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Author Topic: Hallicrafters S-120 Bandspread calibration?  (Read 1509 times)
KX4QP
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Posts: 364




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« Reply #15 on: March 05, 2019, 04:06:01 PM »

I think WWV is off the air? I thought the gov pulled the plug a few months ago amidst much protest.

Last time I looked, their web site still claimed they were broadcasting.  They dropped the weather reports that used to go out hourly, but the time base and such were still going out.  The funding drop was in the 2019 budget request, which I don't know if Congress has even yet managed to accept (or how they amended it).  But at 10 kW,

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.

Actually, I've verified that, as documented in the manual, the bandspread tunes the same sense as the main tuner.  Instructions are to set BS to 100; the main tuner will indicate the actual tuned frequency, and tuning BS down toward zero tunes the frequency lower.  I can set the BS at 100, set the main needle at 4.0 (ish), and tune down through the entire 80 m amateur band; phone first (multiple overlapping SSB conversations, usually -- I think my selectivity isn't good enough to separate them), then often one or two data exchanges, and CW near the bottom (this lands around 20-40 on the BW scale).  Since I have yet to receive anything I can identify as an amateur emission in 20 m, I can't say how the tuning rate varies in that band (the schematic shows four tank circuits for BS, selected in various combinations by the band selector switch).  Same is true for 160; I'm not sure of 40 m (I've heard stuff that I think was in the amateur band, but couldn't always be sure of the band before understanding the operation of the bandspread; nor could I identify the signal).

I get very clear signals from the Christian SW stations in Tennessee and Florida (they're 50 KW or higher, each), and I mentioned the AM signal in Spanish that's right around 10 MHz; a couple times I've picked up broadcasts that online sources said came from China and India (once again, likely very strong state-run stations).

BYU, I like the idea of using a second, highly selective receiver to tune the IF leakage from the primary.  I think the bandspread knob on the S-120 is plenty slow -- the CW portion of 80 m covers about a third of the BS scale.  Now I need to look around for a suitable longwave receiver.
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KB2WIG
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« Reply #16 on: March 05, 2019, 04:55:48 PM »



I copied 10 MC WWV and WWVH "now".


klc
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WW7KE
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« Reply #17 on: March 05, 2019, 08:44:44 PM »

I think WWV is off the air? I thought the gov pulled the plug a few months ago amidst much protest.

WWV and WWVH are still alive and well, both plainly audible in AZ on 5 MHz. 

There was talk about shutting them down, but I doubt it ever went anywhere.  I believe it would require an act of Congress to get rid of them, and I just couldn't see that happening even before last November's election.  So far, it hasn't happened.
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He speaks fluent PSK31, in FT8...  One QSO with him earns you 5BDXCC...  His Wouff Hong has two Wouffs... Hiram Percy Maxim called HIM "The Old Man..."  He is... The Most Interesting Ham In The World!
KX4QP
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Posts: 364




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« Reply #18 on: March 06, 2019, 04:38:36 PM »

Glad to hear folks are still hearing WWV/WWVH.  Neither one comes in here on any frequency, at least with the terrain, antenna, and receiver I have in hand at present.  I live in north-central North Carolina, in a bit of a valley, with hills and what the locals call mountains to the west/northwest.  Even though there are also mountains between me and Ottawa (where CHU is located), it's about 1/3 the distance -- and it is pretty noisy signal.

Once I can get an outdoor antenna up, this S-120 might well bring in WWV on 5 MHz, at least (it seems to do best between 1.5 and 5 MHz).

I'm curious, since they're on 60m(ish), 30m(ish), and 20m(ish), what sort of receivers do folks use?  Will your ham sets tune their (slightly out of our bands) frequency, or are you using SDR or "world radio" receivers?
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N3DT
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« Reply #19 on: March 06, 2019, 05:30:04 PM »

Yes, WWV is still on the air from Ft Collins CO. 2500, 5000, 10000, 15000, and 20000. Not long ago it was on 25000 also. Then there's CHU (SSB) from Ontario I think, on 3330, 7850, 14670. I copy most of them at some time of the day here near DC depending on the conditions. Yes, get yourself a half decent antenna and some kind of feed line so you can get the house noise lower than the antenna signals. Interesting about the bandspread usage. If I set my S38E at 100 on the bandspread, the main dial was way off. With a half decent antenna you should be able to hear the time signals. The SW broadcast stations are very powerful and will overwhelm anything else.

I used to live in Greenbelt when WWV was at Goddard, almost right in my back yard. I have the QSL they sent me for reporting on when they switched over from Goddard to Ft Collins.
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WB6BYU
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« Reply #20 on: March 06, 2019, 08:46:39 PM »

You certainly need a better antenna - that 100 MHz folded dipole is going
to look more like a short circuit across the feedline than an antenna.  Try
disconnecting one side of the feedline from the radio.


Regarding the low frequency receiver, the traditional "Q5'er" was a BC-453,
as there were thousands available surplus in the 1950's, with little other
demand for them.  (They may have been part of the aircraft AN/ARC5 or
SCR-274N "Command Set" installations, which also provided inexpensive
80m and 40m transmitters for many hams on a budget.). I don't see them
as often at swap meets these days, but I know some are still around:  I
might ask my friend how many he has stashed in his basement...
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AC2EU
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WWW

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« Reply #21 on: March 07, 2019, 06:38:23 AM »

I would be *VERY* careful when trying to marry a "hot chassis" set like the s-120 to a standard power system such as what might be used with a BC453.  It's not as if those beacon receivers are plug n play, either, they were designed to run on 28V using a dynamotor for HV DC.  If you find one that is complete with the dynamotor, you will need to clean and re-grease the mechanicals and have a 24 to 28 v @ 10A  power supply to run it. IF there is no dynamotor, a more conventional supply can be built to run off of AC power. In others words, it's not that easy to do, not to mention the capacitor replacements that will inevitably be needed for peak performance or any performance at all due to component aging. 
Believe me , I know, I have restored complete ARC-5 and SCR274 systems.

The Q5er idea works pretty well on the old radios with cold chassis ( with isolating power transformers) but mixing it with a hot chassis radio raises some safety concerns. Even if you were to do this, you still don't have a product detector.
I would feel a whole lot better about it if you used a 50 watt isolation transformer on your s-120 whhich can be obtained at Mouser for about  $15. They are small enough to fit inside a s-38, so I suppose it would fit inside a s-120 also.
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KX4QP
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Posts: 364




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« Reply #22 on: March 07, 2019, 03:03:02 PM »

You certainly need a better antenna - that 100 MHz folded dipole is going
to look more like a short circuit across the feedline than an antenna.  Try
disconnecting one side of the feedline from the radio.

I don't have a feedline as such -- it's just the 300 ohm flat wire (like old TV antenna wire), which goes into molded-on connectors at the T and ends.  At present, at the suggestion of others on this thread, I've got both wires hooked to the same (only) ANT connector on the back of the S-120; the other screw is marked GND.  I previously had just a single wire of the antenna connected, and truthfully, changing it made little or no difference in reception.

The Q5er idea works pretty well on the old radios with cold chassis ( with isolating power transformers) but mixing it with a hot chassis radio raises some safety concerns. Even if you were to do this, you still don't have a product detector.
I would feel a whole lot better about it if you used a 50 watt isolation transformer on your s-120 whhich can be obtained at Mouser for about  $15. They are small enough to fit inside a s-38, so I suppose it would fit inside a s-120 also.

I've heard the S-120 called "hot chassis" previously, and checked for this before powering it up the first time -- my multimeter reports open circuit (on the megohm scale) between both cord prongs and the chassis.  I had intended to install a polarized plug to ensure the chassis connected to neutral, but apparently whoever replaced the filter caps and selenium rectifier with modern parts also rewired the power cord so it no longer connects to the chassis (rectified voltage goes directly to the switch terminals on the volume control, as I recall).  After finding there was no longer a hot chassis, I left the cord untouched.  I suppose I could run a second test, with the receiver powered up in both orientations of the plug, to very there's no voltage from the chassis to the outlet ground.

However, I hadn't read the description of a low-frequency receiver as an auxiliary filter as indicating a direct connection between the two receivers, in any case.  I got the impression that the coupling was more like an antenna on the LF set picking up the IF from the (fiberboard) back panel of the S-120.  Looks like a lot of work to make one of those old aircraft radios work at all, anyway; I can likely get a ham-bands transceiver spruced up for similar effort (and possibly not much more money), and have a much more functional set.  I bought the S-120 because it was cheap, had tubes, and had shortwave bands on the dial (not knowing it was a bottom line model).  I didn't actually expect to be able to just plug it in and run it, I thought I was going to have to recap it and replace the rectifier myself.  It's still got some old wax and paper caps (three, as I recall), but I don't know if replacing them would improve anything in terms of sensitivity or signal-to-noise.
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AC2EU
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« Reply #23 on: March 07, 2019, 03:40:03 PM »

I don't want to belabor the point, but if there is no transformer, it is still a hot chassis. That's OK and there are ways to minimize the hazard such as replacing the chassis to case cap with a modern safety cap an appropriately wired polarized or 3 prong plug (better ) .  Even with these options, if you have a very old or mis-wired ( it happens more often than you might think!) house, things could go snap bang.
Still I would not go interfacing the innards of that radio with the outside world without isolation.

I hope you don't find out what I'm talking about the hard way.  Sad  Cry
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KX4QP
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Posts: 364




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« Reply #24 on: March 08, 2019, 06:17:47 PM »

I hope you don't find out what I'm talking about the hard way.  Sad  Cry

There's nothing in reach from the radio that's conductive and grounded.  I've got a couple metal shelf brackets, but they're mounted with drywall anchors; there is no plumbing in this room (and all the plumbing in this mobile home is plastic pipe anyway).

There's 110 V plate voltage under the chassis, of course, but that's rectified and (fairly well) filtered DC -- it would give a painful shock or a burn, but it's nothing like as hazardous as 60 Hz.

As I've understood it, an isolation transformer mainly limits the current available, in addition to blocking DC voltage, since it turns 120V to some lower figure, then back to 120V (required to operate the radio), still at 60 Hz.  If there's 60 Hz available, the presence or absence of a transformer makes little difference -- 12 V will still run that critical 30 mA across your chest, if you make good contact, and if that's 60 Hz, and there isn't a defibrillator handy (and someone to operate it), you're just as dead as if you grabbed the bus bars in your electrical box.

I've got a club meeting in the morning, I'll try to remember to ask my Elmer about isolation transformers.  I really don't see how they help for the hazard of 120VAC that might find its way to the radio chassis.
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N3DT
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« Reply #25 on: March 08, 2019, 07:03:28 PM »

The isolation transformer removes the direct connection from the AC mains to the ground of the S120. It doesn't change the AC voltage delivered.
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AC2EU
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WWW

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« Reply #26 on: March 08, 2019, 07:25:51 PM »

I was referring to interfacing other devices such as the Q5er where you are bringing wires outside the s-120 box. Also the chassis to case cap can fail or have high leakage which could potentially put 120V on the case! At least change that thing out with a modern safety cap if you don't believe in isolation transformers.

If transformers were so bad, why do you suppose they used them in the more expensive radios?
An isolation type is a 1:1 ratio with about 95% efficiency. It isolates the you from the power grid, so if 120V did appear at the case, there is no current path to ground through other equipment or YOU.
Virtually all modern power supplies use transformer isolated designs!
Also, you CANNOT service an AC/DC radio like the s-120 on a professional AC powered equipment  bench WITHOUT an isolation transformer. ( well, you can, but bad things could happen...)

Please do ask your Elmer about this stuff. I hope he can do a better job of explaining the nuances of this problem that I have!
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WW3QB
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« Reply #27 on: March 09, 2019, 01:39:26 PM »

I had a S-120 as a child. For nostalgia purposes I got another one a few years ago. I could measure significant voltage, but little current on the case to ground, if the power plug was plugged in a certain (bad) way. I put a polarized plug to be sure the case had no voltage on it. Please be sure you put a polarized power plug on this radio. The bandspread, as explained before, just spreads the tuning. It is nearly impossible to tune with just the main tuning. It really is a very poor radio.
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KX4QP
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Posts: 364




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« Reply #28 on: March 09, 2019, 03:18:30 PM »

Also the chassis to case cap can fail or have high leakage which could potentially put 120V on the case!

Chassis to case?  The steel chassis is mounted directly to the steel case; there can't be a capacitor between them.  There is a cap (C11, 0.01 microFarad) between the plate voltage line (labeled as 110V in the schematic) and the system ground, but I don't see anything that verifies that's the chassis (I suspect it is, however; it's a different symbol from the antenna/tuner ground).

Quote
Please do ask your Elmer about this stuff. I hope he can do a better job of explaining the nuances of this problem that I have!

My Elmer, and at least two others at my club meeting, were largely unconcerned.  They first noted that "hot chassis" radios were made and used with a good safety record for thirty-plus years; they also concurred with me that, lacking any continuity between cord conductors and chassis/case, there is no path for supply voltage to ground through the operator (and even when these radios were made with that path, they maintained their safety record despite a 50% chance that a randomly connected plug would put 120V on the case).

I don't know that any of us were aware of a capacitor between plate supply and the case, however; I'll double check that and especially verify that it isn't one of the old, leaky, paper and wax type (if it is, I can replace it).  Even if it were, however, the plate voltage (as previously noted) is rectified and filtered DC, which makes it far less lethal than 60 Hz (accidental contact is likely to result in a burn, rather than heart fibrillation).

At the least, I think I understand what an isolation transformer is supposed to do, now: effectively, it limits the points of additional contact that can lethally shock you if you poke inside a powered radio.  Seems to me running the radio on a GFCI outlet or extension would do the same job as well.

BTW, I'm happy to report that, for the first time since getting this old radio, I've been able to tune WWV (on 15 MHz) with the 100 MHz folded dipole I'm using (borrowed from an FM stereo receiver).  I was sitting here listening to the ticks and tones and station IDs while I replied, but the band appears to have closed; the station is no longer audible (it's getting dark out, and 20m is supposed to be better in daylight).  I've also managed to hear SSB, CW, and data on 40m band (first time I've gotten anything in that band).
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AC2EU
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WWW

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« Reply #29 on: March 09, 2019, 06:53:01 PM »

Quote
don't know that any of us were aware of a capacitor between plate supply and the case,
Me either. The cap is between the floating chassis potential and the case, not the plate supply.

Once you get a better antenna setup, you should be hearing quite a bit more.

Have fun.
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