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Author Topic: Hallicrafters S-120 Bandspread calibration?  (Read 1517 times)
KX4QP
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« on: March 03, 2019, 05:34:29 AM »

I've been listening to ham bands on my S-120 (it's all I've got at present that will receive in the HF bands).  It actually seems to do reasonably well with an FM antenna from an old stereo receiver; especially on 80 m, I get lots of CW and some SSB phone on any given night (and the "BFO" -- regen IF stage -- does a reasonable job of making CW audible and SSB intelligible).

One question I have, however, is whether there's any data on the calibration of the Bandspread scale.  It runs from 0 to 100, and also affects the tuning frequency (turn down the bandspread and the tuning center goes down as well).  It does a fine job on CW when set at zero, though I think it's still not as narrow as it should be for CW listening (I can almost always hear two stations at difference pitches on 80m if I'm getting anything).  For SSB, I find I have to turn it up to around 30 to get reasonable voice sound.

I don't know whether that implies that I'm getting around 3 kHz spread at that 30 setting, or if that's just a coincidence; clearly, zero is wider than the 50-150 Hz I'd want for best separation of CW.  Anyone know what the range is?  The manual (available for download) doesn't given any idea.
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K9YLI
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« Reply #1 on: March 03, 2019, 07:49:32 AM »

how band spread works..  you set the main dial to the  maker on the  band in question. usually  on   halli's  there is a  small round  marker  .. from   'band spread 0"   which way  does the  number change onthe band spread dial.
if   up band, then the   dot is at   say  14000 or  7000 or  3600..
when you set the main tuning on the dot  then  the  band spread       spreads out that  little  black  bar  to  the width of the   band spread.
main tuning  for  20 meters is about  a quarter inch..  the band spread widens it out to   several inches.

again    calibratin of the  numbers on band spread  relay on main tuning being on the    round  dot.
  On  short wave listening,, set  band spread  log scale to  50..
 then  main tuning will be  way off  but when you hear a station, you can  tune  up  or  down band  a bit with   band spread to sneak between statins.

practice  practice  practice.
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AC2EU
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« Reply #2 on: March 03, 2019, 08:32:09 AM »

All good tips, but beyond band spread, the CW separation comes down to having a prodect detector and narrow bandwidth Ifs and/or filters.
However, what is good for CW, is not good for SWL.  Voice is almost unintelligible through a CW filter.  You have to be able to select these options according to what you are trying to receive. This is the difference between a communications receiver and a shortwave radio.
The S-120 is a compromise to keep within a price range.
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WB6BYU
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« Reply #3 on: March 03, 2019, 08:48:40 AM »

I think you are confusing "Bandspread" and "Bandwidth".

"Bandspread" is a fine tuning control.  That's why it shifts the marked frequency.
Typically you would set Bandspread to zero, set the main tuning control to the top
end of a ham band, and then use the Bandspread control to tune through the band
at a slower tuning rate than the main dial provides.  It only changes the receive
frequency, not the width of the receiver passband.

I suspect that the only Bandwidth adjustment you have on that radio is the BFO
control in the regenerative IF stage.  The closer you set it to the critical point
there the stage just starts oscillating, the narrower the receiver bandwidth will be.
(Just as with a regenerative receiver.)   I don't know how sharp you can make it,
but you can experiment with an AM broadcast station and see if you can hear the
reduced audio bandwidth when the control is set to where it is just oscillating.
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KX4QP
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« Reply #4 on: March 03, 2019, 01:02:13 PM »

I think you are confusing "Bandspread" and "Bandwidth".

"Bandspread" is a fine tuning control.  That's why it shifts the marked frequency.
Typically you would set Bandspread to zero, set the main tuning control to the top
end of a ham band, and then use the Bandspread control to tune through the band
at a slower tuning rate than the main dial provides.  It only changes the receive
frequency, not the width of the receiver passband.

I suspect that the only Bandwidth adjustment you have on that radio is the BFO
control in the regenerative IF stage.  The closer you set it to the critical point
there the stage just starts oscillating, the narrower the receiver bandwidth will be.
(Just as with a regenerative receiver.)   I don't know how sharp you can make it,
but you can experiment with an AM broadcast station and see if you can hear the
reduced audio bandwidth when the control is set to where it is just oscillating.

The manufacturer's manual for the S-120 seems to say that the Bandspread controls bandwidth to some extent, as well as acting as a fine tuning (tuning center moves down in frequency from the default bandspread setting of 100, where the dial nominally matches the frequency tuned -- in this instance, it's off a couple needle widths). 

That said, lacking a true BFO, I have to set the "BFO" regen control in oscillation (above the "crashing sound" region) to even hear CW.  As with any regen, however, when set too high this reduces sensitivity as it narrows tuning, and as I've noted above, I can't make it narrow enough to fully separate CW stations that are close together in frequency.  I do hear them at different pitches, which lets my ear (mostly) separate them.

I do, however, find that SSB is almost unintelligible with the Bandspread set too low -- seems to work better around 30 on its scale, where CW works right down to 0 (though it's a bit hard to tell where I'm tuned, since in 80m the needle is around 4.2 MHz when I'm tuned to the CW segment, and CHU at 3.33 MHz comes up close to 4.0 on the scale at that bandspread).  AM transmissions are even worse; even with regen off or used only below oscillation for the sensitivity boost, when bandspread is set very low, AM (whether broadcast or shortwave) is almost unintelligible.

Looking at the schematic, it appears the bandspread system is an entire parallel tuner, connected to the first grid in the 12BE6 mixer tube, while the primary tuner goes to the third grid in the same tube (second and fourth are connected to plate voltage, like a pentode's screen, and the 5th is internally connected to the cathode like the suppressor in a pentode).  I'm not sure whether that acts as a variable width roofing filter, or only changes the primary frequency (the coils in the bandspread aren't coupled the to the RF transformers in the main tuner, as far as I can see).  I don't see, however, why that control would be labeled one thing if it only does another.
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AC2EU
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« Reply #5 on: March 03, 2019, 03:12:55 PM »

Quote
Looking at the schematic, it appears the bandspread system is an entire parallel tuner, connected to the first grid in the 12BE6 mixer tube, while the primary tuner goes to the third grid in the same tube (second and fourth are connected to plate voltage, like a pentode's screen, and the 5th is internally connected to the cathode like the suppressor in a pentode).  I'm not sure whether that acts as a variable width roofing filter, or only changes the primary frequency (the coils in the bandspread aren't coupled the to the RF transformers in the main tuner, as far as I can see).  I don't see, however, why that control would be labeled one thing if it only does another.
   

The band spread is nothing more than a fine frequency control . Think of the main tuning as "coarse" and band spread as "fine".
A previous poster explained it pretty well. If you refer to the manual , it will tell you where to set the main to get the best use of the band spread to move though the marked section of the main dial with more precision. Bandwidth is a whole other matter...
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KX4QP
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« Reply #6 on: March 03, 2019, 03:51:16 PM »

Okay, I don't have any evidence that Bandspread does anything other than fine tune -- and I can think of one reason it might have that label instead of, for instance, "Fine Tuning" -- because it effectively "spreads" the band you're tuning, to take several turns of the Bandspread knob to cover a small fraction of the selected band.

I'm still working on getting actual communications hardware in hand -- money limits and collectibility of tube equipment work against me.  I may wind up going back to my original plan of building a regen receiver with plug-in coils for band selection.  It's my understanding that a true regen can be made almost arbitrarily narrow if you have an RF amp stage to prevent re-radiation of oscillation (which is still necessary to hear CW and reconstruct SSB).  It would probably cost as much to build a superhet better than the S-120 for this purpose as it would to buy a tube-era multi-band transceiver -- and I don't want to modify the only working SW radio I have (because at the minimum, I'd have none for a period of weeks while I work on it).

At a minimum, I need better sensitivity, better selectivity (ideally an adjustable filter to separate CW and SSB transmissions or selectable filters < 200 Hz and ~2.5 KHz wide for those purposes), and lower internal noise.  A real BFO and product detector would be desirable, but the parts count (along with cost and troubleshooting effort) keeps going higher and higher on that path; I've seen demonstration videos that suggest a well made regen with two or three tubes can do the job I need.
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N8AUC
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« Reply #7 on: March 03, 2019, 05:03:09 PM »

I used to have an S-120 years ago. It was better than nothing, but not by much, and mine was rather deaf
above 20 meters. There is no calibration to the bandspread tuning. It's just 0-100, and the actual frequency
spread varies by band. The S-120 only has one IF bandwidth, mainly because it was a rather low budget
receiver when it was new. The bandspread is nothing more than a fine tuning control and it has no effect
on the receiver bandwidth.

My best advice to you is to get a crystal calibrator, also called a marker generator. Preferably one that does
multiple outputs. Back in the day, this was a standard piece of equipment that all hams had. Better receivers
had them built in. Many transceivers had them built in as well. Sometimes the crystal calibrator was sold as an
extra cost option (Drake did this with the 2-B if I remember right). It's how you knew what frequency you were
tuned to (or near) in the days before radios had digital frequency displays.

There was a schematic and parts list for one in the 1983 ARRL Handbook (see page 16-12 and 16-13) that had
selectable outputs at 100KHz, 50KHz, and 25KHz. An etching pattern for a PC board is also there.
It used a 100KHz crystal, a 4001 CMOS quad 2-input NOR gate, a 4013 CMOS dual D type flip-flop, and an
LF-353N dual op-amp. I built one of these many years ago, and it worked quite well. All you had to do was
zero beat the oscillator to WWV, and you really were "close enough for government work".

FAR circuits also sells a bare PC board for a variant of this circuit that uses TTL parts which aren't quite as
sensitive to damage from ESD (electrostatic discharge), and they often include a copy of the original
article so you have the schematic and parts list.

73 de N8AUC
Eric
« Last Edit: March 03, 2019, 05:23:15 PM by N8AUC » Logged
N8AUC
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Posts: 587




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« Reply #8 on: March 03, 2019, 05:30:25 PM »

The way bandspread tuning worked, is that you would first set it at zero.
Then tune the main tuning to the bottom of the band you wanted to listen to.
For 40 meters, you'd tune the main tuning to 7 MHz. And the way you knew
you were at 7MHz, was that you'd use your calibrator to generate a signal that
you would tune to zero best. Once you did that, you'd tune around the band
using your bandspread tuning, which was a much finer resolution tuning control
than the receiver main tuning was.

Hope this helps!

73 de N8AUC
Eric
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N8AUC
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Posts: 587




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« Reply #9 on: March 03, 2019, 05:47:18 PM »

Update...
Rather than find all those old parts to build a marker generator,
QRPGuys as one as a kit for $15. Can't beat that.

https://qrpguys.com/k7qo-marker-generator

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K9YLI
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Posts: 1315




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« Reply #10 on: March 04, 2019, 07:43:21 AM »

I may be reading between the lines here.   I have used  BFO to tune  SSB..
Remember  SSB  uses   lower  side on  40 and 80 and  uppper on 20 and above.
So the   BFO  has to be set the opposite side of   zero beat    for the different     upper/lower  bands.
Using the  'wrong' side will make  SSB   almost  readable.   If you find this, put  bfo on  the other side of  zero beat.    Once you get it right,  make with a pencil
and you should be able to  tune  ssb   with just the main tuning functions..
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KX4QP
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Posts: 364




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« Reply #11 on: March 04, 2019, 04:03:43 PM »

I may be reading between the lines here.   I have used  BFO to tune  SSB..
Remember  SSB  uses   lower  side on  40 and 80 and  uppper on 20 and above.
So the   BFO  has to be set the opposite side of   zero beat    for the different     upper/lower  bands.
Using the  'wrong' side will make  SSB   almost  readable.   If you find this, put  bfo on  the other side of  zero beat.    Once you get it right,  make with a pencil
and you should be able to  tune  ssb   with just the main tuning functions..


Sadly, the S-120 doesn't have a real BFO -- it has a sort of regen taken off the IF amp tube.  It'll oscillate, but it'll do so equally for two stations 910 kHz apart (I don't see anything in the circuit diagram that would suggest there's an upper/lower filter anywhere).  I guess if I get distorted voice (which I've so far gotten only on 80m band) I just need to tune up by 910 kHz (for LSB) to let the "BFO" fill in the correct sideband.  So far, I've only gotten amateur signals on 80m and once or twice on 40m.  The radio picks up on higher bands -- I get a Spanish SW broadcast station that seems to be in Spain (2500 miles from North Carolina, near enough) at around 10 MHz, and a couple from, I think, Chile (transequatorial?) on the tuner's top band (10-30 MHz), but those are probably 50 kW or stronger.

@N8AUC, just ordered one of those.  I've already got a frequency counter/crystal tester kit (as yet unbuilt).  Unfortunately, I can't zero beat with WWV to test this; I have yet to get WWV on the S-120 on any of its frequencies, likely due to my inadequate antenna.  It's as far away as Spain, but there are more mountains in between.  I might be able to pick out a harmonic that will zero beat to CHU (3.33 MHz), which I can get pretty reliably after dark...
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AC2EU
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WWW

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« Reply #12 on: March 04, 2019, 06:53:57 PM »

I think WWV is off the air? I thought the gov pulled the plug a few months ago amidst much protest.
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N3DT
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« Reply #13 on: March 04, 2019, 07:18:02 PM »

I'm pretty sure BYU is right. The bandspread on Hallicrafters when set to 0 and tuned up to 100, tunes backwards from the main dial. That is if you set the main dial to 7000 and the bandspread to 0 and then start tuning the bandspread up towards 100, you are actually tuning lower than 7000. Like BYU said, put the BS at 0, the main tuning to the top (high freq end) of the amateur band and the BS will tung down through the band. It's up to you to figure out where the 7000 is on the BS dial if you start at 7300 and tune down. A 1KHz marker will tell you. It's probably not very linear either. And I doubt that the 0-100 will cover the whole band, it will probably cover about 1MHz or more so you don't really get much bandspread. Higher the freq, the worse it gets. They're barely usable above 40M. But 40M is almost always usable.

I used to use my S38E when I was a novice and it's a challenge to tune in CW, you can easily hear several stations at the same time, but with diff tones unless they're the same freq. However it is do-able but not pleasant. Consider it a challenge if that's what you have to do.

I doubt there is any bandwidth adjustment on your 120.

With a strong 1KHz generator you will probably get lots of image frequencies in there too which will confuse the issue big time. Image rejection in these radios was maybe 20dB. Images will be weaker, but it may be hard to tell.

You really need something better, sorry to say. The Hammarlund HQ series were much better.
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WB6BYU
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« Reply #14 on: March 05, 2019, 06:14:36 AM »

Quote from: ZEISSIKON

...I guess if I get distorted voice (which I've so far gotten only on 80m band) I just need to tune up by 910 kHz (for LSB) to let the "BFO" fill in the correct sideband...



As youvtune through an SSB signal (especially with the Bandspread control, which gives
you a better sense of relative frequency) you'll find that you get usable audio on the
higher frequency side of the signal on 40 and 80m, but on the lower side of the signal
on the higher ham bands.  Also, the tuning is quite touchy to get intelligible audio,
especially compared to tuning in an AM signal.


Sounds like you need a better antenna: try connecting both wires of your folded dipole
to the same antenna terminal on the back of the set. Or run a wire out the window.


One old technique for improving selectivity was to use a WWII surplus aircraft receiver
that tunes 200-500 kHz:  a wire stuck through a hole in the cabinet picked up the IF
signal from the main receiver, which was tuned in with the second receiver, which had
much better selectivity due to the 70 kHz IF.  This approach requires no modification
to the primary receiver, and uses the BFO in the second receiver.

You can also build various types of audio filters to go between the receiver and your
headphones, either active circuits (tubes, transistors, or ICs), or passive ones that
just use coils and capacitors (though the coils need to be much larger than those
for RF use).

One problem with sharp selectivity is that you need a slow tuning rate to get the
desired station centered in the passband.
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