January 19 2000

Collins is one of the best known radio manufacturers - both of professional equipment and earlier also of top-class ham radio gear.

There's 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.

In the 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.

On most 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.

Compared 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.

Regarding 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.

Here 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.

Some 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.

For optimum 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.

There 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.