On Fri, 14 Nov 2003 08:36:36 -0800, "Bob" wrote:

An excellent approach would be to look at the waveforms with an oscilloscope
to see just what's going on there.

What would a 7Mhz waveform look like with a 3.5 sub-harmonic say 6 db
down? Does it display as two seperate, distinct waveforms or one
distorted one?

--

"I expect history will be kind to me, since I intend to write it."
- Winston Churchill

Farhan, I took your advice about tuning out the subharmonic, although
I used a dummy load instead of a 12" wire. I did find a tuning point
at which the subharmonic went away. The article says to tune it so
that there is a minimum of illumination at the PA indicator lamp, and
a maximum of illumination at the RF indicator lamp. I found a point
where there is NO illumination at the PA indicator lamp, while the RF
lamp glows brightly. Prior to this test, I always tried to have at
least some illumination at the PA indicator lamp. Perhaps no
illumination is best for this particular incarnation of this rig,
given that it still has some problems. I have no scope and no
wavemeter (yet) and not a lot of knowledge, so I rely a lot on trial
and error, probably too much.

The tuning point described above is also the point where any
self-oscillation stops, the subharmonic is silent, and the output is
the greatest as measured via a voltmeter across the dummy load,
rectified with a diode and capacitor as suggested by KC6WDK. I'm now
getting 12 v there.

Now I'll continue to try to find the source of the subharmonic, and
also the chirp, which is still a little troublesome. I am also going
to try some suggestions made by others who responded.

I realize that I'm rather handicapped, having a 1-watt rig that uses
crystals, and having only a "Technician with Morse Code" license. I
missed passing the General exam by one question. :(

Thanks to all.


On 14 Nov 2003 16:02:58 -0800, (Ashhar Farhan)
wrote:

(John Sandin) wrote in message ...

The problem is, she also heard a signal about half as
strong at a subharmonic (3562 kHz). Does anybody out there know what
might typically cause this to happen with a transmitter like mine? It
didn't occur to me to try a random wire at the receiving end as a
comparison. My receiver is an Icom R71A.


if she has received the signal across the town, then you are putting
out a pretty strong sub-harmonic. a couple of things have to be
checked up:

1) are u sure that the signal is actually due to the crystal? it might
be that the oscillator is freely running on its own. to check that,
test that the signal is stable, if u move ur hand around the coils or
touch them, the signal should not shift its frequency. if it does
that, then the crystal is not really a part of the oscillation.

2) if you have figured out that the crystal is oscillating properly,
next, you might have mistuned your PA to 3.5MHz instead of 7MHz. Try
retuning the PA, disconnect the antenna from the receiver attach a
foot of wire to the output of the tx and tune the tx for maximum
output on 7MHz. Then switch the rx to 3.5MHz and see if you can 'tune
out' the 3.5MHz trace.

these are quick fixes. the larger problem that you are faced with
really is that you don't exactly know what is going on. the above two
solutions are based on diagnosing the problem emperically.

what you really need is a wavemeter. If you have another variable cap
lying around, it might be a good idea to hook one up. It will serve
you for years. It takes about 4 components to wire it up. You can
caliberate it easily using a regular TX for 3.5 and 7. Now you can
couple the wavemeter to each of your TX coils and tune it to the
correct frequency.

if we are to assume that the crystal is actually 7MHz, i see no way
that it can give sub-harmonics in a proper design. the only culprit
could be the tuned circuits.

- farhan


-John Sandin KC0QWE

Remove the "T" to respond by e-mail

Farhan, I took your advice about tuning out the subharmonic, although
I used a dummy load instead of a 12" wire. I did find a tuning point
at which the subharmonic went away. The article says to tune it so
that there is a minimum of illumination at the PA indicator lamp, and
a maximum of illumination at the RF indicator lamp. I found a point
where there is NO illumination at the PA indicator lamp, while the RF
lamp glows brightly. Prior to this test, I always tried to have at
least some illumination at the PA indicator lamp. Perhaps no
illumination is best for this particular incarnation of this rig,
given that it still has some problems. I have no scope and no
wavemeter (yet) and not a lot of knowledge, so I rely a lot on trial
and error, probably too much.

The tuning point described above is also the point where any
self-oscillation stops, the subharmonic is silent, and the output is
the greatest as measured via a voltmeter across the dummy load,
rectified with a diode and capacitor as suggested by KC6WDK. I'm now
getting 12 v there.

Now I'll continue to try to find the source of the subharmonic, and
also the chirp, which is still a little troublesome. I am also going
to try some suggestions made by others who responded.

I realize that I'm rather handicapped, having a 1-watt rig that uses
crystals, and having only a "Technician with Morse Code" license. I
missed passing the General exam by one question. :(

Thanks to all.


On 14 Nov 2003 16:02:58 -0800, (Ashhar Farhan)
wrote:

(John Sandin) wrote in message ...

The problem is, she also heard a signal about half as
strong at a subharmonic (3562 kHz). Does anybody out there know what
might typically cause this to happen with a transmitter like mine? It
didn't occur to me to try a random wire at the receiving end as a
comparison. My receiver is an Icom R71A.


if she has received the signal across the town, then you are putting
out a pretty strong sub-harmonic. a couple of things have to be
checked up:

1) are u sure that the signal is actually due to the crystal? it might
be that the oscillator is freely running on its own. to check that,
test that the signal is stable, if u move ur hand around the coils or
touch them, the signal should not shift its frequency. if it does
that, then the crystal is not really a part of the oscillation.

2) if you have figured out that the crystal is oscillating properly,
next, you might have mistuned your PA to 3.5MHz instead of 7MHz. Try
retuning the PA, disconnect the antenna from the receiver attach a
foot of wire to the output of the tx and tune the tx for maximum
output on 7MHz. Then switch the rx to 3.5MHz and see if you can 'tune
out' the 3.5MHz trace.

these are quick fixes. the larger problem that you are faced with
really is that you don't exactly know what is going on. the above two
solutions are based on diagnosing the problem emperically.

what you really need is a wavemeter. If you have another variable cap
lying around, it might be a good idea to hook one up. It will serve
you for years. It takes about 4 components to wire it up. You can
caliberate it easily using a regular TX for 3.5 and 7. Now you can
couple the wavemeter to each of your TX coils and tune it to the
correct frequency.

if we are to assume that the crystal is actually 7MHz, i see no way
that it can give sub-harmonics in a proper design. the only culprit
could be the tuned circuits.

- farhan


-John Sandin KC0QWE

Remove the "T" to respond by e-mail