[Premium-Rx] Racal 177x PSU

[John Nelson]

>Next week I will sart with the PSU modification, I will replace the bridge
rectifier diode D2 in a fresher place. You can see the steps here:<

Replacing D2 may be worth doing, especially if one can find a Schottky
bridge, but there is another PSU modification that's even more important.
All the RA177x family are excellent from an RF point of view but the
youngest is now around thirty years old and many have led hard lives in
professional service. Unfortunately some aspects of their mechanical and
electrical design were not conducive to longevity. For example, the main
wiring harness between the mains transformer, the rear-mounted PSU board,
the reservoir capacitors and the rear heatsink carrying the PSU pass
transistors was executed in PVC-insulated cable which in service became very
hot. Over the years this caused the plasticizer to leach out and vigorously
attack any copper it could find. The result is extensive radial cracking of
the insulation and corrosion of the wiring and associated terminals. The
cure is to rewire the entire harness in modern PTFE cabling, which is a
chore but not inordinately difficult and allows easy replacement of the
reservoir capacitors at the same time. 

A little forced-air cooling goes a long way with these receivers. A small
fan (I use 68 x 68mm) adjacent to the rear heatsink and arranged to blow air
on to it drastically reduces the temperature around the PSU area. If in the
original Creeth case, the fan can be directly mounted on the slotted
vertical section over the heatsink.  

Several other electrolytics inside the unit will have dried out over time
and need replacing, and some of them (e.g. C27 and C29 in the RA1778) are
difficult to access without fairly extensive dismantling. I've never
bothered changing the 150uF capacitors on the IF boards and elsewhere,
having never come across a leaky or reduced-value example, but plenty of the
470uF 25V components have required replacement. 

It's worth remembering that the printed-circuit boards in the RA177x family
were not manufactured to modern standards and the tracks and pads are
relatively fragile. It is all too easy to break them away from the substrate
if too much heat is applied whilst soldering or desoldering. The PM335
first-mixer board is a particular case in point. The mixer amplifier output
transistors (TR2 and TR5) have marginal heatsinking and run very hot. They
quite often fail if the receiver is mounted in a rack or case where there is
inadequate cooling, and replacement almost always involves concomitant
repairs to the PCB. 

For best results from AM broadcasting the eight-pole McCoy 6K8 filter seems
to be the best one to have. The McCoy 200Hz filter is excellent for CW. 

73 John
GW4FRX