Racal Preselection & Protection unit - UK Vintage Radio Repair and Restoration Discussion Forum

vintage_8bit · 24th Oct 2021, 3:24 pm

Hi. I have just obtained a Racal RA 17L. (Might ned some help with that later), anyway it came with an extra 19" rack sub assembly. Looking at the attached images can anyone identify it & tell me what its purpose is? I'm sure it will not be of any use to me but interested in what it does. Colin

Sean Williams · 24th Oct 2021, 3:29 pm

Hi Colin,

This is inserted in the antenna lead before a receiver - it is another tuned circuit which helps keep unwanted signals out of the RF section of the receiver.

Quite a useful bit of kit, and will also possibly contain a quantity of amplification, which is sometimes also helpful on the higher frequencies.

The protection unit function comes into its own when you are trying to operate a receiver on a multi-transmitter site - the extra rejection of out of band signals really saves the receiver, especially if there are several tens of volts of stray RF floating around.

Radio Wrangler · 24th Oct 2021, 3:39 pm

RA17s suffer a number of spurs and images. Its little internal preselector isn't enough. This unit fixes that problem.

David

vintage_8bit · 24th Oct 2021, 6:05 pm

Thanks gents. I was hoping to get rid of it due to its size. Noting the comment "Quite a useful bit of kit" and David's post, I'm not sure what to do with it now. I expect the RA 17 will more than suit my needs on its own. I wonder if there's a manual for the preselector unit somewhere? Colin

Radio Wrangler · 24th Oct 2021, 6:44 pm

The preselectors are a lot harder to find than RA17s, so be sure you really will never want one before selling it.

David

Dave757 · 24th Oct 2021, 9:15 pm

Hi Colin,

51 page technical Manual at www.pa3esy.nl/index-gb.htm
Select receivers from menu and go down to Racal.

Kind regards
Dave
G0ELJ

vintage_8bit · 24th Oct 2021, 9:41 pm

Thanks for the link. I have had a good read & now have the manual, only a couple of valves in it (M8136's) and a bridge rec. I'll hang on to it for now then. It will be a few weeks before I get round to the Ra17. Now where can I hide it....

Refugee · 25th Oct 2021, 2:25 am

If you want to do some serious short wave listening it will almost certainly get rid of the hover board charger noise from the aerial feed to pretty well any short wave receiver.

vintage_8bit · 25th Oct 2021, 2:03 pm

Ok. Change of plan. I'll move on my Marconi r.f. TF 1066B/1 genny to make space for it. I will put the RA 17 on the shelf & when time permits look at the Preselector first. I have an Eddystone S640 I can try in on. Colin.

turretslug · 26th Oct 2021, 12:57 am

As Crocodile Dundee might have put it, "this is a preselector", made to a specification rather than a price by folk who had established a reputation for knowing what they were at. Just the mechanics of the device with its sturdy partitioned case and the six ganged tuned circuits with their worm drives etc. would have cost a small fortune. The valves are professional-spec ECC82s connected in cascode mode acting as low-noise, strong-signal capable buffers with only as much gain as is sensible. They don't make them like that any more and they don't grow on trees, think carefully before moving it on, provided you've got the space!

Radio Wrangler · 26th Oct 2021, 9:50 am

Then there's also the SSB adaptor to look out for......

David

vintage_8bit · 26th Oct 2021, 11:04 am

I'm looking forward to getting it on the bench, spurred on by all these positive comments.

In the past & on other communications receivers I've just used the on-board BFO to listen to SSB. What does the SSB adapter do differently? Colin

Radio Wrangler · 26th Oct 2021, 12:40 pm

It's an ssb-dedicated IF filter, a proper product detector etc. There is a big improvement in audio quality and clarity. I used an RA117 for many years, with just the BFO for SSB demod, but you begin to hanker for something easier to use and clearer to listen to. In the end I never came across an SSB unit (there are several variants including ISB) so I went and bought a new ICOM IC765.

Draker4c · 26th Oct 2021, 4:21 pm

Hi Colin
You have a pm.
Hugh G1AUR

Synchrodyne · 27th Oct 2021, 6:19 am

Some information on the Racal SSB adaptors may be found here: http://www.chavfreezone.me.uk/racal/...-adapters.html.

I suppose that you could say that the addition of an SSB adaptor took the RA17 into the capability realm of point-to-point SSB/ISB receivers of the time, I’d guess with a combination performance that was probably somewhere above that of the single-box Marconi HR22, but below that of the full-height rack HR21. In those days point-to-point receivers required an AFC system to track the pilot carrier, something that with steadily improving transmitter stability had become optional by the later 1970s (Marconi H2540 era). One of the Racal SSB adaptors was configured for broadcast relay use, with 6 kHz audio bandwidth, and could be used to provide synchronous detection of HF AM signals, thus minimizing selective fading distortion.

Cheers,

Radio Wrangler · 27th Oct 2021, 10:43 am

The target market for the RA17 were military and governmental organisations. At the time, these people used CW Morse code, FSK for teleprinters, and AM for speech. There wasn't any significant interest in spending money on SSB capability.

So the RA17 got narrow filters and a BFO for Morse, that 1.2kHz filter and the BFO for FSK to feed a teleprinter via a 'terminal unit' that FSK de-modulated audio tones, and the wider bandwidths with the BFO off to do AM.

But SSB was a growing thing. It's spectral efficiency and its power efficiency were attractive, so Racal brought out SSB adaptor boxes with a whole duplicate final IF in them, with a balanced 'product' detector and a stable BFO preset to the right frequencies.

So the RA17 straddles the rise of SSB in military/governmental radio systems.

SSB was invented as a method of cramming multiple conversations onto a single phone line and dates back to the 1920s, but it took a while from invention to getting rolled out in any quantity. These systems were called FDM, frequency division multiplex, and eventually were cramming up to 2600 channels onto a single coax line in the 1970s. They ended as digital techniques replaced them.

It was some radio amateurs who tried using SSB techniques over the air instead of over wires. One prominent exponent of the time was called Arthur (Art) Collins. Yes, that Collins, he went on to found Collins radio. Bell had got what they wanted for their phone lines, they had much less interest in radio things.

So, Collins receivers have SSB capability from early on, It's where they came from.

Racal sets after the RA17/RA117 have SSB facilities built in. When they did the cute little RA1217 and RA217, their first transistor sets, they got full-blooded SSB filters product detectors and accurate oscillators for carrier reinsertion. Filters became optional an so you could order all sorts of combinations, you didn't have to have the SSB filters. As early transistor jobs, the fornt ends of these sets are disappointingly easy to overload.

The RA1772 came along and the front end trouble was definitely fixed. It was an abrupt and large jump in performance.

David

vintage_8bit · 28th Oct 2021, 10:50 am

Thanks for the last two post, very interesting. It reminded me about the teleprinters & terminals we had at work year's ago. Just used locally to communicate with units under test. Colin

Bill MCSL · 28th Oct 2021, 6:53 pm

The MA197B was originally designed for the Royal Navy for "Common Antenna Working" ie to permit a single wideband antenna to feed a number of preselector / receivers, tuned to different frequencies. Typical use would be on board a ship, where transmitters were very close to receiving antennas, hence the "protection" part of the title. The various units would be daisy-chained together.

At the frequency to which the preselector is tuned the input impedance is 50 ohms. At all other frequencies it is very high. Thus all other frequencies will be reflected from the first preselector, and passed on via the daisy chain with quite low loss.

Bill

vintage_8bit · 3rd Nov 2021, 12:23 pm

Well after a slow power up the electrolytics seem good and as far as I could tell with my random wire antena connected, and looking at the output on my s.d.r. it appears to work. I can see a humped band of signals & noise traveling up and down in frequency as I change the tuning on the preselector.

Re comment from Bill"(The MA197B was originally designed for the Royal Navy for "Common Antenna Working")" Following up on intermittent output, I found that solder had never flowed to the center conductor on the aerial input socket. I wonder if that ever caused past operators problems. Specially as it could of been used in the path of an aerial daisy chain. Colin.

24th Oct 2021, 3:24 pm	#1
vintage_8bit Heptode Join Date: Feb 2013 Location: Rainham, Kent, UK. Posts: 525	Racal Preselection & Protection unit Hi. I have just obtained a Racal RA 17L. (Might ned some help with that later), anyway it came with an extra 19" rack sub assembly. Looking at the attached images can anyone identify it & tell me what its purpose is? I'm sure it will not be of any use to me but interested in what it does. Colin Attached Thumbnails

24th Oct 2021, 3:29 pm	#2
Sean Williams Dekatron Join Date: Feb 2005 Location: St.Ippolyts, Hitchin, Hertfordshire QRA IO91UW Posts: 3,517	Re: Racal Preselection & Protection unit Hi Colin, This is inserted in the antenna lead before a receiver - it is another tuned circuit which helps keep unwanted signals out of the RF section of the receiver. Quite a useful bit of kit, and will also possibly contain a quantity of amplification, which is sometimes also helpful on the higher frequencies. The protection unit function comes into its own when you are trying to operate a receiver on a multi-transmitter site - the extra rejection of out of band signals really saves the receiver, especially if there are several tens of volts of stray RF floating around. __________________ Engineers make things work and have spare bits when finished

24th Oct 2021, 3:39 pm	#3
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,858	Re: Racal Preselection & Protection unit RA17s suffer a number of spurs and images. Its little internal preselector isn't enough. This unit fixes that problem. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

24th Oct 2021, 6:05 pm	#4
vintage_8bit Heptode Join Date: Feb 2013 Location: Rainham, Kent, UK. Posts: 525	Re: Racal Preselection & Protection unit Thanks gents. I was hoping to get rid of it due to its size. Noting the comment "Quite a useful bit of kit" and David's post, I'm not sure what to do with it now. I expect the RA 17 will more than suit my needs on its own. I wonder if there's a manual for the preselector unit somewhere? Colin

24th Oct 2021, 6:44 pm	#5
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,858	Re: Racal Preselection & Protection unit The preselectors are a lot harder to find than RA17s, so be sure you really will never want one before selling it. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

24th Oct 2021, 9:15 pm	#6
Dave757 Heptode Join Date: Apr 2009 Location: Scratby, Norfolk, UK. Posts: 649	Re: Racal Preselection & Protection unit Hi Colin, 51 page technical Manual at www.pa3esy.nl/index-gb.htm Select receivers from menu and go down to Racal. Kind regards Dave G0ELJ

24th Oct 2021, 9:41 pm	#7
vintage_8bit Heptode Join Date: Feb 2013 Location: Rainham, Kent, UK. Posts: 525	Re: Racal Preselection & Protection unit Thanks for the link. I have had a good read & now have the manual, only a couple of valves in it (M8136's) and a bridge rec. I'll hang on to it for now then. It will be a few weeks before I get round to the Ra17. Now where can I hide it....

25th Oct 2021, 2:25 am	#8
Refugee Dekatron Join Date: Apr 2012 Location: Worksop, Nottinghamshire, UK. Posts: 5,553	Re: Racal Preselection & Protection unit If you want to do some serious short wave listening it will almost certainly get rid of the hover board charger noise from the aerial feed to pretty well any short wave receiver.

25th Oct 2021, 2:03 pm	#9
vintage_8bit Heptode Join Date: Feb 2013 Location: Rainham, Kent, UK. Posts: 525	Re: Racal Preselection & Protection unit Ok. Change of plan. I'll move on my Marconi r.f. TF 1066B/1 genny to make space for it. I will put the RA 17 on the shelf & when time permits look at the Preselector first. I have an Eddystone S640 I can try in on. Colin.

26th Oct 2021, 12:57 am	#10
turretslug Dekatron Join Date: Nov 2011 Location: Surrey, UK. Posts: 4,394	Re: Racal Preselection & Protection unit As Crocodile Dundee might have put it, "this is a preselector", made to a specification rather than a price by folk who had established a reputation for knowing what they were at. Just the mechanics of the device with its sturdy partitioned case and the six ganged tuned circuits with their worm drives etc. would have cost a small fortune. The valves are professional-spec ECC82s connected in cascode mode acting as low-noise, strong-signal capable buffers with only as much gain as is sensible. They don't make them like that any more and they don't grow on trees, think carefully before moving it on, provided you've got the space!

26th Oct 2021, 9:50 am	#11
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,858	Re: Racal Preselection & Protection unit Then there's also the SSB adaptor to look out for...... David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

26th Oct 2021, 11:04 am	#12
vintage_8bit Heptode Join Date: Feb 2013 Location: Rainham, Kent, UK. Posts: 525	Re: Racal Preselection & Protection unit I'm looking forward to getting it on the bench, spurred on by all these positive comments. In the past & on other communications receivers I've just used the on-board BFO to listen to SSB. What does the SSB adapter do differently? Colin

26th Oct 2021, 12:40 pm	#13
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,858	Re: Racal Preselection & Protection unit It's an ssb-dedicated IF filter, a proper product detector etc. There is a big improvement in audio quality and clarity. I used an RA117 for many years, with just the BFO for SSB demod, but you begin to hanker for something easier to use and clearer to listen to. In the end I never came across an SSB unit (there are several variants including ISB) so I went and bought a new ICOM IC765. __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

26th Oct 2021, 4:21 pm	#14
Draker4c Pentode Join Date: Jun 2017 Location: Basildon, Essex, UK. Posts: 188	Re: Racal Preselection & Protection unit Hi Colin You have a pm. Hugh G1AUR

27th Oct 2021, 6:19 am	#15
Synchrodyne Nonode Join Date: Jan 2009 Location: Papamoa Beach, Bay of Plenty, New Zealand Posts: 2,944	Re: Racal Preselection & Protection unit Some information on the Racal SSB adaptors may be found here: http://www.chavfreezone.me.uk/racal/...-adapters.html. I suppose that you could say that the addition of an SSB adaptor took the RA17 into the capability realm of point-to-point SSB/ISB receivers of the time, I’d guess with a combination performance that was probably somewhere above that of the single-box Marconi HR22, but below that of the full-height rack HR21. In those days point-to-point receivers required an AFC system to track the pilot carrier, something that with steadily improving transmitter stability had become optional by the later 1970s (Marconi H2540 era). One of the Racal SSB adaptors was configured for broadcast relay use, with 6 kHz audio bandwidth, and could be used to provide synchronous detection of HF AM signals, thus minimizing selective fading distortion. Cheers,

27th Oct 2021, 10:43 am	#16
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,858	Re: Racal Preselection & Protection unit The target market for the RA17 were military and governmental organisations. At the time, these people used CW Morse code, FSK for teleprinters, and AM for speech. There wasn't any significant interest in spending money on SSB capability. So the RA17 got narrow filters and a BFO for Morse, that 1.2kHz filter and the BFO for FSK to feed a teleprinter via a 'terminal unit' that FSK de-modulated audio tones, and the wider bandwidths with the BFO off to do AM. But SSB was a growing thing. It's spectral efficiency and its power efficiency were attractive, so Racal brought out SSB adaptor boxes with a whole duplicate final IF in them, with a balanced 'product' detector and a stable BFO preset to the right frequencies. So the RA17 straddles the rise of SSB in military/governmental radio systems. SSB was invented as a method of cramming multiple conversations onto a single phone line and dates back to the 1920s, but it took a while from invention to getting rolled out in any quantity. These systems were called FDM, frequency division multiplex, and eventually were cramming up to 2600 channels onto a single coax line in the 1970s. They ended as digital techniques replaced them. It was some radio amateurs who tried using SSB techniques over the air instead of over wires. One prominent exponent of the time was called Arthur (Art) Collins. Yes, that Collins, he went on to found Collins radio. Bell had got what they wanted for their phone lines, they had much less interest in radio things. So, Collins receivers have SSB capability from early on, It's where they came from. Racal sets after the RA17/RA117 have SSB facilities built in. When they did the cute little RA1217 and RA217, their first transistor sets, they got full-blooded SSB filters product detectors and accurate oscillators for carrier reinsertion. Filters became optional an so you could order all sorts of combinations, you didn't have to have the SSB filters. As early transistor jobs, the fornt ends of these sets are disappointingly easy to overload. The RA1772 came along and the front end trouble was definitely fixed. It was an abrupt and large jump in performance. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

28th Oct 2021, 10:50 am	#17
vintage_8bit Heptode Join Date: Feb 2013 Location: Rainham, Kent, UK. Posts: 525	Re: Racal Preselection & Protection unit Thanks for the last two post, very interesting. It reminded me about the teleprinters & terminals we had at work year's ago. Just used locally to communicate with units under test. Colin

28th Oct 2021, 6:53 pm	#18
Bill MCSL Diode Join Date: Jun 2016 Location: Whitby, North Yorkshire, UK. Posts: 2	Re: Racal Preselection & Protection unit The MA197B was originally designed for the Royal Navy for "Common Antenna Working" ie to permit a single wideband antenna to feed a number of preselector / receivers, tuned to different frequencies. Typical use would be on board a ship, where transmitters were very close to receiving antennas, hence the "protection" part of the title. The various units would be daisy-chained together. At the frequency to which the preselector is tuned the input impedance is 50 ohms. At all other frequencies it is very high. Thus all other frequencies will be reflected from the first preselector, and passed on via the daisy chain with quite low loss. Bill

3rd Nov 2021, 12:23 pm	#19
vintage_8bit Heptode Join Date: Feb 2013 Location: Rainham, Kent, UK. Posts: 525	Re: Racal Preselection & Protection unit Well after a slow power up the electrolytics seem good and as far as I could tell with my random wire antena connected, and looking at the output on my s.d.r. it appears to work. I can see a humped band of signals & noise traveling up and down in frequency as I change the tuning on the preselector. Re comment from Bill"(The MA197B was originally designed for the Royal Navy for "Common Antenna Working")" Following up on intermittent output, I found that solder had never flowed to the center conductor on the aerial input socket. I wonder if that ever caused past operators problems. Specially as it could of been used in the path of an aerial daisy chain. Colin.