[Milsurplus] ARC-38 chopper issues

[J. Forster]

The thing was likely made by Airpax, Bristol, Bulova/American Time
Products or Burr-Brown.

The guts of vthe thing is almost certainly a spring and mass like a
one-legged tuning fork. The mass will likely be a magnet. Sort of like
this:

|
|           v |--|
|-------------|  | //////
|           ^ |--| |     |
|                  o     o


The mass and spring are resonant at the driving frequency and the mass
moves up and down, making and breaking contacts.

===

The reason I asked about TWO coils is that some choppers have a drive and
a sense coil and are driven by an amplifier and self oscillate. Bulova,
American Time Products, frequency standards work this way.

===

If there is only one coil, it is likely resonant like a 60 or 400 Hz
Frequency Meter as seen on generators.

What I'd try, assuming you have a pinout of the thing is:

Drive the coil from a variable frequency & amplitude AC source, like an
audio generator. You may need a transformer to boost the amplitude. They
don't draw much power. Start at the nominal voltage & frequency for the
unit.

With an AC supply from 60 Hz, like a filament transformer and limiting
resistor, run current from each contact to common. Something like 24 VAC
limited to say 25 mA would be a decent starting point. DO NOT use DC. It
can arc.

Monitor the voltage across the chopper contacts you are powering w/ a
scope and look for reliable contact closure. Sync the scope from the coil
drive signal.

If you don't get reliable closure after 5 or 10 minutes, try increasing
the drive voltage and diddle the frequency a bit.

Repeat for other contacts.

Good luck.

-John

==========




> 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
>
>
>
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