By David W. Knight. G3YNH
Part 1 of 5:
The Racal RA17 was arguably one of the finest short-wave radio
receivers ever built using valve technology, so much so that it
set the standard for receiver performance well into the 1970s.
The reason for its success lay in its excellent frequency stability,
low noise, and EMP hardness; all of which proved to be a difficult
act to follow using early transistor technology. I first met,
and fell in love, with the RA17 in 1969, when I was given the
job of setting one up as a training exercise; but not being a
Government department, I was unable to own one for myself until
they came onto the surplus market in quantity in the late 1970s
and early 80s. Even today, the RA17 is still a good receiver,
but it lacks many of the features that modern working practice
demands. It is relatively future-proof however; and it is possible,
without modifying it in any way if so desired, to provide adapters
to make good many of its shortcomings.
If one is to use the RA17 (rather than put it on a very strong
shelf and admire it) there is room for improvement in the following
areas:
Frequency Readout: The film-scale readout and 100KHz comb-spectrum
calibrator was a tremendous improvement over earlier communication
receiver designs, but it is a wonder that by the late 1960s, Racal
did not modify the RA17 / RA117 to incorporate digital readout.
It seems that the company was busy trying to promote the all-transistor
RA217; and had no desire to update the venerable classic, even
though it was superior to its replacement. The RA17 however, has
an interpolation receiver VFO output at PL11 (next to the VFO
valve V12) and this can be taken straight to the input of a DFM.
The 1MHz internal reference is also available on the back panel,
so that the DFM and receiver can both be locked to the same reference
and calibrated simultaneously.
Reception Modes: The RA17 has no facility for receiving
FM, and its provision for receiving SSB and CW consists of a BFO,
injected straight into the AM detector according to best 1930s
practice. The problem with this type of BFO implementation is
that it overloads the AGC system, forcing the use of manual RF
gain control, and makes it impossible to obtain readings of RF
level. The lack of proper SSB reception facility was of course
addressed by Racal in the form of the RA63 SSB Adapter; but RA63s
are difficult to obtain. A perfectly satisfactory alternative
is to take the RA17 IF output and feed it into a simple product
detector. When I bought my RA17, I deliberately chose one of the
specialised models (RA17C18) which has no BFO, so that I would
be forced to build it a product detector before I could use it
(I was also a student at the time, and RA17Ls were more expensive).
Audio quality: The RA17 bucked the communication receiver
trend by having a loudspeaker on the front panel. This feat of
ergonomic brilliance was marred however, by the audio quality,
which for moderate periods of listening, constitutes a cruel and
unusual form of punishment. It is best to bypass the receiver
audio system altogether, and to bypass the AM detector, which
is merely a diode rectifier. It is of course, standard practice
in most AM radios to use a simple diode detector, but nowhere
is there evidence that this is a good idea. A phenomenal improvement
in AM quality is possible by using a precision rectifier stage,
which also gives a response almost down to DC and a truly linear
representation of RF level.
RA17 Adapter overview:
The RA17 Adapter discussed in this article combines digital
readout, precision AM detector and level meter, product detector,
NBFM detector, audio stages, and CW filter into one unit. The
original was designed and built in 1981, using technology which
was perhaps a little old-fasioned even then, but it should serve
to illustrate the necessary principles for those interested in
using more up-to-date techniques. A block diagram is given below.
An audio filter for CW is provided in this design because the
RA17C18 used by the author does not have an IF crystal filter.
The frequency counting system measures the interpolation receiver
VFO and calculates IF offset to display the KHz part of the listening
frequency. The MHz part is obtained from the RA17 'Megacycles'
dial as usual (a discussion of how to implement full frequency
readout is given in part 4). When the variable frequency carrier
insertion oscillator is in use, the counter measures both VFO
and CIO and displays the listening frequency corresponding to
zero-beat. The demodulation mode switch (S2) has five positions:
CW, SSB, AM, FM, and Auxiliary audio input. SSB and CW are the
same except that, in the CW position a filter is inserted into
the feedback loop of the audio power amp. The audio line amp does
not have a CW filter, since the principal purpose of the line
output is to permit tape-recording of telephony signals (and nowadays
to feed RTTY, FAX, etc. into a computer sound card). All three
detectors are permanently connected to the IF input, the FM detector
also providing a centre-zero tuning meter, and the AM detector
an RF level meter; CIO injection is however suppressed except
in SSB and CW modes. The CIO injection switch (S3) has three positions,
'Fixed', 'Variable', and 'Read'. In the 'fixed' position, the
CIO is fed with an exact 100KHz signal derived by dividing the
1MHz reference by 10. This position is used for calibrating the
receiver onto frequency standards broadcasts, and for finding
the exact carrier frequency of AM stations. The 'Variable' and
'read' positions are the same, except that in 'read' mode, the
counter is switched to display the frequency of the CIO. To decode
USB transmissions, the receiver bandwidth is set to 3KHz, and
the CIO is set to 98.2KHz. To decode LSB, the CIO is set to 101.8KHz.
The 'read' setting is also used for calibrating the CIO onto the
centre of its tuning dial. In retrospect, the procedure for changing
sidebands is rather slow, and it would have been better to have
provided two extra positions: 'USB' and 'LSB', with an additional
crystal oscillator and two crystals for the IF-1.8KHz and IF+1.8KHz
feeds.
» Part 2
D.W. Knight. 1981, 2000