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Vintage Amateur and Military Radio Amateur/military receivers and transmitters, morse, and any other related vintage comms equipment.

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Old 2nd Nov 2022, 2:18 pm   #61
Radio Wrangler
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Default Re: Improving Selectivity In Vintage Receivers

Spaced coupling usually comes down to an empirically chosen separation, but it works well, providing you have enough space in your screening can. It also is a reasonably 'flat' coupling factor. Top or bottom coupling via capacitors or inductors tend to slope the filter skirts, so if you want filters symmetrical in linear frequency, then you have to employ different coupling techniques between some sections in order to cancel the slope.

Back in the 30's, spaced coupling saves a component or two. Tapping was also good.

Nowadays you can buy decent inductors off the shelf from coilcraft, but not tapped ones. We've lost a useful tool. Spaced coupling and tapped coils still have a number of advantages.

David
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Old 2nd Nov 2022, 2:25 pm   #62
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Default Re: Improving Selectivity In Vintage Receivers

Of course some receivers used mechanically-variable spacing of the IFT windings so you could choose between a sharp peak, something a bit flatter/wider, or the double-humped wider bandwidth of overcoupling if you wanted hifi reception.

I also remember seeing something in a German marine radio where the two IF coils in a can were wound on ferrite pot-cores and largely screened from each other by Mumetal, but there was a mechanically-movable ferrite rod between them that could be rotated by a panel control to vary the coupling.
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Old 2nd Nov 2022, 2:47 pm   #63
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Default Re: Improving Selectivity In Vintage Receivers

When we're taking about "vintage receivers" on this thread, are we including normal, domestic, AM sets? You see, I'm unashamedly easily confused and when I see a reference to vintage receivers (especially on here) I immediately think that the subject is vintage domestic receivers (because nine times out of ten, it is), but I'm getting the strong impression that the thread is about comms receivers? Will someone confirm please?!
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Old 2nd Nov 2022, 3:41 pm   #64
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Default Re: Improving Selectivity In Vintage Receivers

It started as a branch from a thread about the Eddystone 840C which is probably as selective as you would need for a broadcast receiver but a bit too broad for a communications receiver.

My own interest is in an HRO which is probably somewhat more selective than the Eddystone 840 but still rather lacking for SSB reception.
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Old 2nd Nov 2022, 4:10 pm   #65
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Default Re: Improving Selectivity In Vintage Receivers

I don't see why domestic receivers cannot be included, at least to get the conversation started, and it could branch to form a separate threads if it get's going (just as Jon says, this thread branched off the one on the Eddystone 840).

A common feature is that many/most domestics have 455kHz IF, so again, the possible use of inexpensive ceramic filters comes in to play. Something of a compromise for the purists perhaps, but offering a relatively easy and cheap way of going some way to improving selectivity.

Their use in valve receivers has been quite limited previously, but they show up in a number of solid state receivers.

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Old 2nd Nov 2022, 4:14 pm   #66
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Default Re: Improving Selectivity In Vintage Receivers

So it's not really about domestic receivers. I'm fine with that, just wanted to know. But saying that, with the dearth of AM stations around these days, I've never found selectivity to be a problem.
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Old 2nd Nov 2022, 4:19 pm   #67
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Default Re: Improving Selectivity In Vintage Receivers

I think you've misread my post #65; we've not excluded domestic Rx.

This thread emerged from one about Eddystone 840's, and by accident, seems currently to be focussed on HRO's.

As suggested, people could start a conversation on here about domestics, and spin it off as a separate thread if it gets going.

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Old 2nd Nov 2022, 4:30 pm   #68
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Default Re: Improving Selectivity In Vintage Receivers

I had seen that post Baz, but the thread is mostly about comms receivers. All understood now, thanks.
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Old 2nd Nov 2022, 4:33 pm   #69
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Default Re: Improving Selectivity In Vintage Receivers

Quote:
Originally Posted by G6Tanuki View Post
Of course some receivers used mechanically-variable spacing of the IFT windings so you could choose between a sharp peak, something a bit flatter/wider, or the double-humped wider bandwidth ...
Eddystone used this arrangement in several of its receivers. I have an S750 - double superhet - that has a front panel control which varies the spacing between the primary and secondary windings in each of the two LF IFTs.
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Old 2nd Nov 2022, 4:36 pm   #70
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Default Re: Improving Selectivity In Vintage Receivers

It's the borderland.

Sets which try to do somewhat more than is needed for a domestic broadcast entertainment receiver, but don't go as far as high performance mode-specific filters for Dingle Sideband, Morse and data modes. You could use them for shortwave broadcast and some of themhave multiple or variable bandwidth for AM speech and maybe Morse.

The AR88 and HRO mentioned have switched bandwidths making them OK for AM music, AM speech and can bring in a quartz crystal to make them quite decently narrow morse receivers. They pre-date the adoption of other modes.

So there is a fair old mixture of vintage receivers out there. Sets for government/military, sets for spooks, sets for radio amateurs, sets for maritime use, sets for aviation ground stations, sets for shortwave listeners, and then all the domestic broadcast jobs.

Preferred modes and roles moved around a lot over the years.

If you wanted to categorise them, you'd need a two-dimensional matrix to spread them out by what and by when.

Some broadcast receivers of the posh sort sported variable bandwidth so you could bask in the audio quality of a locak station in 'wide' or go narrow to winkle out something distant.

I have a couple of Racal receivers with switched banks of mode-specific filters for CW, SSB two bandwidths of AM and also filters for teleprinter modes.

My AR88 has five bandwidthe, two by switching coil coupling, and three with a quartz crystal added to make it sharp for separating Morse signals.

My Eddystone EA12 moves coils in IFTs to vary coupling to do AM and to make a fairly poor go of SSB. A crystal comes in for morse. THis set is right from the beginning of SSB on the amateur bands in the UK and is a half-way step towards getting it right.

To complete the variants, My Icom 7700 mixes everything to switched crystal filters near 60MHz, then mixes down, digitises and does all the real selectivity in DSP. With this I can play with bandwidths, offsets, slopes and notches all at once with shapes for all the current modes. Gross overkill? Yup. Works a treat, but not vintage for many years to come.

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Old 2nd Nov 2022, 4:37 pm   #71
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Default Re: Improving Selectivity In Vintage Receivers

Thinking on the HRO IF transformer. With the spacing they have chosen between windings the bottom coil is not vary far into the screening can.

The can mounts on the painted steel chassis which will only be about half a coil diameter away. At the top in the attached photo. Seems a bit close to me...
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Old 2nd Nov 2022, 4:53 pm   #72
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Default Re: Improving Selectivity In Vintage Receivers

I think the HRO was designed mainly by Mr Millen at National; I guess you could drop him a line and ask about that .

When you start taking the cans off, you're certainly left in no doubt that the HRO is a vintage receiver!

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Old 2nd Nov 2022, 5:53 pm   #73
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Default Re: Improving Selectivity In Vintage Receivers

The solder reminds you that it was last molten about 84 years ago. This one is from 1938.
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Old 6th Nov 2022, 4:22 pm   #74
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Default Re: Improving Selectivity In Vintage Receivers

Had a better look at the first tuned circuit inside the HRO crystal filter box.

Two identical windings (looking very much like the ~2.2mH found in the grid tuned circuit for 1st IF).

The upper "pie" tuned by 2x 100pF to ground, the lower pie an untuned coupling winding from the mixer valve anode. The spacing of the windings is as close as practicable in this case.
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Old 6th Nov 2022, 4:35 pm   #75
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Default Re: Improving Selectivity In Vintage Receivers

Yes, looks just like the one out of mine. Are you still waiting for the arrival of your FET or is there any progress there?

On the question of how we like our SSB to sound, I recall now a thread on VRRR about adding audio processors to vintage receivers, typical some circuitry limiting the passband and I thing doing some degree of expansion?

However, a chap came on with a post about what I think were called DSP filters (digital) with a link to a website enabling you yo listen to their effect. If was amazing and might well be worth adding to a vintage receiver. I'll see (later) if I can find the thread.

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Old 6th Nov 2022, 4:44 pm   #76
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Default Re: Improving Selectivity In Vintage Receivers

I haven't done anything about FETs. It's not a direction I want to go unless I absolutely have to. I would prefer to match the 455 ceramic filter with passive components if possible.

I think that will probably mean losing the crystal filter or getting involved in a lot of nasty switching to retain it. For all the use it is (unless you are a real CW fiend) I doubt it's worth it.

Presently thinking on how to test the modified filter box. I would like to measure the response before attempting to graft it into a working HRO.
I suppose an oscilloscope might be OK on the 1st IF grid connection. The extra 30pF might have to be lost by a compensating adjustment of the trimmer.

How to drive it at the mixer end of things is what I wonder now. Sig gen + series resistor to "pad it up" perhaps? Ideas very welcome.

The impedance here is critical to get right or the driving impedance presented to the ceramic filter will be wrong. It's the effect of that on passband ripple etc which is the whole point of the measurements after all.

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Old 6th Nov 2022, 4:58 pm   #77
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Default Re: Improving Selectivity In Vintage Receivers

I took the decision to lose the existing xtal filter some time ago.

If a new filter arrangement (ceramics) can be made to work purely with passives, that's fine, but I'm happy to put FET's or BJT's in (or even a valve).

I agree that some kind of a "test rig" would be very good. Could a wobbulator be at the heart of it?

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Old 2nd Dec 2022, 9:04 pm   #78
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Default Re: Improving Selectivity In Vintage Receivers

Im slowly assembling the Rx from QST 1966 originally discussed here https://www.vintage-radio.net/forum/...d.php?t=195648

The original uses one single tuned circuit at 455kHz followed by a normal IFT. I wondered whether using an IFT in place of the single tuned circuit would improve selectivity, including it at shown in the attached PDF. Of course, the plan is to use ceramic filters in place of the two crystals.

I suspect there may be some subtleties lacking in my arrangement; if anyone could advise Id appreciate the input.

B
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Old 3rd Dec 2022, 10:11 pm   #79
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Default Re: Improving Selectivity In Vintage Receivers

That circuit uses a four-crystal lattice filter, split into two sections separated by some IF stages to buffer one from the other, and also to give a better gain-loss profile down the strip, helping dynamic range.

To make it work, you will need four carefully manufactured crystals, all on different frequencies. The pushpull arrangement helps to cancel the case capacitances (Czero) of the two crystals and usually relies on planting the series resonance of one element on the parallel resonance frequency of the other. So you can view the circuit as needing two pairs of crystals. If you make the second pair the same as the first pair, then the in'band ripple of the filter is exaggerated - doubled over one section. So proper design has different quartz design in the two stages. One pair have their frequencies closer together than the other pair, so the people making the crystals need to be able to control the spacing of the series and parallel modes.

To give you an idea of what's involved, find the construction articles for the G2DAF receiver. G R B Thornley did two versions. The later one used a Kokusai mechanical filter, but the earlier version used a homebrew crystal lattice filter with the series/parallel planting trick.

Back in that day you could find ex-military FT243 crystal units for a variety of channels, and Thornley wrote of which channels to hunt for and how to re-grind them by hand to get them on the wanted frequencies. He also included the design of test jigs for measuring the things. Let's just say it was a large undertaking. The Kokusai was seen as a large improvement in making this state of the (then) art receiver achievable.

If you go down this path, you get a reasonable (but not excellent) filter for SSB and not much use for any other mode.

You've entered the mode-specific filter world, and you'd likely want other filters to be available as well.

The old FDM telephone system used stacked SSB channels on coax cables or over microwave links. Channel filters were BIG business and Bell Labs were in on the act, to try to get costs down. Out of Bell Labs came the concept of the ladder filter. It fixed a number of unpleasentnesses of lattice filters and could be made where all the crystals were essentially similar. The amateur fraternity didn't notice for a while, and then as awareness grew, latched onto the concept with a death-grip. It works and it saves a lot of trouble.

The ladder filter as usually done in amateur circles produces an asymmetric response best suited for only one of the sidebands, upper or lower depending on receiver architecture. You can afford to throw a lot of crystals at it and get a classic mode-matched response. There are techniques for making the thing symmetrical, and with plenty of stages you can get a goos USB/LSB filter by shifting the CIO.

One snag is that the bandwidth you want pushes the centre frequency into the 3-12MHz region. You have to find a way to live with this.

A motorist in a joke stopped to ask directions from a pedestrian and got the answer "Sure, if I was wanting to go there, I wouldn't start from here"

My inclination would be to leave an HRO unmolested as an example of a very important waypoint in receiver evolution and well-suited to the modes in use in its era. In the fifties and sixties, there were people with HROs, AR88, CR100 and the only viable path to serious SSB listening was to try to soup the old receiver up. From a vintage radio viewpoint, butchery ensued.

If you want to explore receiver development, why not start a receiver from scratch? Maybe steal a 455kHz IF feed from an existing receiver?

Or why not restore a receiver from the age when SSB had become usual?

It's that 'where do you start from?' question again. If you have a puppy, but want a pony, surgery may not be the best answer.

There are SSB era receivers out there which are quite good. The Eddystone EA 12 is useable but not very good on selectivity, but demand makes them expensive. There are RA1772s around which are good. I got given an RA1792 for free, but they are a bit of an underperformer.

The change over to SSB was around the time of the change to transistors. I have an RA1217 which has true USB and LSB crystal filters, is as cute as hell, but the front end is easily overloaded, so has to be driven carefully.

As I see it, there are two options I'd consider. Start with something with the modes and filters wanted, and restore it to good condition, or get stuck into homebrew and have some fun exploring receiver architectures and stage design.

Getting crystals to specified parameters to make lattice filters used to be expensive and difficult. It is now much more so.

Old scrappers of amateur radio transceivers, especially ones with problems can be affordable and sources of crystal mode-specific filters

At the radio rallies of the eighties, I bought up a lot of 1.4MHz crystal filters which were to marine main receiver specs. CW, teleprinter, AM narrow, AM wide. I also got given some 68.6MHz high IF roofing filters. I've got a good supply to play around with.

The G-QRP club have some stocks of 6-pole 9MHz SSB filters and the matching USB/LSB carrier crystals. Advertised on the back cover of Sprat. I measured them for George and they're quite good.

Anywy, those crystal pairs in that circuit are up to something quite complicated, and dropping in off the shef ceramic elements is unlikely to be a success. You'd need some special custom-made ceramic elements to get decent results.

Don't read this a gloom and despondency, there are several other starting points which can get you to interesting places.

David
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Old 3rd Dec 2022, 10:28 pm   #80
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Default Re: Improving Selectivity In Vintage Receivers

Well, I am just "playing" with that little receiver originally described in the April '66 QST as a bit of fun.

As reported in the thread https://www.vintage-radio.net/forum/...d.php?t=195648, the original (WWII) crystals used in that radio are just unobtainable now, but people are reporting tolerable results using the small ceramic filters.

I'd go to my TS530 and IC718 (sorry) for serious listening. The QST Rx is just a "little winter project". That being so, do you think the extra transformer would work in the configuration shown or have I missed something?

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