[Premium-Rx] Racal RA1792 PSU problem?

[John Nelson]

I've recently been involved in the restoration of a number of RA1792s. Apart
from the usual crop of problems which one would expect to find in receivers
that have been out of service for some time, one particular oddity in
respect of the A10 PSU has emerged about which I'd welcome any observations
from other users. 

Operated off-load for measurements and testing, all these PSUs run almost
silently. On load, some are quiet and remain relatively cool whilst others
run noticeably warmer and generate a considerably higher level of mechanical
hum, which seems to originate in the transformer. The measured values of the
three main reservoir capacitors C1, C4 and C7 are usually a good 10% higher
than the marked values, as expected, and the ripple voltages inspected with
an oscilloscope don't show any marked disparity between one unit and
another, from which I assume that the ripple currents aren't excessive. The
capacitors don't run warm either. All the rail voltages check out and the
receivers' BITE doesn't choke on low voltages when the units are used for
their intended purpose.  

I suppose the laminations could just be looser in some transformers than in
others, especially since some of these receivers probably ran 24/365 for
many years in professional service, but it would be interesting to hear
other opinions and any suggestions as to what else might be checked. 

73 John
GW4FRX