Racal RA17 MkII

 Below I'm going to cover some general notes then I'll describe a couple of repairs (first & third examples)

First is my 1st example then later here's my second example

 In some ways the RA17 receiver, introduced in the 50's, was ahead of its time but in an odd way it harks back to the twenties. The latter is due to the fact that to tune a particular station requires the use of four knobs! It was ahead of its time as it uses some of the techniques employed by modern digital receivers to overcome frequency drift, which in the wholly "analogue" RA17 is all but absent.

In the 30's when superhet receivers were being introduced to replace TRF, or straight designs, they were heralded by advertisements stating that a single knob was used for tuning. Gone were the days when first a tuning dial was positioned and then a reaction control was adjusted before one could sit back and listen to your favourite broadcasts. The latest TRF sets had been an advance on early models as the former had ganged tuning of several stages whereas the really old models needed a whole row of dials to be set and twiddled before a single station could be heard. The only way to cope with this was to write down the details of all the dial positions for each favourite station, and to this end each dial carried a plate calibrated 0-180 degrees.

Anyway back to the Racal. First you set the number of Megahertz (this is equivalent to a wavechange switch); then you set the Kilohertz knob to approximately where you expect the station to be. Then you set the aerial tuning bandswitch to the correct range and twiddle the associated tuning control. With luck you should hear the background noise peak. Then you return to the Megahertz knob and re-peak the background noise with that. Finally you return to the tuning dial and look for the station. Of course with practice these steps are second nature but it is certainly not as easy to use as an ordinary superhet. Is all this really necessary you might ask, and why was the Racal so successful in its field if it demanded all this fiddling?

Well it had two important features; first the dial was accurate to about 5Khz and if you knew where to look you could sit and wait with confidence for a signal to appear; second the receiver doesn't drift so once tuned the set could be operated solely by adjusting its subsidiary controls.. bandwidth, RF gain, AF gain etc. All this is now commonplace with the most elementary of modern sets because of the "digital revolution". Now that frequency synthesisers are used, the features that were once unique to the Racal are redundant. In a similar way to the Philips receiver, which was the ultimate in TRF design, described elsewhere on this site, the Racal reached the pinnacle of "analogue" superhet development.

The last great advance in analogue receiver design, adopted (for the first time in a commercial receiver) by Racal was the "Wadley Loop". This allowed for complete compensation of the the chief circuits that drifted. The design of the basic tuning VFO for which compensation was not possible was given special treatment and as its range is in the low HF region this circuit could be made extremely stable and to all intents and purposes, drift free.

In a nutshell the "triple conversion" Racal uses a tunable VHF oscillator (the MegaHertz knob), a tuneable low HF oscillator (the KHertz knob), several multi-section filters, numerous mixers, harmonic generators and the like, together with twenty-odd valves and of course lots of knobs to cover the range from "zero" to 30MHz. I say zero because signals can be tuned almost down to "no" KHz. Not quite... because of a huge heterodyne present at this frequency (or should I say "lack" of frequency?). Performance drops off below about 500KHz because the antenna variable tuning arrangement doesn't go that low and one has to rely on the untuned (or wide) position of the switch.


 Above is a picture of the Racal now in my collection. It's a MkII which started life at GCHQ, probably in Cheltenham, possibly close to the office where I once used to work but maybe just down the road at a second site in the same town. Most of the GCHQ receivers were rack mounted and the Racal was probably the first type available in those days suitable for this mode of operation.

When I'd found a suitable military style "Plessey Plug", fitted it to a mains lead and plugged it in I was rewarded with a lot of background noise and absolutely no sensitivity at the aerial socket. I connected a long wire aerial (actually the wires to the intercom between the workshop and the house) and carefully tuned around 1.2MHz. With a little difficulty, using the BFO, I managed to resolve a faint heterodyne. By carefully tweaking all the knobs on the front of the set I eventually heard "Virgin Radio" with an awful signal to noise ratio.

The only real worry I had was that the main tuning control was very rough. At each revolution there was a very lumpy feel and generally the mechanism felt rough. I peered inside and saw some very inaccessible gearwheels. Was it damaged teeth or was it a damaged no-longer-obtainable slow motion component? The whole satisfaction of the receiver depends on a nice feel to the tuning as to go from one end to the other of the enormous tuning scale demands scores of revolutions of the knob! I turned the knob backwards and forwards lots of times looking for a corresponding telltale movement in the gears.. but there wasn't any juddering. Then the penny dropped. The knob had been bumped. I found an Allen key, slackened off screw in the centre of the knob and pulled it out a little. When I'd tightened the Allen screw, tuning was as smooth as silk. The cause was the back of the knob, which has a web design, catching on the little clips holding the transparent plastic rubbing plate in position. These rubbing plates prevent the paint around the knob from wearing out.

Back to the Racal's deafness. After removing the top aluminium cover I felt the temperature of the valves. All were lit, and closer examination under the black cans, revealed that most were either new or hardly used (the lettering was clear and fresh looking and hadn't burnt on). As I wobbled some of the more accessible valves the noise level changed and when I wobbled a particular, very hot E180F, Virgin Radio suddenly jumped to half scale on the signal strength meter. I removed two valves and applied switch cleaner to their sockets. The E180F had gold plated pins so I didn't bother scraping these! I could now hear the odd weak station on the medium waveband. I removed the aerial and tried a signal generator. Sensitivity was attrocious. I could just hear 10 millivolts of test signal across the whole range but nothing less, and 100millivolts was needed to give a workable S-meter reading.

Time for a bit of detective work. First check the odd valve on the valve tester or by substitution, then check all the resistors and then perhaps look at the alignment. All the important valves were OK, as I'd guessed. At one point I thought I'd found a discrepancy in the wiring to one of the mixers but hopes were dashed when I discovered the British and American versions were pinned differently... G2 and G3 were reversed.

A first study of the underside of the chassis revealed something interesting. Someone had been here before! I looked at the last IF strip which is at 100KHz and found lots of paralleled resistors. Now these old carbon "rod" resistors always go off. I think it may be corrosion or weakened grip of the brass end caps under the ceramic material. I don't think it's heating and cooling because I have an enormous number of brand new unused resistors of this type and very few are within spec. Most resistors climb in value, none get less. A simple way to compensate is to solder a second resistor across the old one. The Racal is not easy to work on so removing a resistor is not always easy and paralleling is a convenient option. Resistors for valve circuits are not too critical so I don't worry too much unless the measured value is say 700k instead of 470k. One can quickly place a second component across the offending one and if something worthwhile happens change the original, otherwise crack on. All the decoupling capacitors appeared to have already been changed so I didn't bother with these.

After removing all the lower screening panels, of which there are numerous, and changing a couple of resistors in one of the compartments, sensitivity improved slightly. I don't yet have a manual or a circuit diagram, and as the next step is quite critical I had to be careful. This step involves twiddling. There are literally dozens and dozens of trimmers and many of these are associated with filters. If the characteristics of the filters are altered the job may turn into a major exercise to get the receiver back in business so I first marked the position of every one of the trimmer capacitors in the filters. This was easy as air-spaced types are used and a fine pen gave a mark to which one can revert to in the event of a dead end.

I initially checked the 100KHz IF strip and using a signal generator set to 100KHz (without wobbulating it) and with the crystal filter set to its narrowest setting peaked the trimmers. Now Virgin Radio was clear and the meter read over half scale.

Next using the signal generator set to 1.2MHz, and connected to the aerial socket, I tackled one of the filters. There are two fitted with manual adjustments. Both have a row of trimmers going down the chassis then turning a corner and continuing across for a distance. I wobbled the trimmers for each of the sections of the first filter and got a corresponding wobble on the S-meter. Next I tackled the second filter. I gingerly "wobbled" the first trimmer. The S-meter wobbled in sympathy. The second responded, the third, the fourth and so on. The first one round the corner didn't have any effect no matter how much I rotated the rotor and neither did the next but one. The others responded with a sympathetic S-meter wobble. Now because all the sections of the filter basically look identical, it follows logically that if some sections tune nicely, then the others should as well. Across each trimmer was an immaculate-looking red 33pF siver mica capacitor. I removed the one across the first suspect section and fitted a replacement. The section now responded to a wobble with the trimmer set to half-mesh. I went to remove the second suspect capacitor and found that although the first end had a nice blob of solder on its lead it wasn't actually soldered to the trimmer. I removed it and fitted a replacement anyway. Now that section peaked nicely with the trimmer rotor about half mesh.

I backed off the signal generator and found to my relief that a strong signal was now tunable with the generator set to below 1microvolt. Sensitivity for half-scale indication had changed from 100 millivolts to better than 1 microvolt! I connected an aerial and found that my six inch crocodile clip test lead was now producing a better Virgin Radio signal than previously had the long wire. In fact, bringing my hand near to the lead caused the S-meter to press against its stop.

I refitted all the covers and de-upended the Racal back on its feet and connected a length of wire. On 21MHz I heard an OK1 talking to a W8. He said the W8 was S7 but I could only receive him at S3 but that's good enough for me on a short length of wire stuck in the back!

My father-in-law who was G3AQY, bought a Racal RA17 when they were first introduced. The price was astronomic. That receiver is still in the family. My brother-in-law has it now, complete with a manual for which he is now searching. Once I've borrowed it I'll be able to set up the filters properly. Thankfully there wasn't anything seriously wrong with my purchase. Looking back.. judging from the amount of resoldering, the last owner presumably spent fruitless hours looking for the elusive fault. Virtually every solder joint had been seen to... to the extent that one could be excused for believing that this example was the first prototype! However.....each of the metal screens covering the filters had been taped over with masking tape plus cellotape as if he'd steeled himself not to touch the infamous filters. If only he had, and resorted to "intelligent" twiddling, he may have been able to enjoy the delights of this wonderful receiver.


Now I'll discuss my third example which arrived in August 2020 from Dave G3SUL who mentioned that it had a few problems. 


 The first thing that needs to be done is to lubricate the tuning mechanism. The Mc/s tuning is so stiff that the knob is slipping on its shaft. I tightened it up to check exactly what was going on and discovered that there's probably a gear whose grease has hardened to the extent its acting like a rubbery glue. If I help the Mc/s dial to rotate the mechanism feels better which sort of points to the dial shaft or its immediate gears, but to gain access to the gearbox etc will mean detaching the front panel, something I've not attempted before. The panel appears to be secured by 8 large 2BA screws, a whole pile of knobs, a couple of jack sockets and two switches, plus the meter and loudspeaker.

Not only is the Mc/s tuning stiff, but spinning the Kc/s tuning knob results in a scraping noise which seems to be the result of a stiff vertical idler shaft around which is fed the filmstrip. You can see some wear on the filmstrip numbers so this needs to be sorted out before any serious damage is caused. Dave, G3SUL, also mentioned that one of the valveholders needs looking at. Other than this, the chassis looks very clean, without the corrosion often seen on other examples eg. my second example...


 Above you can see all the knobs have been removed. The circular knobs require a 3/16 inch Allen Key which fits centrally into a screwed cup which grips the control shaft whilst the others each use a pair of 5/64 inch Allen Screws... except the Lock knob which has a pair of 1/16 inch Allen Screws. The speaker and meter toggle switch nuts (use an 18mm spanner) are now detached and some fixing screws removed (there are eight in total, six have captive nuts but the two lower ones are fitted with a washer and nut which you can access once the underside metal cover has been removed (see below). I can recommend these Allen Keys which will also do for many HP equipments. Never use a metric key for Imperial Allen Screws (and vice versa if it comes to that). Before removing the last front panel fixing screws detach the two meter leads and prepare to unscrew the loudspeaker.


 The panel lifts off once the fixing screws are removed, leaving the loudspeaker fastened to the panel via four 6BA screws. These should be removed and the speaker can be balanced as shown further down the page.






 Once the panel has beens detached and the Mc/s dial has been removed you can access the drive mechanism to the Mc/s tuner. The mechanism can be detached after removing the four 4BA nuts and washers in the corners of the fixing plate.

I hadn't noticed this until prompted, but all three gears plus the chain were made by Meccano not far from where I used to live in the Liverpool suburbs in the 40s through to 1965.

 Below, a view of the mechanism. It was clear at this point that the input and output shafts were completely gummed up. The idler wheel which sets the chain tension was also sticky.


 In order to free the shafts I had to remove the fixing hub for the Mc/s dial. This is fixed in place with a pair of 0.05 inch Allen Screws. These are locktighted so take care not to use an ill-fitting key. The hub needed to be removed so I could apply switch cleaner at the point where the shaft goes through the plain bearing. Resting the assembly on its gears for a few minutes allowed the shafts to start to free up. I then operated the shafts fully from end to end of their travel until they freed up completely then applied a little oil. The idler gear wheel wasn't too bad and freed up easily. It's a good idea to remove old grease and certainly any fresh oil on the metalwork in case this attracts dirt or drips off later. I chose not to completely disassemble everything and completely clean and regrease the mechanism.

The spring on the right is used to apply pressure on the idler gear shaft to maintain proper chain tension. The right hand end of the spring is secured by a bent solder tag held in place by the 4BA nut. This does seem a little Heath Robinson to me but it works.



 Above (right) a view of the Mc/s tuner drive shaft fitted with anti-backlash gears. I dripped switch cleaner and oil between the gear pair in case the gears were sticky and I was lucky to find the shaft rotated freely. Below the gears is the tuning lock mechanism which appears to be OK. I also dripped a little oil on its shafts. I then dripped oil on the Kc/s tuning bearing and anti-backlash gears (left). Below.. the Kc/s mechanism is very noisy in operation and I found this was partly due to dry bearings top and bottom of the metal idler shaft used to line up the scale.


 Above, the idler now cleaned of ink deposited from the scale lettering (note the blurry red markings), and top and bottom bearings exposed for oiling. The two bearings are very noisy and could be replaced but this would entail a lot of work so I just applied oil and spun the shaft. Hopefully the bearing noise will reduce. Before reassembly I'll remove surplus oil and apply a little grease. Reassembly of the Mc/s and Kc/s mechanisms is going to need reference to the Racal Servicing Manual and I'm not refitting the front panel until I'm happy with the dial settings.



On top of the two reels of filmstrip you can see this triangular plate supporting an idler gear. This fits between a pair of gears (one on each reel) to couple them together. Before refitting the idler gear plate the front reel has has to be rotated to provide tension in the filmstrip.

I found the idler gear was also suffering from gummed up grease. In fact I now suspect all the moving lubricated parts in this receiver were treated with the same grease.

Tension adjustment seems a bit hit and miss as first the gear nearest the front of the receiver is rotated to tension the filmstrip with this idler plate pushed clear of the gears, then once tension is about right you slip the idler into place, locking the gears together. There's also an adjusting screw which when slackened will allow the idler gearwheel to be moved but I'm not sure this was the better method. I'm not entirely happy with the filmscale winding as it seems to catch on the lower edge of the front reel and I also noticed the left sprocket wheel was noisy. Instead of smooth tuning there's a plasticky ticking noise from the filmstrip which isn't present on either of my other examples.

I found absolutely no free movement at the worm drive (almost seized?) so I'll examine the top bearing next.



 Pieter, PA3HDU suggested looking on the back of the receiver for a plate indicating a rebuild. Sure enough this example was refurbished in Week 27 of 1976.

You'll note one of my pet hates here.

I learned that "M"=Meg and "m" =milli.

Megacycles is OK but the audio power output is more than a bit optimistic!

When I looked under the chassis I spotted the output transformer. It seems Parmeko used the same labelling "MW" as the Racal labellers.



 The mains setting is realistic for my QTH at 245 volts.

 Signs of a change?





 Above left is the bearing cap at the top of the Kc/s tuning shaft which carries a worm drive mating to the tuning condenser in the VFO. The bearing it covers (above) was packed with a nasty black putty-like stuff. The cap carries a hex screw fitted into a locking nut. I suspect this nut is 0BA as my 7/16" spanner was a poor fit. The hex screw needs a 1/8th Allen key. I could shift the two 4BA screws holding down the cap easily but the nut was extremely tight. As soon as I'd started removing the bearing cap the Kc/s tuning freed up like magic. Left is the bearing cap with its mating depression surrounded by the black gunge before cleaning. After cleaning and greasing the cap and bearing I fitted the cap back in place so that end float was virtually indiscernible. The tuning knob now spins freely and some (but not all) of the objectionable filmscale noise has gone (see below).



 This picture shows the front filmscale spool. You can see the scale is pressing (albeit very slightly) on the lower edge of the spool so that it makes a scraping noise as it winds or unwinds. Maybe wear or mechanical tolerancing has caused the problem? I would have thought that there should have been a method of altering the level of the spool. It might also be a tensioning problem as the instructions in my manual for fitting the scale are rather vague. I've also noticed the tuning weight changes from light at the 100Kc/s end to a bit heavier at the 900Kc/s end.


 I encountered a real problem when I was experimenting with flywheel tuning. The adjustment at the top of the tuning shaft needs careful adjustment to remove end float. If the screw is too tight the flywheel is damped. I happened to spin the tuning too close to the low end of the scale... the STOP warning flashed past followed by a loud clunk and I could no longer operate the tuning knob. It seemed to be seized. Looking at the picture on the right, that squarish thing was hitting the top or bottom of the slot when I tried the tuning knob and nothing would persuade the tuning knob to tune without slipping.

It turned out the tuning limit mechanism had jammed. That squarish thing moves backwards as the tuning goes from zero to 1000Kc/s and instead of having an endstop at the front of the slot the designers relied on the rear face of the front flywheel to limit tuning. My problem was that the thing was jammed so tightly that I needed to use a lever in one of the holes in the flywheel edge to free the jam and to do this I had to remove the front plate for access.

The plate carries a small bearing which turned out to be stiff from gummy grease just like the others.


 Before deciding on what to do about swapping the tubular condensers I turned on the power and, with a long wire, checked the performance. The set seemed very lively and no real smell of burning (previously noticed), but I noticed the Mc/s scale was a bit out. Rather than change the tuning I'm going to rotate the dial gear clockwise a few teeth. When I'd refitted the chain gear assembly I'd set it so the vertical line near the "0" was vertical but I think this is wrong because the line at the "29" end is coming up short. Tuning 40m revealed the nearest band indication was 6 and with "0" showing I was receiving a strong medium wave bradcast at about 1.2MHz instead of 198KHz. Tuning across a strong AM signal showed the filters were pretty good with no sign of multiple responses but with the skirts maybe not as good as they could be, however the long wire might be producing exceptionally strong signals that would explain this. Below, listening to 40m CW.


 After ten minutes, I'd sorted out the Mc/s number which is now almost right, just a little right of the marker line. I glued the escutcheon in place because it was only secured to the panel by the cursor adjuster. I also cleaned and glued the scale window perspex in place and cleaned the escutcheon. A thin coating of 3-in-one resulted in a nice shiny finish. All the smaller knobs needed a residue of a greenish paint removing from them and the chunky switch knobs needed relining. I used some white typing corrector fluid, let it dry, then scraped off all but the centre line.. Finally I refitted the front panel. I discovered the loudspeaker wires weren't actually soldered so wrapped them around the speaker connections and switched on.

Results were disappointing to say the least.... complete silence although the S-Meter responded slightly to tuning, but then I remembered that Dave, the last owner, telling me there was a duff valveholder and sure enough the RF amplifier was cold. After a waggle the S-Meter showed plenty of signal from Radio 4.. but still no audio. I jiggled all the switches and then discovered the speaker on/off switch was upside down.... and after turning the switch to "OFF" there was loads of sound. After trying several bands I worked out that on a really strong 40m SSB signal (9A4Y in Croatia) I could turn the aerial attenuator to MAX and still hear decent audio. During tests I'd reset the BFO knob so it read correctly at "0". Setting it to -2Kc/s resulted in nice resolution of 40m SSB.

I don't think I need to worry about the decoupling condensers after all, just correct a few minor details. Invert the speaker switch, solder the speaker wires and perhaps check over the two filters. These seem fairly good but not perfect and at the same time maybe tune the oscillator so that all the range Mc/s number are centrally displayed. An annoyed feature is there's a beat note on Radio 4 which I suspect is on exactly 200KHz and means you have to slightly detune from 198KHz to get rid of it. I seem to recall hearing this beat note on a previous RA17... The thing must be internally generated because turning the aerial attenuator switch to the first attenuated setting makes the birdie get relatively a lot stronger than Radio 4. In the non-attenuated setting the birdie is relatively unobtrusive when Radio 4 is slightly detuned but objectionable if attenuation is used.

 I've just finished the minor jobs... not so minor though was turning the loudspeaker switch the right way up. I had to remove most of the front panel fixing screws, the IF gain and BFO knobs and the BFO valve in order to extract the switch and turn it through 180 degrees. The speaker wires were then only just long enough to reach the speaker. After trying out the receiver for a short time I noticed that if you were tuning at around 900Kc/s the tuning/scale would creep slowly down in frequency. I very slightly turned the bearing cap screw clockwise. As pressure increased the scale stopped creeping and the tuning friction hadn't really changed.


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