Advice on SX-101A
 

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

On 9/20/05 2:54 PM, in article , "Antonio
Vernucci" wrote:

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.

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



You should measure the L.O. Signal at the 1st mixer. Low injection voltage
can cause lots of strange intermod problems.

Don

Antonio Vernucci wrote:


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.

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

Tony....

I think there is something wrong. The manual I downloaded from BAMA says
the oscillator frequency is ABOVE the signal frequency on ALL bands!

73, Roger


--
Remove tilde (~) to reply

Remember the USS Liberty (AGTR-5)
http://ussliberty.org/

Tony....
=20
I think there is something wrong. The manual I downloaded from BAMA =
says
the oscillator frequency is ABOVE the signal frequency on ALL bands!
=20
73, Roger

Hi Roger,

what you say is interesting.

I have an original SX-101A manual, which I presume should be the same as =
the BAMA copy. Could you please tell me at which page you read that? In =
my manual I was unable to find any mention of the oscillator frequency.

I am pretty sure of the oscillator frequencies, as I measured them both =
with a frequency meter and with an oscilloscope (the oscillator waveform =
period on 20 meters is somewhat longer than on 40 meters, no question =
about that). And the strong 9.5 MHz phantoms frequencies are exactly =
what they should be with an halved oscillator frequency.

On the other hand my receiver was owned by a person who never put his =
hands in it, but even hypothesizing that someone did something in it, =
what could he have done to make the oscillator frequency exactly half of =
what one would expect to be, and on 10, 15 and 20 meters only? Halving =
the oscillator frequency would cause the dial scale to be no longer =
correct; matching is instead fairly good (with normal tolerances for a =
receiver of that kind).

73

Tony I0JX / K0JX

Roger,

I carefully re-checked the manual and I found the text you mentioned.

It literally reads: "The 1st conversion oscillator operates at a =
frequency higher than the incoming signal by an amount equal to the =
first intermediate frequency of 1650 kc/s"

That sentence clearly states that the beat frequency is higher than that =
of the incoming RF signal but, in my opinion, it does not absolutely =
also imply that the beat frequency corresponds to the fundamental =
oscillator frequency.

In other words, it is true that the beat frequency is always higher than =
the incoming RF signal, but the beat frequency could be obtained by =
taking the second harmonic of the oscillator frequency. And the sentence =
would still be meaningful.

I am taking that interpretation as I am 101% sure that, on 10, 15 and 20 =
meters, the beat frequency is twice the fundamental oscillator =
frequency.

73

Tony, I0JX=20

Antonio Vernucci wrote:
Tony....

I think there is something wrong. The manual I downloaded from BAMA says
the oscillator frequency is ABOVE the signal frequency on ALL bands!

73, Roger


Hi Roger,

what you say is interesting.

I have an original SX-101A manual, which I presume should be the same as the BAMA copy. Could you please tell me at which page you read that? In my manual I was unable to find any mention of the oscillator frequency.

I am pretty sure of the oscillator frequencies, as I measured them both with a frequency meter and with an oscilloscope (the oscillator waveform period on 20 meters is somewhat longer than on 40 meters, no question about that). And the strong 9.5 MHz phantoms frequencies are exactly what they should be with an halved oscillator frequency.

On the other hand my receiver was owned by a person who never put his hands in it, but even hypothesizing that someone did something in it, what could he have done to make the oscillator frequency exactly half of what one would expect to be, and on 10, 15 and 20 meters only? Halving the oscillator frequency would cause the dial scale to be no longer correct; matching is instead fairly good (with normal tolerances for a receiver of that kind).

73

Tony I0JX / K0JX


Hi Tony....

It's on page 15 under RF ALIGNMENT. It's the last sentence before the
actual proceedure. I have no idea what could be wrong with your receiver
but to use the second harmonic for mixing is very unusual. The only time
I've seen it used is when a manufacturer was trying to add a "VHF" band
to an early receiver and couldn't get reliable oscillation at the
fundamental frequency.

73, Roger

--
Remove tilde (~) to reply

Remember the USS Liberty (AGTR-5)
http://ussliberty.org/

Antonio Vernucci wrote:
Roger,

I carefully re-checked the manual and I found the text you mentioned.

It literally reads: "The 1st conversion oscillator operates at a frequency higher than the incoming signal by an amount equal to the first intermediate frequency of 1650 kc/s"

That sentence clearly states that the beat frequency is higher than that of the incoming RF signal but, in my opinion, it does not absolutely also imply that the beat frequency corresponds to the fundamental oscillator frequency.

In other words, it is true that the beat frequency is always higher than the incoming RF signal, but the beat frequency could be obtained by taking the second harmonic of the oscillator frequency. And the sentence would still be meaningful.

I am taking that interpretation as I am 101% sure that, on 10, 15 and 20 meters, the beat frequency is twice the fundamental oscillator frequency.

73

Tony, I0JX


Hi Tony....

The "beat" frequency is generally the difference (IF) frequency. In a
receiver the "beat frequency oscillator (BFO) is used to generate a
"beat" frequency in the audio range for CW reception. My copy of the
manual clearly states that the OSCILLATOR frequency is HIGHER than
the signal frequency on ALL bands.

73, Roger

--
Remove tilde (~) to reply

Remember the USS Liberty (AGTR-5)
http://ussliberty.org/

Hi Tony....
=20
The "beat" frequency is generally the difference (IF) frequency. In a
receiver the "beat frequency oscillator (BFO) is used to generate a
"beat" frequency in the audio range for CW reception. My copy of the
manual clearly states that the OSCILLATOR frequency is HIGHER than
the signal frequency on ALL bands.
=20
73, Roger

Well, I used the wrong term - beat frequency- due to my limited command =
of the English language.

In summary, what I am trying to say for the sake of justifying what I =
measure here is that the sentence "the oscillator frequency is always =
higher than the signal frequency" should have been more precisely =
written "the frequency used to convert the incoming RF signal to IF is =
always higher than that of the RF signal". In this way the sentence =
would become compatible with using the second harmonic of the oscillator =
for converting the RF signal (on 20 meters the measured fundamental =
oscillator frequency is lower than the RF signal, but its second =
harmonic is anyway higher).

Anyway, I agree with you that using the second harmonic of the =
oscillator is rather odd, but it is a fact that, in addition to the =
frequency meter and the oscilloscope measurement results, the phantom =
carriers coming from the 9.5 MHz band are exactly where they should be =
if the second oscillator harmonic is used.

It looks like a kind of a mistery. I hope that an SX101A owner will take =
care to measure the oscillator frequency at his first convenient =
occasion.

73

Tony, I0JX


the SX-101A, the frequency that converts the RF signal into the IF =
signal is always higher than the RF signal. This is absoultely true, on =
the other hand the receiver dial shows the same sense on all bands (if, =
on a certain band, the frequency that converts the RF signal into the IF =
signal would be lower than the RF signal, then the relevant dial would =
go in the opposite direction).

Agreed.

"Antonio Vernucci" wrote in message
...
Hi Tony....

The "beat" frequency is generally the difference (IF) frequency. In a
receiver the "beat frequency oscillator (BFO) is used to generate a
"beat" frequency in the audio range for CW reception. My copy of the
manual clearly states that the OSCILLATOR frequency is HIGHER than
the signal frequency on ALL bands.

73, Roger

Well, I used the wrong term - beat frequency- due to my limited command of
the English language.

In summary, what I am trying to say for the sake of justifying what I
measure here is that the sentence "the oscillator frequency is always higher
than the signal frequency" should have been more precisely written "the
frequency used to convert the incoming RF signal to IF is always higher than
that of the RF signal". In this way the sentence would become compatible
with using the second harmonic of the oscillator for converting the RF
signal (on 20 meters the measured fundamental oscillator frequency is lower
than the RF signal, but its second harmonic is anyway higher).

Anyway, I agree with you that using the second harmonic of the oscillator is
rather odd

[SNIP]

Oh, I don't know. The Heath HR-10 ham receiver, while admittedly in a lesser
league, did this on 15 and 10 meters and admitted it. I would strongly
suspect that many of the lower end sets did this, but didn't fess-up.

[SNIP]
=20
Oh, I don't know. The Heath HR-10 ham receiver, while admittedly in a =
lesser=20
league, did this on 15 and 10 meters and admitted it. I would strongly =

suspect that many of the lower end sets did this, but didn't fess-up.=20
=20

I am not sure of their rationale. Perhaps the stability of 30 MHz =
oscillator is, in practice, more than twice worse than that of a 15 MHz =
oscillator, so they preferred to work at lower frequency and take the =
second harmonic. They did not even care to filter the oscillator =
frequency so as to remove the fundamental and leave only the harmonic.

By the way I put a parallel LC circuit in series with the 20 meter RF =
input transformers, and, adjusting its for resonance around 9.6 MHz, the =
BCs disappear completely. Adjustment is quite critical and the resulting =
BCs attenuation is tremendously high.

73

Tony I0JX / K0JX