowymsey.org