Aligning a Racal RA17

Here's the tricky filters


 It's been several years since looked at my Racal receiver.
At the time I checked and replaced some resistors and capacitors.
I have a vague idea there's something not quite right. Tuning a strong station reveals three peaks, like three stations side by side. Sensitivity isn't great either and generally it seems a bit docile. Also I notice that the megacycle dial has to be set half way between the numerals.
So I reckon it needs re-alignment.
Since I now possess a spectrum analyser with built in tracking generator I am able to sort out the notorious Racal band pass filters.
There are two complex filters. One centred on 40Mc/s and the second 37.5 Mc/s.
The former is a problem as it needs to have a pass band of around plus and minus 650KHz. ideally it should pass frequencies within the range 39.35 to 40.65 Mc/s.
When I last looked at the RA17 I'd discovered it was very deaf. This turned out to be a faulty silver mica capacitor in the 40Mc/s band pass filter.
At the time I just twiddled each of the eight trimmers (having marked their original settings) and discovered one didn't show any signs of peaking. The others when turned right or left had an effect on the signal strength of a station but one didn't. I fitted a new fixed capacitor of identical marked value and found the associated trimmer now peaked a signal.

The set then worked quite well. That was perhaps ten years ago…

I was now quite keen to see the shape of the response of the 40Mc/s filter.

click here to for practical info
After removing V5 and V9 and the 1Mc/s crystal, I connected the DSA815 output to Test Point 2 and its input to Test Point 3 using miniature crocodile clips together with a third to a local chassis connection.
I set the centre frequency of the DSA18 to 40Mc/s and the span to 5 Mc/s, giving 500KHz horizontal divisions.
The top of the curve peaked at around -67dBm and the noise level was around -90dBm (these are relative figures and dependent on the high impedance amplifier I'm using).

A word of warning here. As spectrum analysers are very expensive and modern models often have a relatively low maximum DC limit at input and output it is advisable to isolate these from any inadvertent HT from the RA17. Test points 2 and 3 are basically OK in this respect but one can never be too careful.

Note that I used a homebrew high impedance (greater than 1Mohm) amplifier at the input and a DC blocker (2 x 0.22uF 500v capacitors in series with back to back 15V zener diodes to ground at their junction) at the output. This inserts quite a lot of loss into the measurement process but as the loss is constant over a very wide range of frequencies this is inconsequential if the internal amplifier in the DSA815 is used and the tracking generator is set to a high enough level (0dBm)

Surprisingly the shape wasn't too bad, however, although the top was more than 20dB higher than the skirts the centre frequency was way over to the right. In fact it was nearer a 41Mc/s band pass filter rather than a 40Mc/s filter.

Using the spectrum analyser I experimented by twiddling the eight trimmers to see the effects of each. Some shifted a peak to the right and some peaked to the left. Some made the top of the response wider and some made it narrower, but nothing clear cut, all very vague and woolly.
After half an hour or so I got the filter centred on exactly 40Mc/s. Initially the top was half a meg wide being nice and flat from 40.3 to 40.8 Mc/s but not wide enough in respect of the RA17 spec. A bit more fiddling and the top got wider. Now, the top was a Meg wide having a peak to the left at 39.5 Mc/s and a peak on the right of 40.5 Mc/s with a rather saggy centre, but was now a respectable 3Mc/s wide at the noise floor.
The next day I read a bit about overall alignment and decided to sort out the misalignment of the Megacycle dial.
This dial is on the shaft of the capacitor that tunes VFO1 which tunes in the range 40 to 60 Megacycles. The figures on the dial represent the number of whole megacycles to which the RA17 is tuned in the relationship where "1" equals a VFO setting of 41.5Mc/s and "29" equals a setting of 69.5 Mc/s.
The reason for the wide response of this filter now becomes meaningful. If the VFO drifts due to warm-up, or if it shifts due to mechanical vibration affecting the oscillator valve or due to an imperfect setting of the megacycle knob the set should still work.
In fact if one slowly tunes the megacycle knob whilst tuned to a strong station the response of the 37.5 megacycle filter can be observed.
In my case there are two peaks (39.5 Mc/s and 40.5 Mc/s) and a dip of around 2 or 3 dB in the centre at 40 Mc/s and this shape can clearly be seen as a weakish station is slowly tuned using the megacycle knob.
The position of the knob is best set in the zone in between the two peaks. If any drift occurs the signal will increase slightly in strength.
First though let me explain what I found when investigating the megacycle dial offset.
Station peaking resulted in the megacycle dial ending up half way between the numbers.
This could be due to the dial being fixed in the wrong place on the capacitor shaft or the VFO not tracking properly.
I decided to investigate the latter.
This is where I had a brainwave. I have a communications receiver that tunes the VHF range and can be quickly set by keying in a number. I connected the aerial input of the VHF receiver to a length of cable terminated with crocodile clips and clipped these to the Racal. The ground clip to the chassis and the live clip to a wire passing close to the VFO valve, V5.
I then tuned the VHF receiver to 69.5 Mc/s AM and adjusted the Racal megacycle dial to "29". By slowly tuning the latter backwards and forwards I was able to hear the 69.5 Mc/s Racal VFO. Setting the latter to "1" allowed me to hear the 41.5 Mc/s signal also, however finding the exact signal was not too easy as a mixing process took place and resulted in a whole set of medium wave signals emerging from the VHF receiver loudspeaker. In fact using dial setting "2" turned out to be easier ("2" is equivalent to 42.5 Mc/s).
After some experimentation it was apparent that the Racal VFO was out of sync with its dial.
Basically the VFO frequency range is shifted with respect to the dial so that instead of 41.5 Mc/s at "1" the frequency is about 41 Mc/s or 41.5 Mc/s ends up half way between "1" and "2".
Clearly, if the frequency needs to be aligned, I need to adjust and track the VFO so it lines up with the dial. Where is the VFO trimmer and coil tuning slug?
The answer is; underneath the metal box carrying V13 and V15. This box carries the calibrator and is connected via a coax cable and an octal socket. A securing screw needs to be slackened off and the calibrator pulled away and lifted clear to access the VFO settings.
Once this is done the trimmer capacitor and coil tuning slug adjuster can be seen.
The latter is locked with a nut and I could see from distortion of the screw slot that it's been tackled before.
Using the VHF receiver I was able to fiddle with the trimmer and the tuning slug to bring the VFO into line with the dial settings.
The trimmer adjustment is extremely critical as we're dealing with frequency changes of around 10Kc/s, but I was able to set this correctly by persevering.
For reference I checked the VFO frequencies at either side of the black "1" marker.
The leftmost edge was 41.47 Mc/s and the rightmost edge was 41.55 Mc/s, a range of 80Kc/s.
41.5 Mc/s ended up almost in the centre of the black "1" mark.
At the other end of the Mc/s dial the leftmost "29" was 69.41 Mc/s and the rightmost was 69.57 Mc/s (a range of 160Kc/s).
If I'd carried on fiddling the settings could have been more accurate, but the final settings were good enough.
Once I'd finished the VFO settings I plugged the calibrator back in and tried the RA17 on an aerial.
Performance was now an order of magnitude better and the megacycle dial was within the black markings for all settings.
I confirmed the BFO was aligned correctly by switching the receiver to the BFO calibration setting and confirming that zero-beat was at the zero position of the BFO adjustment knob. Tuning to 5, 10, 15 and 20 Mc/s gave me very weak reception of standard frequency stations and these corresponded exactly to the zero position of the Kc/s dial setting (adjusting where appropriate the sliding plastic dial marker to conform to the relevant receiver calibration marker).
I checked the "drift-ability" by moving the Mc/s knob whilst tuned to a strong medium wave station. This is essentially moving the VFO frequency downwards and upwards from the centre frequency and observing signal strength. The station was lost at settings of 41.31 and 41.83 Mc/s. The frequency difference of about half a meg, to a large extent represents the pass band of the 40 Mc/s band pass filter.

Next I need to confirm the 37.5 Mc/s band pass filter is acceptable, the 100 Kc/s IF amplifier is OK and overall sensitivity is within spec.

During my experiments it became pretty obvious that the modern listening environment was extremely poor. When I used to listen to radio in the 50s the background level between stations was really quiet and you could virtually hear a pin drop. In those days the short waves were full of fascinating tunes. Every broadcast station had it's own signature tune, Radio Moscow, Radio Peking, Radio Sofia and the Voice of America, all had their own very recognisable tune.
As the years rolled on, in order to comfortably continue listening one had to switch off fluorescent lights and rely on ordinary lighting. That's not true any more as modern electric lamps use integral switch-mode power supplies, even worse than tubes.
Add to this a host of other extraneous noises and short wave listening can be impossible.
Telephone lines carry broadband signals, dozens of electronic items on standby (including computers), hosts of plug-in switch-mode power supplies radiating interference, and mains wiring circulating all sorts of weird noises.
The answer now, I suspect is to erect a vertical aerial as far away as possible from telephone wires and mains wiring, and to feed this via good quality coax.

Another solution may be to employ a tuneable frame aerial or a ferrite rod mounted high up and away from buildings. Both can be rotated to reduce interference. To make best use of these techniques would need a method of tuning. Maybe a varactor diode connected to a potentiometer located adjacent to the receiver?

The next step was to re-check the 37.5Mc/s and the 40Mc/s filters as I didn't care for the slight double hump response when tuning the Mc/s knob. A second exercise squared off the top at the expense of a slight narrowing of the pass band of the filter. It now has a width of 1.0Mc/s at the -6dB points instead of the recommended 1.3Mc/s. At 20dB down the width is now 1.8Mc/s and at -40dB it's now 2.4Mc/s.
I might have another go at this if it fails to work out in practice.

The 37.5 Mc/s filter has more adjustments but can be narrower than that for 40Mc/s (recommended at 300kc/s).
I tried adjusting it using the spectrum analyser but I wasn't happy with the loss of gain for a relatively small increase in bandwidth, so I just peaked all the trimmers using a signal generator set to 5Mc/s connected to the aerial socket.
I also tweaked the 100Kc/s slugs in the final IF amplifier, but these were relatively flat so just peaked them up.

During proceedings I checked overall sensitivity. First at the low end of a band, choosing 29.000Mc/s. I found the set could easily hear 0.5 microvolts modulated at 1 Kc/s.
I then tuned to 30.000Mc/s and discovered the receiver was completely deaf at this setting.
The sensitivity was OK up to about 29.6Mc/s then the test signal just vanished.
A look at the circuit diagram indicated the likely problem was VFO2 which tunes 2.1 to 3.1Mc/s.
I connected the DSA815 to a convenient point which was a socket close to V11 and set the RA17 back to 29.000Mc/s and adjusted the analyser centre frequency to 2.5Mc/s with a 2Mc/s scan width.
The display showed a good solid signal at 3.1Mc/s at a level of -36dBm (this figure is relative) but as I tuned the RA17 up in frequency the signal started to drop in amplitude as its frequency decreased until, at 2.5Mc/s it just disappeared into the noise. Just before it vanished it stood at -48dBm with the RA17 tuned to 29.600Mc/s.
I hunted around and found a used EF91 and plugged this in place of the original valve.
After a minute or so the signal reappeared but this time it vanished at around 2.1Mc/s when it stood at -44dBm. A second hunt through my boxes of valves revealed a brand new EF91. This did the trick. The signal tuned right across its range starting at -32dBm and dropping to -38dBm.

Another test and I found that at 1.5Mc/s I could comfortably hear a quarter microvolt modulated signal and with the BFO switched on I could just hear a signal of only 0.1 microvolt.
The set now tunes stations nicely with no spurious humps and the Mc/s dial smoothly increases and decreases signals with each Mc/s setting within the width of the associated black mark.

However, I'd like to play around with the set before putting it all back together and maybe making more adjustments?


Return to RA17

There's also a page or two on sorting out a second RA17.

I bought this around 10 years after the one described above, that readers might find informative

click here