January 19 2000
is one of the best known radio manufacturers - both of professional
equipment and earlier also of top-class ham radio gear.
lots of information available about Collins equipment in the Internet,
in many "Boat Anchor" pages. I have not repeated here
information that's readily available, for example in the Collins
Virtual Radio Museum.
1940's and 1950's Collins manufactured serveral versions of the
51J receivers. In Finland, I have seen models 51J-2, 51J-3 and
51J-4, the latter being more common. The differences between models
are relatively minor, except for the inclusion of mechanical filters
in model 51J4, which brought a substantial improvement in selectivity.
Otherwise, the 51J4 is almost identical to the 51J-3.
bands the 51J-4 operates as a double conversion superheterodyne.
The first conversion uses a fixed (crystal controlled) oscillator,
with band-switched crystals. The first IF is variable; the second
conversion uses VFO injection, outputting a fixed 500 kHz second
IF. The receiver covers 0.5 to 30.5 MHz; on the lowest bands,
single conversion is used. The second IF includes the mechanical
filter(s) and a variable "phasing" control, providing
an adjustable notch - at least when the radio is properly aligned.
It is not advisable to try to align the receiver without the full
service manual and proper test equipment.
to Racals, the 51J series receivers are straightforward to use
and tune. This is due to the very complex tuning mechanism - a
nightmare to service and align if a previous owner has tampered
with it without proper knowledge.
mechanical construction, the Collins is lighter, using sheet metal
construction vs. Racal's die-cast frame.
Shielding is not as good
as in Racals, but then again the principle of operation is very
different. For example, the Racal's first IF is much higher, requiring
more shielding. Mechanical filters are often missing in used 51J-4's;
if all filters are missing, the receiver can be made operational
by replacing a filter with two resistors and a capacitor - selectivity
will then of course not be any better than in the 51J-3.
are views of the Collins 51J-4 from underside, from the rear and
behind the front panel. In the latter picture you can see how
the dial string is threaded, should it happen to break. The mains
transformer of this receiver has been replaced with a Finnish
one and the rectifier tube has been removed.
modifications may be desired in 51J-4's intended for day-to-day
listening. The rectifier tube can be replaced with silicon diodes
equipped with series resistors. The mains side wiring should be
checked and modified to be more compliant with current safety
standards. Bare solder lugs in the mains circuit should be covered
with shrink tubing. The phone jack can be replaced with a stereo
jack for use with modern headphones. A tape recording output can
be added by soldering a 0.047 uF capacitor and a 100k resistor
in series to the AF gain potentiometer's hot end, and then bringing
the signal out via a shielded cable to a connector in the back
panel. To improve sensitivity on band 1 I have added a ceramic
10pf/1000V capacitor in parallel with C117.
sensitivity, the voltage at the RF-gain potentiometer's ground
end should be checked. If it is not between 1.3 and 1.4 volts,
the resistor in series with the potentiometer should be changed.
In one 51J-2 receiver, the voltage was right when I added a 1.5
kilo-ohm resistor in parallel with the series resistor. I have
also changed the AGC time constant by adding a 1 uF/400V capacitor
in parallel with C205B.
are lots of additional improvements one can make. In one 51J-4
I made substantial changes, like the transistorized VFO pictured
here. When adding transistor circuits you'll of course need to
add a suitable power supply.