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Author Topic: Digital Frequency Display for boat anchor receiver?  (Read 1226 times)
G3RZP
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Posts: 1313




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« Reply #15 on: July 22, 2019, 03:42:25 AM »

Quote
I'm not sure on that one, whether the offset needs to be flipped on some bands (don't recall the mixing scheme, but this often needs to be done).

Pardon me while I boggle -- I can't fathom why a radio designer would do that, though I'm sure there's some good reason and it probably has to do with money.

One reason for going from high side injection to low side on (usually) the higher bands is because the strays involved in a multi band HF receiver give tracking difficulties at the top of the range. One marine receiver used harmonic injection on the top range because of that - it covered 15kHz to 28MHz. Plus it isn't always possible to use the circuit that gives best tracking of strays - in a superhet oscillator, from the viewpoint of strays and their effect on tracking, a C and L in series is not always the same  as the identical L in series with the identical C if one of them has one end grounded.
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KB1GMX
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Posts: 1824




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« Reply #16 on: July 22, 2019, 09:55:57 AM »

The S120 is a simple (minimal) 5 tube (aa5 lineup) receiver with single
tuned RF and 455khz IF.

Generally the LO is -455khz.  The 12BE6(same as 6be6) will pull with
tuning adjustment and agc.

BFO is by oscillating IF.  It is unstable as it varies with the setting of the
screen resistance and AGC.

Selectivity is via two IF cans (total of 4 tuned circuits) and is
AM bandwidth (about 10khz).

Images on bands above 10mhz are notable, one has to be careful or use
external preselector.

Sensitivity is not a real issue for 10-40m, though above its a bit on the
weak side.

As to "hot" chassis the physical chassis is not coupled to either side of
the power line unless someone changed it.  It is AC coupled as RF
ground as part of the RF circuits.  If the power cord has DC continuity
to chassis find out why!

I have two of them one stock and the other was my project radio.
The project radio has no tuning above band C (coils were missing).
My mods were a real product detector with BFO for CW/SSB
(12at7/6ar8), added IF 5699 and ceramic filter for 5khz wide IF,
improved audio and a real transformer power supply.    The result
is a fine AM broadcast/SWL radio and really good for 160M AM.

I plan to find another and replace the tubes with FETs and
transistors for 12V operation.  It may get more mods than that.
Its a fun chassis to mod and they can be had very cheap.

Allison
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KX4QP
Member

Posts: 404




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« Reply #17 on: July 22, 2019, 03:42:21 PM »

One of the ways that we use to use to perk up the IF section, was to employ and external IF Amplifier/Detector. I have a old BC-453, that I bought for $5, and used with my HE-30. The BC-453 is tune-able from 190KHz to 550KHz. That might not seem useful, until you realize that it tunes to 455KHz, which is the IF in the HE-30 and the S-120. The BC-453 internally, has a RF Amp, Mixer, Two 85 KHz IF stages, and a Detector/BFO. At the time, it turned a so-so receiver into a really hot receiver. Of course, to make it work you need a power supply and the knowledge to hack into your receiver. But that was then.

Here is a link to something called The Novice Q5er. This is another way we use to get useful performance from a receiver without spending a lot of money.

Hmm.  Seems like if I had a BC-453, I wouldn't even want/need the S-120, I'd just build that Q5er to get my 80/40/20m signal into the command set receiver.  Should handle SSB, too, given a real BFO.  Too bad those are a lot higher than $5 these days, but I just saw one on eBay for about what I have into my S-120 (including shipping).

The S120 is a simple (minimal) 5 tube (aa5 lineup) receiver with single
tuned RF and 455khz IF.

Generally the LO is -455khz.  The 12BE6(same as 6be6) will pull with
tuning adjustment and agc.

BFO is by oscillating IF.  It is unstable as it varies with the setting of the
screen resistance and AGC.

Selectivity is via two IF cans (total of 4 tuned circuits) and is
AM bandwidth (about 10khz).

Images on bands above 10mhz are notable, one has to be careful or use
external preselector.

Sensitivity is not a real issue for 10-40m, though above its a bit on the
weak side.

As to "hot" chassis the physical chassis is not coupled to either side of
the power line unless someone changed it.  It is AC coupled as RF
ground as part of the RF circuits.  If the power cord has DC continuity
to chassis find out why!

I have two of them one stock and the other was my project radio.
The project radio has no tuning above band C (coils were missing).
My mods were a real product detector with BFO for CW/SSB
(12at7/6ar8), added IF 5699 and ceramic filter for 5khz wide IF,
improved audio and a real transformer power supply.    The result
is a fine AM broadcast/SWL radio and really good for 160M AM.

I plan to find another and replace the tubes with FETs and
transistors for 12V operation.  It may get more mods than that.
Its a fun chassis to mod and they can be had very cheap.

Allison

Wow.  So, basically, a couple tubes and the front panel knobs left from the original S-120?

Sounds like another vote for "put this radio in another room, and buy/build a better one for ham use."
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G3RZP
Member

Posts: 1313




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« Reply #18 on: July 23, 2019, 07:15:10 AM »

Allison

Quote
Generally the LO is -455khz.

Do you mean the signal is 455kHz below the LO?

If the LO is 455 below the signal, it has to cover  85 -1195kHz, which would need an enormous tuning gang.....

$69.95 in 1962 - what's that in today's money? The Rolling Meadows plant (now Northrop) probably banged them out for $30!
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KB1GMX
Member

Posts: 1824




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« Reply #19 on: July 23, 2019, 09:04:13 AM »

Yes, the LO is below the IF at about 455khz give or take aging and
tuning for 3 of the 4 bands am broadcast is the exception.

Never played much on AM broadcast with it as there noting
worth hearing there.  So the interest was 1.6 to 10mhz range.

You could tune it up so all of the bands have the LO 455khz above
and likely never know the difference.

It does have a large main tuning cap and a smaller bandspread
tuning cap. Also AM broadcast and the next band up 1.6 to 4.5mhz is the
only one that has enough front end selectivity (1 tuned circuit) to say its
relatively image free. the 4.4 to 11mhz band (C) gets pretty poor
for images at the high end.  the Band D is 11 to 30mhz and tunes
too fast and anything strong enough to be heard at 16mhz will
be heard in two places 900khz apart.

All the higher bands have the LO below the band presumably for
improved stability.  That its stable is also unlikely.  that is how both
of mine were tuned but the service manual does not specify if that
is correct.

Pull a manual, its out there and free.  Its good for restringing the
dial cords and  a schematic.


"Wow.  So, basically, a couple tubes and the front panel knobs left from the original S-120?"

When I look at it the basic radio is good for that.  You have a pair of
tuning mechanisms and a set of simple tuned circuits and two IF cans
mounted for you the rest is utterly forgettable.  Closest thing to a ARC5
for people that like to play and can be had a garage sales and fleas for 5$.

Preserved more or less:
All the tuned circuits that were there and still mounted.  Tuning caps and dial. 
The 12BE6 became a 6be6 and 12ba6 a 6ba6 followed by a 5699 then 6av6 with
6aq5 for audio so I could go from a hot chassis to a transformer powered chassis.
Then added a 12at7 and 6ar8 in a 2x1.5x3 minibox for the BFO and product
detector.  So it acquired three tubes more than original and parallel 6.3V heaters,
120V transformer and 1n4007 diode for a roughly 130V bus for the tubes.  The
goal was to have fun and see how far I could go with it and fit it all inside
(excluding the box with transformers).  The selectivity is narrow enough that for ham
AM use its fine but broadcast suffers some sideband cutting.  Makes a marginal SSB
RX (wide filter and strong signals pull the mixer.).

Would I do it again. No.  I'd solid state it and modify the tuning as: A band for 160M,
B band for 80/75M, C band for 40M, and D band as 20M and add front end selectivity
for fewer images. 

If it were rare I'd not do that but they are common.

Allison
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KM1H
Member

Posts: 5504




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« Reply #20 on: July 23, 2019, 05:29:04 PM »

Reminds me what I did to a pair of cheap used SX-140's a few decades ago, pretty useless as built.
First went double conversion with 262 kHz IF's from auto radios, added a product detector also.
Real nice on AM and useable on CW.

The other used a 455 kHz second conversion with sharp and wider KIWA ceramic IF filters; much more convenient to use on crowded bands.

Both used at our summer home on coastal Maine where room for gear was tight. A rebuilt Knight T-150A did the TX honors 80-6M. Kids grown, home and gear sold; kept the D-104!

Carl
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WW7KE
Member

Posts: 948




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« Reply #21 on: July 23, 2019, 09:19:23 PM »

$69.95 in 1962 - what's that in today's money?

Roughly $500.  You can get a very good receiver for that these days.

Quote
The Rolling Meadows plant (now Northrop) probably banged them out for $30!

The S-120 predates the Rolling Meadows plant, which opened later in the 1960s.  It was the successor to the S-38 line, was manufactured at the old 5th Avenue plant in Chicago, but was discontinued before moving to Rolling Meadows.  It was the last consumer-grade shortwave receiver manufactured by Hallicrafters in the US.
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