[Milsurplus] ARC-38 chopper issues

[Ray Fantini]

The 400 cycle choppers are a Collins device. They are in a small metal tube about two and a half inches long and sealed. They have a seven pin base. Do not have a unit in front of me so do not have the part number.  The vibrating part of the chopper works fine, you cannot mistake when one is running so am assuming that the contacts have failed. I do have one where the motor or whatever is inside the unit died. The choppers take a small sample voltage provided by a centering circuit or the output of a discriminator and use that to produce a 400 cycle AC signal that is amplified and used to drive a 400 cycle servo motor with two sets of windings. One winding is a reference winding and the second is feed from the servo and the direction of the motor is controlled by the phase difference developed by the amplified output of the chopper so the signal on the chopper contact is very small and low current. In normal operation a positive voltage on the input of the chopper causes the servo to track in one direction, a negative voltage causes the servo to track the opposite direction and zero volts results in the motor stopping. Those engineers were very clever back in 1950! Although I have not worked on one would assume the T-195 uses the same systems to auto tune, also have to wonder about the GRC-106 and if there are choppers in their? I am using a external instrument grade 400 cycle inverter to provide power for all the 400 cycle servo circuits, the chopper coils themselves run from 6 volts AC provided by a transformer from the 115/400 cycle bus.
Ray Fantini KA3EKH


>>> "J. Forster" <jfor at quik.com> 10/2/2009 11:35 AM >>>
What is failing? The coils or the contacts? If you drive the thing w/ an
audio oscillator, can you feel it vibrate? Are there 1 or 2 driver coils?

If it's the contacts, running it for a while and switching some current
through it, might clean the contacts.

Is it a stock part W/ a MFGrs name & number?

Best,
-John