I have recently got a very good single-owner Hallicrafters SX-101A, =
mechanically and electrically intact. The radio is very sensititive on =
all bands and appears to still be well calibrated, despite it has not =
been used for more than 30 years.

A problem I noticed, especially on 14 MHz, is the presence of several =
strong AM broadcast (BC) signals across the band. Interesting to note =
that the receiver preselector control yields no change at all in the BC =
signal strength, whilst it yields a very clear peak in the 14-MHz =
background noise.

The first idea which came to my mind was poor image reception. With an =
IF frequency of 1,650 kHz, the image should be plus/minus 3,300 kHz from =
the actual 14-MHz receive frequency (i.e. in the 10.7 or 17.3 MHz =
ranges) . But, on a separate receiver, I could hear no BC signals at all =
on the expected frequencies.

I then connected a signal generator to the SX-101A and I realized that =
the receiver, when on 14-MHz, is very sensitive to a carrier in the 9.5 =
MHz range.

So, I reconnected the antenna to the SX-101, and, with the aid of a =
separate receiver tuned around 9.5 MHz, I was immediately able to =
identify the BC stations heard on the 14-MHz band with the SX-101A.

I found the following frequency relationhips:

- apparent BC carrier frequencies on SX-101A: 14139, 14189, 14309 kHz
- corresponding BC real carrier frequencies: 9545, 9570, 9630 kHz

The frequency difference between any two 14-MHz carriers is exactly =
twice the real one (i.e. that in the 9.5 MHz range).

I do not believe in intermodulation, because attenuating the input RF =
signal (using an attenuator) cause an equal decrease of the BC signals =
and the real 14-MHz signals (i.e. no improvement of the =
wanted-signal-to-interference ratio).

I believe more in something that has to do with second harmonics, but I =
have not yet been able to derive the equation.

Questions

1) any idea on the mechanism by which BC signals in the 9.5 MHz range =
get through so strong?
2) did you have a similar experience with your SX-101A. Or, in other =
words, is it a design problem or an adjustment problem?

Thanks and 73

Tony, I0JX / K0JX

I found a possible solution:

- with the receiver tuned at 14,139 kHz, the local oscillator works at =
14,139+1,650=3D15,789 kHz
- should the local oscillator also have a component at half its =
frequency (i.e. 7,894.5 kHz), that component would convert an incoming =
9,545 kHz signal to the 1,650 kHz IF
- this method also applies to the other two frequency cases=20

Any ideas on the reason and on the cure?

73

Tony I0JX / K0JX

If no other fix comes along ,do this ;connect a series resonant circuit
to ground in the if path .use a small coil form and a variable cap
,tune it with grid dipper before you install it.. This circuit should
be a sharp trap and can easily be tuned to the interfering frequency to
remove the culprit..This may not be top notch engineering,but, it
always works for me..We used to call it a suckout trap... GL W4PQW

If no other fix comes along ,do this ;connect a series resonant circuit
to ground in the if path .use a small coil form and a variable cap
,tune it with grid dipper before you install it.. This circuit should
be a sharp trap and can easily be tuned to the interfering frequency to
remove the culprit..This may not be top notch engineering,but, it
always works for me..We used to call it a suckout trap... GL W4PQW

Antonio Vernucci wrote:
I found a possible solution:

- with the receiver tuned at 14,139 kHz, the local oscillator works at 14,139+1,650=15,789 kHz
- should the local oscillator also have a component at half its frequency (i.e. 7,894.5 kHz), that component would convert an incoming 9,545 kHz signal to the 1,650 kHz IF
- this method also applies to the other two frequency cases

Any ideas on the reason and on the cure?

73

Tony I0JX / K0JX

I'd use an O'scope to look at the local osc. - see if the waveform is
"funky"... If it's not a nice sine-wave - that could be the source of
your "sub-harmonic"... which might cause the image you're picking up. Be
careful when probing with a scope - you'll want to use some sort of weak
/ high impedance coupling - certainly not a direct connection with a 1:1
probe, etc. If the local is producing some odd waveform - check the
power supplies / decoupling / tuning of the osc. Someone may have
cranked a slug or trimmer cap into some weird tuning that "sorta"
works... but not where it should be.

best regards...
--
randy guttery

A Tender Tale - a page dedicated to those Ships and Crews
so vital to the United States Silent Service:
http://tendertale.com

I'd use an O'scope to look at the local osc. - see if the waveform is=20
"funky"... If it's not a nice sine-wave - that could be the source of =

your "sub-harmonic"... which might cause the image you're picking up. =
Be=20
careful when probing with a scope - you'll want to use some sort of =
weak=20
/ high impedance coupling - certainly not a direct connection with a =
1:1=20
probe, etc. If the local is producing some odd waveform - check the=20
power supplies / decoupling / tuning of the osc. Someone may have=20
cranked a slug or trimmer cap into some weird tuning that "sorta"=20
works... but not where it should be.
=20
best regards...
--=20
randy guttery

Hi Randy,

sorry for my late reply, but I have been out of town.

Thanks for the advice. By making some measurements, it was easy to =
determine what the problem is.

As expected, on 80 and 40 meters I measured the conversion oscillator =
frequency to be 1,650 kHz (i.e. the IF value) higher than the receive =
frequency, .

Conversely, on 10, 15 and 20 meters, the frequency meter indicated that =
the oscillator fundamental frequency runs at HALF the figure one would =
expect. For instance, when the receiver dial is at 14.000 kHz, the =
oscillator runs at 7,825 kHz and the converter tube then works on its =
second harmonic at 15,650 kHz (equal to 14,000 + 1,650). Measuring the =
oscillator waveform period with an oscilloscope, it was easy to confirm =
that the fundamental is at 7,825 kHz. The waveform is not sinusoidal and =
then has a rich harmonics content.

This is just the Hallicrafters design approach, not a problem of my =
receiver. Probably they found it easier to build a high-stability =
oscillator at a lower frequency and exploit the second harmonic.

But, with the oscillator fundamental at 7,825 kHz, the receiver will =
receive both 14,000 kHz and, even better, 9,475 kHz, unless the RF stage =
provides a sufficient block for the latter frequency. =20

Unfortunately, in Europe we have terrific BC signals in the 9.5-MHz =
range, that pass through the receiver RF stage tuned coils, =
independently of the frequency they are tuned at. Problem is that their =
ultimate rejection is too low, and peaking the preselector does not help =
at all.

The next step will be to try putting a 9.5-MHz band stop filter at =
receiver input.

73

Tony, I0JX

If no other fix comes along ,do this ;connect a series resonant =
circuit
to ground in the if path .use a small coil form and a variable cap
,tune it with grid dipper before you install it.. This circuit should
be a sharp trap and can easily be tuned to the interfering frequency =
to
remove the culprit..This may not be top notch engineering,but, it
always works for me..We used to call it a suckout trap... GL W4PQW

Hi Randy,

sorry for my late reply, but I have been out of town.

Thanks for the advice, and you suggestion is just what I am going to do =
next.

As a matter of fact, by making some measurements, it was easy to =
determine what the problem is.

As expected, on 80 and 40 meters I measured the conversion oscillator =
frequency to be 1,650 kHz (i.e. the IF value) higher than the receive =
frequency, .

Conversely, on 10, 15 and 20 meters, the frequency meter indicated that =
the oscillator fundamental frequency runs at HALF the figure one would =
expect. For instance, when the receiver dial is at 14.000 kHz, the =
oscillator runs at 7,825 kHz and the converter tube then works on its =
second harmonic at 15,650 kHz (equal to 14,000 + 1,650). Measuring the =
oscillator waveform period with an oscilloscope, it was easy to confirm =
that the fundamental is at 7,825 kHz. The waveform is not sinusoidal and =
then has a rich harmonics content.

This is just the Hallicrafters design approach, not a problem of my =
receiver. Probably they found it easier to build a high-stability =
oscillator at a lower frequency and exploit the second harmonic.

But, with the oscillator fundamental at 7,825 kHz, the receiver will =
receive both 14,000 kHz and, even better, 9,475 kHz, unless the RF stage =
provides a sufficient block for the latter frequency. =20

Unfortunately, in Europe we have terrific BC signals in the 9.5-MHz =
range, that pass through the receiver RF stage tuned coils, =
independently of the frequency they are tuned at. Problem is that their =
ultimate rejection is too low, and peaking the preselector does not help =
at all.

73

Tony, I0JX

"Antonio Vernucci" wrote

Unfortunately, in Europe we have terrific BC signals in the 9.5-MHz range,
that pass through the receiver RF stage tuned coils, independently of the
frequency they are tuned at. Problem is that their ultimate rejection is too
low, and peaking the preselector does not help at all.

*** Tony,out of sheer curiousity,what are the BC signals in the 9.5 MHz
range?

Brian Goldsmith.

*** Tony,out of sheer curiousity,what are the BC signals in the 9.5 =
MHz=20
range?
=20
Brian Goldsmith.=20

Brian,

these are very strong BCs speaking languages sometimes difficult to =
identify.

Examples:
9,330 kHz speaking French S 9+40
9,345 kHz speaking unidentified language S 9+30
9,355 kHz arab music S 9+40
9,375 kHz arab music S 9+40
9,390 kHz seems to be dutch S9+60+++
9,410 kHz BBC world service in english S 9+60
9,420 kHz arab music S 9+50
9,440 kHz speaking english S 9+40
9,460 kHz speaking unidentified language S 9+60
9,480 kHz speaking portoguese S 9+60
9,495 kHz speaking unidentified language (arab?) S 9+60++++
etc.
etc.
etc.

Don't you hear them in the US? Lucky man.

73

Tony, I0JX

If no other fix comes along ,do this ;connect a series resonant =
circuit
to ground in the if path .use a small coil form and a variable cap
,tune it with grid dipper before you install it.. This circuit should
be a sharp trap and can easily be tuned to the interfering frequency =
to
remove the culprit..This may not be top notch engineering,but, it
always works for me..We used to call it a suckout trap... GL W4PQW

Hi Randy,

sorry for my late reply, but I have been out of town.

Thanks for the advice, and you suggestion is just what I am going to do =
next.

As a matter of fact, by making some measurements, it was easy to =
determine what the problem is.

As expected, on 80 and 40 meters I measured the conversion oscillator =
frequency to be 1,650 kHz (i.e. the IF value) higher than the receive =
frequency, .

Conversely, on 10, 15 and 20 meters, the frequency meter indicated that =
the oscillator fundamental frequency runs at HALF the figure one would =
expect. For instance, when the receiver dial is at 14.000 kHz, the =
oscillator runs at 7,825 kHz and the converter tube then works on its =
second harmonic at 15,650 kHz (equal to 14,000 + 1,650). Measuring the =
oscillator waveform period with an oscilloscope, it was easy to confirm =
that the fundamental is at 7,825 kHz. The waveform is not sinusoidal and =
then has a rich harmonics content.

This is just the Hallicrafters design approach, not a problem of my =
receiver. Probably they found it easier to build a high-stability =
oscillator at a lower frequency and exploit the second harmonic.

But, with the oscillator fundamental at 7,825 kHz, the receiver will =
receive both 14,000 kHz and, even better, 9,475 kHz, unless the RF stage =
provides a sufficient block for the latter frequency. =20

Unfortunately, in Europe we have terrific BC signals in the 9.5-MHz =
range, that pass through the receiver RF stage tuned coils, =
independently of the frequency they are tuned at. Problem is that their =
ultimate rejection is too low, and peaking the preselector does not help =
at all.

73

Tony, I0JX