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 Post subject: Using 455 mechnanical filters in receiver
PostPosted: Jan Sat 13, 2018 7:28 pm 
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Joined: Mar Sun 14, 2010 2:51 am
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Location: Newport, Oregon, 97365 U.S.A.
After I get myself a desoldering tool ( FR-300 ), i'll have myself some riches in the form of several 455 kHz 2.1 BW Collins mechanical filters
out of old SSB boat radios. But I realized, there's really not an easy way to use these except with hamband - only receivers. I have a
Nat'l NC-173 ( Actually, it's a model "NBS" ) and there's really no graceful way I can think of dropping such a filter into a gen cov
receiver such as this without ruining it for general reception. I am not into major revisions of classic receivers. At the price I got these
at, I suppose I could put one in something like the Lafayette HA-800 ham band receiver. If anyone has ideas for using the filters in
general coverage receivers I would appreciate hearing about how. 2.1 kHz of course is way too sharp for broadcast reception.
tnx, Hue


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 Post subject: Re: Using 455 mechnanical filters in receiver
PostPosted: Jan Sat 13, 2018 8:25 pm 
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Hue,

General coverage receivers are great for AM, however it is often for SSB reception that they have lower performance. A fixed filter of the appropriate bandwidth would help them for SSB or CW. The advantage of any filter is to improve selectivity and reduce noise. Many general coverage receivers have a filter, however the Collins filters are very fine units with steep skirts and low loss. I would suggest they would be most useful in good general coverage receivers that lack a crystal filter. They are also sought after by Collins restorers and are used in home brew receivers. Their 455 KHz center frequency allows them to work with many IF's--normally inserted between the mixer and first IF tube.

Norm

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 Post subject: Re: Using 455 mechnanical filters in receiver
PostPosted: Jan Sat 13, 2018 8:37 pm 
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Location: Newport, Oregon, 97365 U.S.A.
You don't address the issue of how to insert them in general coverage receiver. You don't want to do bandcruising with a 2.1 kHz filter.
The classic boatanchor receivers have the selectivity string all in a series, not a selectable option of alternate filters, so that's where
the problem comes in. How without extensive mods to provide a switchable in/out high selectivity filter. If you put the filter right
before and feeding into a product detector, you have most of the gain before the selectivity, which is not good design. I am still puzzled.
-H


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 Post subject: Re: Using 455 mechnanical filters in receiver
PostPosted: Jan Sat 13, 2018 9:00 pm 
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Hue,

In general, a filter would be connected to the plate of the mixer with a good high voltage capacitor (.001 MFd) protecting it from the plate DC. Its output would feed the grid of the first IF. There could be a multi-position switch that would choose one of several filters or no filter at all.

In a specific case, My 75 A3 manual shows the filters between the first and second If tubes and they are not protected by a capacitor. In another case, my home brew bandpass filter receiver has the filter after the mixer and it must be protected because it contains a capacitor inside that is not of sufficient voltage rating to survive. That filter is a modern one and was made for solid state radios.

So, to use these in an existing radio, one would interrupt the wire from the plate (mixer or early IF stage) to the grid of the IF tube and run that to a switch (could be external in which case shielded RF cable like RG 58 would be needed) and then from the switch back to the grid of the following tube.

I am not an engineer, so I may have overly simplified the procedure, but with a little experimentation, a person could use these filters. I have a home brew receiver project I am starting now that uses Collins 455KHz filters--and I still haven't acquired any.

Norm

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 Post subject: Re: Using 455 mechnanical filters in receiver
PostPosted: Jan Sat 13, 2018 11:46 pm 
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As noted you want the filter as early as possible in the IF chain. It will have to be switched to keep the receiver's original "personality" in intact and either a relay or diode switching method can be used by adding a single control switch (SPST style, nothing fancy). You can often tie the switching function to the phono input or one of the positions of a single element crystal switch using a little ingenuity to avoid having to add another switch. Other possible ways of adding this switch include using one a push/pull switched ganged with a pot to replace an existing pot or a magnetic reed switch hidden behind the panel and activated by placing a magnet on the panel when the filter is needed.

It is likely you will want (and need) some impedance matching along with some gain to keep the level the same so you may end up needing to add a small card to mount the filter, a single stage of transistor amplification, and other supporting components. Power can be from a 9 volt battery or preferably steal a little current from the heater bus and rectify and filter this for any needed preamp and/or diode switching of the filter. While you are adding this you could also easily add provisions for an additional broad AM filter using one of the fairly low cost ceramic filters.

You could also use the filter as part of a nice external accessory box for the receiver consisting of circuits like a noise blanker, the filter, a product detector, and potentially audio filtering and an amp. then your accessory box could be an add-on external accessory for any vintage receiver with a 455 Khz. IF by tapping off a sample of its IF output and feeding it to the accessory box. Your 455 Khz. filter could also be used as part of an external box to add passband tuning to a newer receiver that already has good crystal or mechanical filters. You basically tap the IF from the receiver, assume for example a Heathkit SB/HW series or one of the early Trio/Kenwood rigs with a 3.395 Mhz. second IF, and feed it to your external "project". The box contains a pair of mixers, the 455 Khz. mechanical filter, and a narrow range tunable oscillator. The tunable oscillator is tunable a few kilohertz either side of 3.850 so it converts the receiver IF down to 455 Khz. and then back up to 3.395 mhz. using the same oscillator to drive both mixers. By shifting the oscillator frequency slightly to either side you are still feeding a 3.395 signal back to the receiver IF chain but you have "moved" the 455 Khz. filter across its bandpass allowing it to reject signals on either side of the tuned receiver frequency. This is the same process used for electronic passband tuning in the more advanced receivers except it is all done inside the receiver.

Be sure and also capture any BFO/carrier oscillator crystals from these junked CB sets because those will make fine replacements for dead or missing elements in single crystal filters used in many vintage receivers. And if you get really lucky you will find one of the older CB sets that also contained a Collins AM filter.

There are many uses for these filters in an old receiver just use them in such a way that there are no irreversible mods to a nice old receiver.

Rodger WQ9E


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 Post subject: Re: Using 455 mechnanical filters in receiver
PostPosted: Jan Sun 14, 2018 1:25 am 
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Roger, an external box is out; my rule is that the receiver must appear and basically function as original. Your point about diode switching
and using the "phono input" position of the function switch is a good one and i'll be studying the receiver manuals on how to work this.
-Hue


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 Post subject: Re: Using 455 mechnanical filters in receiver
PostPosted: Jan Sun 14, 2018 1:54 am 
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Hue,

Also quite a few receivers are set up to receive AM one sideband at a time although with a 2.1 to 2.4 Khz mechanical filter that would still be pretty narrow. Those receivers, like the Hallicrafters SX-96 family, place the carrier a little way down the slope of the bandpass but it cannot be attenuated much without distortion issues. AM received with one sideband only can work very well since it greatly reduces the interference from the other side of the carrier.

I haven't tried this technique with an add-on filter in a vintage receiver but I suspect for it to work well in that scenario you would have to realign the IF slightly away from 455 Khz so that the IF transformers are tuned to a frequency center maybe in the middle of the mechanical filter passband. With the typical fairly broad response of the usual 455 Khz. L/C IF strip I suspect you could then tune the receiver so that the mechanical filter favors either the lower or upper sideband of an AM signal and although this would be voice quality only it could be useful when you are trying to pull out a weak station when there is heavy interference only below or above the desired station.

My Hallicrafters SX-88 doesn't have the switch selectable sideband on AM feature of the later 50 Khz. IF Hallicrafters receivers but it does have a very nice choice of bandwidths through it high Q transformers which are better than the later cheaper offerings. With it I can select which sideband to receive on AM by offset tuning to either side of center so that the carrier is placed at one edge of the IF passband and this is how I use it for ham AM operation on 75 meters. In theory you could tune any AM receiver this way but it only works properly when the IF provides a fairly narrow passband with very steep skirts and if you try tuning this way on a typical AM receiver you will get the usual distortion expected from a improperly tuned AM station. This tuning method works well with most mechanical filter equipped receivers and the R-390A can be used this way to advantage when conditions are poor. A 4 Khz. IF bandpass tuned to receive only one sideband on AM provides the same recovered audio response as a traditional receiver with twice the IF bandwidth. Both tuning and IF shape factor are critical with this method because the carrier needs to be right at the edge of the passband or very slightly (maybe 3 db down the slope) but the shape factor/skirt selectivity must be sufficient that the unwanted sideband is very highly attenuated or distortion will result. Experiment with the best tuning to provide good recovered audio along with sufficient carrier.

Rodger WQ9E


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 Post subject: Re: Using 455 mechnanical filters in receiver
PostPosted: Jan Wed 17, 2018 3:15 am 
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If they are the old FA style filters they can be easily implemented.

Tell us the Collins part numbers. Back in the octal socketed tube days hams would use a small minibox with and octal plug on one end and a tube socket on the other. That was the foundation to make in a plug in adapter for the first IF tube with filter and tube on the adapter.

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 Post subject: Re: Using 455 mechnanical filters in receiver
PostPosted: Jan Wed 17, 2018 3:25 am 
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Location: Newport, Oregon, 97365 U.S.A.
These are 2.1 kHz filters. No one would put these in a gen cov receiver as the only selectivity option.
I took a look at my NBS-1 manual last night. The contacts on the mode switch that allow phono input could be put to
use to enable switching diodes, but there's really no simple way to make the mech filter switchable in/out. Requires some
clever thinking. I don't have a good idea yet.


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 Post subject: Re: Using 455 mechnanical filters in receiver
PostPosted: Jan Wed 17, 2018 3:35 am 
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That is why many hams used a plug in module that occupied an IF tube position. Easily reversed. Miniature relays could be used to remotely bypass the filter. But not all mechanical filters are the same and with out knowing what resonating caps are required it is a moot point.

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 Post subject: Re: Using 455 mechnanical filters in receiver
PostPosted: Jan Wed 17, 2018 4:02 am 
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Location: Newport, Oregon, 97365 U.S.A.
Lack of specs is not a problem. The filters are fully marked and all specs are online. The relay approach is possibly the best as the
diodes may have less isolation. However in these receivers the 1st IF transformer is typically a ( single ) crystal filter and inclusion
of a mechanical filter ahead of this requires some kind of adapter. Or an adapter board right in the area. With a MOSFET input buffer
to isolate the mixer tube from the filter matching transformer. At this point, for me, it's something to think about, but i'm also wondering if it is really worth doing. These classic receivers don't actually have to perform up to modern standards.


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 Post subject: Re: Using 455 mechnanical filters in receiver
PostPosted: Jan Wed 17, 2018 4:40 am 
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Hue,

I would use the Collins mechanical filter in place of the receiver selectivity filter. A switch or relay would choose one or the other. The advantage of the Collins filters is they have very steep skirts. The full 2.1 bandwidth is pretty flat and the skirts fall off very steeply. This is different than the old single crystal filters. I have a 2.4KHz modern crystal filter in a home brew tube radio and it works very well for SSB phone reception and even serves tolerably for AM.

Norm

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 Post subject: Re: Using 455 mechnanical filters in receiver
PostPosted: Jan Wed 17, 2018 4:54 am 
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Location: Newport, Oregon, 97365 U.S.A.
Interesting, Norm, on the use of the 2.4 filter for AM. I'll think about that for some other radio. The only filters in these boat radios is however 2.1, so that's a little too narrow.
The single crystal filter in the boatanchors does a real good job on CW, so I wouldn't want to lose that, and it's more of a receiver mod than I want to do, would change it too much. What got me thing about this is that I have 6 of these boat radios with the
455 filter in them. I could always just sell the boards at a hamfest, along with the 150 watt RF PA board. Or, better said, "try to sell them". -Hue


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 Post subject: Re: Using 455 mechnanical filters in receiver
PostPosted: Jan Wed 17, 2018 5:12 am 
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If the receiver is aligned properly you can use the SSB filter to receive AM one sideband at a time by tuning so that the carrier is either at the upper or lower edge of the Collins filter depending upon which sideband you want to receive. You would likely have to align the receiver IF strip to the center of the Collins filter to avoid problems from a skewed IF bandpass creating problems trying to receive AM one sideband at a time but that would depend upon how narrow the existing receiver bandpass.

The big advantage to receiving AM this way is you can often avoid severe interference that is either just above or below the desired station. Hallicrafters SX-96 and later 50 Khz. final IF family members use this system for AM as does the Heathkit Mohawk which copied a lot from the Hallicrafters design. A 2.4 Khz. SSB filter used this way provides response equivalent to a 4.8 Khz. AM filter tuned to center the carrier in the passband. The response pattern is tight but very usable for speech. When operating 75 meter AM I much prefer using a receiver that provides selectable sideband on AM and it would have been great to have such a setup during the busy days of shortwave broadcasting during the 1970s and earlier where it would have allowed pulling out many of the weaker signals that had a strong neighbor 5 Khz. up or down.

Rodger WQ9E


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 Post subject: Re: Using 455 mechnanical filters in receiver
PostPosted: Jan Thu 18, 2018 5:44 pm 
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For AM reception, the 2.1 KC filter is just too narrow for normal listening on a 173. In fact many sidebanders would think that 2.1kc is too narrow for SSB reception under clear frequency conditions. Really if AM were meant to be normally received with one sideband, why wouldn't it be transmitted with just a carrier and one sideband. Lol.

I have used the Collins Mechanical filter plugin adapters on my SP-600, and National 183Ds successfully. But those adapters have the 6 KC filter fitted.

Kiwa may still make their cascaded LC filter modules, in reasonable AM bandwidths. They also carry Murata filters. Most would install easily into a BA receiver.

Al


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 Post subject: Re: Using 455 mechnanical filters in receiver
PostPosted: Jan Thu 18, 2018 9:08 pm 
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Al,

I agree 2.1 is very narrow for AM but in situations where communications quality is all that is necessary and nothing else will work a narrow filter can be very beneficial There were a few times when I was contesting with my Drake C line that using the 1.5 Khz. filter on SSB allowed me to add to my total through pulling out stations that were otherwise unintelligible. It doesn't sound nice but with the passband tuning equipped R-4C it was usable. With a NC-173 or similar vintage receiver if you install a 2.1 Khz. filter rather than try to tune so that the carrier is in the passband in many situations it will work better to put the receiver in CW mode, tune so the transmitted carrier is down the slope of the passband, and adjust the BFO for proper carrier reinsertion-the so called "exalted carrier" method. Again it isn't something you would want to do all of the time but it is another tool when conditions are rough.

There are times when the ability to receive one sideband on AM is a great advantage. Many times I have been the net control for the Saturday morning regional net on 3885 and using just one sideband allowed me to take check-ins from weak stations that would otherwise be covered by the sidebands from another AM group on 3880 or some fairly obnoxious SSB types that would hang out around 3890. The major concern with receiving AM this way is some of the vintage SSB transmitters provided "compatible AM" that only transmitted one sideband so you have to be careful not to miss them. A Hallicrafters SX-101 or similar is great for this type of operation because once you zero beat a station you can select upper or lower sideband at will with no retuning and the station response will be exactly the same. The system used by Hammarlund won't provide this type of performance because in the real world the values used to shift the frequency of the low frequency IF between upper and lower are never perfect; the Hallicrafters system switches via high or low side injection so once aligned and properly tuned it is perfect.

My SX-88 has great selectivity but Halli didn't put the switch selectable sideband in it so I have to bump the tuning to select either upper or lower. With the very steep slopes of its high Q IF section it works extremely well in this service but it isn't as convenient as the later and lower cost descendants.

A SX-101A with the bandpass set to 5 Khz. has the same recovered AM audio bandpass as a traditional AM receiver with a 10 Khz. bandpass but is much more immune to off frequency interference. I loved operating with my HRO-50T1 paired with the Ranger/Desk KW just like the Johnson print ads showed the station but it also meant disappointing some folks who would have liked to have participated in the net but didn't have the signal strength to make it through the QRM. A lot of the folks who have a good experience getting their "feet wet" with lower power AM from a transceiver are far more likely to get involved and put together a more serious vintage station with a medium to high power high level AM rig. Even with narrow selectivity and one sideband using my SX-101A and later SX-88 I couldn't pull everyone out but I gave them a fighting chance to check in instead of having a frustrating experience.

Rodger WQ9E


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 Post subject: Re: Using 455 mechnanical filters in receiver
PostPosted: Jan Fri 19, 2018 12:15 am 
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We have had this discussion before regarding the Hallicrafters single sideband AM receivers in other threads in this forum. Hallicrafters gave up on real AM transmitters after the HT-20, which was quite early on. And many of their receivers following the SX-28a and SX-42/SX-62 followed suit.

I prefer the way Hammarlund achieves their selectable sideband. I've aligned my 180s and 180as so there's minimal difference when switching from upper to lower.

But....Hammarlund gives the receiver operator a choice to also receive both sidebands. That's a huge advantage for me. In a standard AM detector, the detected audio is the vector sum of the energy from both sidebands. Yes there are cases under extreme QRM that ECSS reception is advantageous. But having a choice is a huge plus.

But getting back to the original inquiry, I would only consider using that 2.1kc filter in a 173, if I had the choice or option of switching it out of the circuit as needed.

Al


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 Post subject: Re: Using 455 mechnanical filters in receiver
PostPosted: Jan Thu 25, 2018 9:13 am 
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One thing to keep in mind is to keep dc off "mechanical" filters. The Collins filters don't like dc as it will bias the magnets at each end of the filter, for the ceramic variety it can cause migration of the plating of the ceramic wafer holders and cause all sorts of grief when that happens. A small cap is used in each case.


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 Post subject: Re: Using 455 mechnanical filters in receiver
PostPosted: Jan Thu 25, 2018 5:55 pm 
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To add to what db said above, modern SSB and bandpass filters have capacitors inside and these are not always high voltage capacitors because the intended application and market is for low voltage situations--and--high voltage capacitors take more space. It is often hard to get information from the manufacturer or seller what DC voltage the filter is rated for--so as a precaution, I would place a capacitor in series on the plate side when used with tube radio circuits.

Norm

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 Post subject: Re: Using 455 mechnanical filters in receiver
PostPosted: Feb Mon 05, 2018 5:12 am 
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Its sounds too simple to work but...Could you just bridge the input and output of a 2.1khz filter with a small capacitance to slightly widen the response? Or some other technique to accomplish the same thing. I suppose I should have looked up crystal filter construction before asking. :wink:


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