[Milsurplus] Collins 618S/ARC-38 project

[Ray Fantini]

A brief review of nineteen fifties servo systems and where I am today,
the Collins automatic tuning systems used in the fifties and early
sixties are true marvels of science. Not to be confused with the old
auto tune systems employed before that only relied on a preset
mechanical memory the arc-38/618S and the T-195 have a true automatic
tuning system. A short description is that you have a servo motor with
two sets of windings, one winding is feed a 400 cycle reference voltage
and the second winding is driven by a servo amplifier that generates a
400 cycle tuning voltage that changes in phase to the reference and
drives the servo motor one way or the other. The servo amplifier is
driven by a mechanical chopper that takes a sample voltage and converts
it to a 400 cycle signal that changes its phase in proportion to the
sample voltage. This voltage is developed by a discriminator circuit
that in resonance has an output of zero volts and will go more positive
or negative the further away from resonance. The 618S1/ARC-38 has two
servo systems. The first drives the variable IF deck and driver tuning.
The second controls the PA tuning for the transmitter. Both use the same
type of dual winding 115 volt 400 cycle servo motor and identical servo
amplifier/chopper modules the discriminators for the IF system is
located on the IF deck and the other is on the PA deck. I can see
reference voltage from each discriminator and see them reach zero if I
manually turn the drive gears manually. both servo amplifier modules
have been substitute out with other units and I would assume that I
don’t have four bad amplifier modules. The 400 cycle choppers
vibrate and appear to work but how do you tell with that sort of thing?
And I have cleaned all the drive gears, rails and sliding components and
put small amounts of oil on them , there is no way I am going to find
the lubrication specified in the book so have been using three in one
oil and would think that’s ok. All of these items turn freely by hand
but have not checked it in inch pounds. If I use an external DC voltage
source to feed the reference input to the chopper amplifier system I can
drive the servos in both directions, with just the output from a VOM in
the Ohms scale you can drive the system up and down and with that being
the case am inclined to say that the discriminators diodes are week and
not generating enough voltage. So that’s where I am today. Every other
aspect of the radio works perfectly, it has full power in transmit,
receiver is very sensitive and all the channel changing works well, just
no automatic servo system. One last thing the 115 volt 400 cycles is
provided by a solid state inverter so don’t think there are any issues
their. As always there are pictures on my web page at www.ka3ekh.net
under “project of the week”
Ray Fantini KA3EKH