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Author Topic: How to tune a particular frequency with homebrew, non-calibrated tuners?  (Read 2190 times)
KX4QP
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Posts: 247




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« Reply #15 on: January 29, 2019, 04:18:38 PM »

Well, I ordered an $8 combination crystal tester and frequency counter kit.  Assuming I can get it together in working condition, that should solve most of the problems.  For that kind of money, it was a no-brainer.

I won't be sure until it arrives how the scale works on the 4" vernier I ordered off eBay (some rotate the entire scale, others have a way to attach a pointer), but it and the smaller verniers I already have do have a "dial" that reads 0-100 as you turn the knob from one end of travel to the other.  With some means to calibrate that against actual megahertz (worst case, put the set into regen oscillation and read the frequency with a wire loop near the tickler, connected to the counter's input), I can either print up a scale, or make a chart of what log reading corresponds to what frequency.

That instrument ought to also let me directly read the frequency of my transmitter when tuning on a dummy load.

I'd have just ordered one in the first place, if I'd known such a thing could be had for under $10.
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KX4OM
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« Reply #16 on: January 29, 2019, 04:32:10 PM »

Those combo boards are based on DL4YHF's freq counter design. I have built several 4 and 5 digit versions of the counters. Wolf's website (search on his call) has several examples built by others. These counters are are aimed at 1 kHz resolution for dial readouts for transmitters and receivers.

Some of the inexpensive 10 digit counters on eBay are 1 Hz resolution.

Ted, KX4OM
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KX4QP
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Posts: 247




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« Reply #17 on: January 29, 2019, 05:00:59 PM »

These counters are are aimed at 1 kHz resolution for dial readouts for transmitters and receivers.

That's pretty much what I'm after.  Not only is that all the accuracy I need for things like meeting a sked on the right frequency, but it's good building practice.  If/when I need a 1 Hz resolution, I'll shop around for that.
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VK6HP
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Posts: 441




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« Reply #18 on: January 29, 2019, 10:03:29 PM »

Reading the thread and your interests, I'm sure you'll have a lot of fun but equally sure that you'll discover for yourself quite a few realities, both in terms of making a usable older-style station and in making operations feasible alongside more modern gear.  But there are plenty of us who enjoy taking time away from bleeding edge technology to enjoy classic and historical radio, so hopefully you'll have plenty of enjoyable contacts.

One suggestion I'd make might seem a bit strange at first in view of your aims but you could look around for a decent receiver, preferably using tubes of course.  You can pick up classic receivers for not too much money and, armed with a crystal calibrator, they offer good bandspread and dial accuracy.  You get a nice box for the shack and perhaps get a functional station ahead of the super-regen receiver but, more importantly, you can use the receiver in a multitude of bench tests, including calibration of your home-brew equipment, assessment of oscillator purity and stability (electrical and mechanical), indication of harmonic and spurious outputs, and a whole host of other things.  

Older receivers come with enough challenges to have their own charm.  But even with very good test equipment, I still find a bench receiver to be very useful

73, Peter.
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G3RZP
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« Reply #19 on: January 30, 2019, 02:44:12 AM »

A still useful piece of equipment that can often be found for under $50 is the BC221 frequency meter from the SCR211. Not only can it be used for frequency measurement, it is also useful for generating a signal of known frequency, checking transmitter for chirp and key clicks and on telephony, checking for gross audio distortion. Has the advantage that it doesn't 'pull' so much as a sensitive receiver when checking transmitter. The Russian copy was supposedly still being made in the early 1970s.
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KX4QP
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« Reply #20 on: January 30, 2019, 04:37:43 PM »

One suggestion I'd make might seem a bit strange at first in view of your aims but you could look around for a decent receiver, preferably using tubes of course.  You can pick up classic receivers for not too much money and, armed with a crystal calibrator, they offer good bandspread and dial accuracy.  You get a nice box for the shack and perhaps get a functional station ahead of the super-regen receiver but, more importantly, you can use the receiver in a multitude of bench tests, including calibration of your home-brew equipment, assessment of oscillator purity and stability (electrical and mechanical), indication of harmonic and spurious outputs, and a whole host of other things.

That's actually a good suggestion I hadn't thought of.  I was thinking existing tube equipment was going to be collector priced, but I just clicked over to eBay and found several multi-band and shortwave tube receivers under $100.  I put in a lowball bid on an old Hallicrafter, we'll see what happens, but there were BIN listings too -- just a little out of the comfort range two days before payday.  When I can spend a little money, I should be able to pick something up that way and, as you note, have a calibrated dial and known working circuit.  I've been watching videos on "restoring" radios like that, too -- which mostly seems to amount to replacing old capacitors that are of types prone to failure, and testing the others, plus cosmetics (which I'm likely to skip since I'm not shopping for the wood cabinets).

I'll still build my own -- but if I have a good receiver, I might build the TX first.
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WB6BYU
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« Reply #21 on: January 30, 2019, 07:23:17 PM »

The receiver local oscillator makes a good signal generator, too.


As a Novice I had a receiver that covered 4 - 11 MHz on one band - the width of the dial pointer was about
equal to the whole 40m band.  (It also had a small tuning knob, which I replaced with a larger one to make
it a bit easier.)  So even with the scale, it wasn't easy to find a specific frequency.  So I had to get clever.

To operate 80m I would set the bandspread knob to zero, swith the Band 1 (AM BC), and tune in one of
our local radio stations at 1470.  Then when I switched to band 2 the radio would be tuned pretty close
to the Novice band, starting at 0 on the bandspread knob (which made a couple turns - it was missing
a scale pointer.)  Rotating the knob about 180 degrees put me close to my transmit frequency.

The transmitter was a single tube affair, so there was no way to turn down the power for a SPOT function.
I certainly could hear it on the dial, but the receiver tended to pull on such a strong signal so I could only
get an approximate position.

On 40m I probably set the bandspread at mid scale and tuned in my transmitter.  It put me somewhere in
the ball park.

It is often possible to find approximate band edges by where signals start (especially in a contest).  I had
to align a transceiver in a logging camp in Alaska this way:  all I had was a 100 kHz crystal calibrator, but
I could determine which marker was which by where the signals changed from CW to SSB.  Once I could
calibrate the dial scale at one point, it gave me a good enough approximation of frequency over the band.

A simple crystal oscillator helps a lot:  you can set the bandspread scale to an appropriate value, tune in
the oscillator with the main tuning knob, and then use the bandspread to tune over a small range.

We didn't bother knowing the exact frequency:  if someone said, "meet me on 3.965", then we'd tune
+/- 10 kHz or more around that, to account for both the difference in dial calibration, but also because
the other station would have to find a clear frequency between the other stations on the band.

Zero beating the other station?  Not with crystal control:  we'd call on our crystal frequency and listen
through the whole 50 kHz band for replies.  (It really caused problems when someone with a transceiver
would call CQ at the top of the band, tune through the band looking for replies, then respond on the
calling station's frequency", because that isn't where we were listening.
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KX4QP
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Posts: 247




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« Reply #22 on: January 31, 2019, 04:31:22 PM »

Well, and so the journey truly begins.  I just won an auction for a Hallicrafters S-120, 1960s vintage 4-tube AM/SW (top of AM up to 30 MHz in three bands) receiver, seller claims it was receiving stations without an antenna at the time of listing (presumably actually with the internal loopstick for AM), which strongly suggests the caps and tubes are at least good enough to operate.  I fully expect to need to replace the main power supply filter, and probably some bumblebee capacitors (a few minutes into a video on restoring this exact model when I paused to check the auction).  Hopefully it'll be in my hands in time to take to the club meeting a week from Saturday, so I can get an Elmer to confirm which caps I need to replace and which are okay even after 55 years (ceramics, good, paper and "bumblebees", bad, but what about electrolytic cans?).  If I've overpaid by a big factor at $25 and about the same for shipping, please don't tell me -- but the four working tubes inside are probably worth at least that.

It's been almost fifty years since I've actually worked on a tube set of any kind (took all the tubes out of my family's B&W TV and tested them to find out why it wasn't working like new -- inconclusive, as I recall, but they bought a color set soon after despite it working as well as before when I was done), so this should be fun.  I've already downloaded the operating manual and schematic.

The dial won't be anything like accurate enough to tune a transmitter by (as described by WB6BYU, the needle will likely obscure most of the 40m CW band, even with the much wider window), but I should be able to follow harmonics from a crystal oscillator up the bands.  If nothing else, I can use it to listen to Radio Cuba, BBC, and Radio Moscow when I get a decent antenna up.
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VK6HP
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Posts: 441




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« Reply #23 on: January 31, 2019, 04:48:42 PM »

Well, that was quick Smiley

One thing you absolutely have to be aware of is that the S-120 looks like it is a transformerless design, meaning that the operating voltages come from circuitry connected directly to the mains.  You have to be extremely careful and, for strong preference, run it via a mains isolation transformer.  I'm not an expert on North American "hot" chassis products but the starting point is to make sure that the "cold" side of the mains (not the active) is connected to the chassis.

Good luck, and be very careful.

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KX4QP
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Posts: 247




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« Reply #24 on: February 01, 2019, 02:22:41 AM »

Well, that was quick Smiley

One thing you absolutely have to be aware of is that the S-120 looks like it is a transformerless design, meaning that the operating voltages come from circuitry connected directly to the mains.  You have to be extremely careful and, for strong preference, run it via a mains isolation transformer.  I'm not an expert on North American "hot" chassis products but the starting point is to make sure that the "cold" side of the mains (not the active) is connected to the chassis.

Good luck, and be very careful.



That's a very good point.  I've heard references to this, including "curtain burner" resistive cords to drop voltage before a simple rectifier/filter power supply.  I think curtain burners were gone by 1960, but I just looked at the schematic, and you're correct, there's no power supply transformer.  One wire of the power cord goes direct to the chassis "ground".  The simple fix for that is to install a new power cord with polarized plug (or put a polarized repair end on the existing cord, at least) to ensure the chassis connects only to the neutral leg (these plugs have a wider prong on one side, so they can be connected only one way).

Looks like the hot wire goes (via rectifier and filter) directly to the plates (110 V plate voltage) on three of the four tubes (the other one is marked for 68 V, not sure yet how they get to that figure), hence such ferocious hum if the filter cap is bad.

I fix power tools for a living, and (seeming alone among the techs in my shop) I'm well aware of the hazards of 60 Hz mains current.  Thirty milliamps is the figure -- the amount that will send your heart into fibrillation, and kill you if someone else doesn't get you restarted.
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G3RZP
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« Reply #25 on: February 01, 2019, 04:27:01 AM »

Quote
The simple fix for that is to install a new power cord with polarized plug (or put a polarized repair end on the existing cord, at least) to ensure the chassis connects only to the neutral leg (these plugs have a wider prong on one side, so they can be connected only one way).

But check that the wall socket is correctly wired - some have found that wasn't the case!
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KX4QP
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Posts: 247




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« Reply #26 on: February 01, 2019, 05:22:56 PM »

Quote
The simple fix for that is to install a new power cord with polarized plug (or put a polarized repair end on the existing cord, at least) to ensure the chassis connects only to the neutral leg (these plugs have a wider prong on one side, so they can be connected only one way).

But check that the wall socket is correctly wired - some have found that wasn't the case!

Yep, I've seen them wired wrong myself.  Easy enough to check.  There's also an easy alteration to the radio (http://www.geojohn.org/Radios/MyRadios/S120/S120.html) to prevent the chassis from being hot even if the outlet is miswired; I'll be doing that.  I might also add the improved BFO from that page, though I've seen other references that suggest what's marked as a BFO is actually a regenerative selectivity enhancer (hence some folks thinking it's a bad BFO) -- that fits the manual references to that control, too; it operates like regen control in terms of tuning assistance, SSB and CW, as well as what sounds suspiciously like "exalted carrier".
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KX4QP
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« Reply #27 on: February 03, 2019, 09:51:19 AM »

 Shocked I'm amazed.  The S-120 arrived today, postal van pulled up and honked.  Yes, that's Sunday delivery, for an eBay item I paid on Friday!  The radio looks really clean, too -- if anything, the eBay photos barely did it justice.  I still need to check for chassis isolation (and at least mark the chassis connected prong, pending cord/plug replacement) and look under the chassis for replaced or original capacitors before I power it up (not so much concerned about letting smoke out, but sellers do exaggerate from time to time and don't always know what they're doing).

Then there's the issue of trying to get a signal inside a metal box (mobile home) or improvising an antenna -- but I'll leave that for after the smoke and hum checks.
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WB6BYU
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« Reply #28 on: February 03, 2019, 11:21:10 AM »

Quote from: ZEISSIKON

...I might also add the improved BFO from that page, though I've seen other references that suggest what's marked as a BFO is actually a regenerative selectivity enhancer...


A set with the regeneration applied at IF rather than at RF was sometimes called a "super-gainer".

I had a set that worked that way:  it had a regenerative IF stage that was adjusted similar to a regenerative
receiver:  on AM it increased gain and selectivty, the same function as a "Q-multiplier" (which was just an
external circuit that added regeneration to an IF stage.  On CW the regeneration control was advanced to
where the stage oscillated to function as a BFO.
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AC2EU
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« Reply #29 on: February 03, 2019, 11:31:35 AM »

I had ( and still have) a s-120 as a kid.
You will find that it doesn't have enough bandspread to dial in stations when the bands get  crowded.
Each bandselect on the s-120 covers a whole lot of MHZ!
Conversely , many of the old communications radios only have only 500Khz spread over the ENTIRE DIAL per amateur band.
Also, The s-120 has a diode detector and BFO rather than a product detector.

All in all SOME receiver is better than none. It's a good starting point to learn the "ins and outs" of radio.
Have fun and welcome to the hobby.
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