[ARC5] Dynamotors.

[Mike Hanz]

On 2/21/2015 12:03 PM, Todd, KA1KAQ wrote:
> On Sat, Feb 21, 2015 at 11:44 AM, J Mcvey via ARC5 <arc5 at mailman.qth.net>
> wrote:
>> I have also found that it is very important to locally ground the the rack
>> to the chassis with the shortest possible length in addition to the power
>> ground . This dramatically improves the s/n ratio and avoids overloading
>> the front with noise.  Those little suckers work great with short mobile
>> antennas!
> Unless they've been removed, there should be a push terminal just like the
> aerial terminal minus insulator, located below and behind the front rack
> clips on each side specifically for this purpose.
>
> Several racks I purchased in a bulk deal from overseas last decade still
> have the original grounding cables attached. They consist of a piece of
> bare steel braid maybe 3-4 inches long with what looks like an over-sized
> phone tip connector (as seen on old headphones) soldered to one end. Those
> not left behind in the aircraft probably disappeared fast on surplus racks
> with receivers converted for AC supplies, but should be simple enough to
> make if you can find or make a suitable pin connector.

Todd's suggestion is spot on.  There was a later evolution of this approach on the postwar equipment that had even better grounding performance, consisting of a short, thin copper bronze strap running from one of the fastening screws on the shock mount used for the airframe support, up to the snap slide stud after the rack was mounted on the mount.  There was a hole on the end of the strap that slipped over the stud, and when the snap was slid over the stud there was a fairly good mechanical contact made to the rack.  While a better approach might have been to terminate the strap on the receiver somewhere using a nut plate, this approach provided a lower impedance ground without making removal more difficult for servicing.  There were two of these straps, diagonally located on the shock mount.  They were usually accompanied by a similar pair on the receiver dynamotor mount to provide a ground for the dyno that was more effective than the single small diameter wire under the motor frame.  The engineers back then were no dummies...

     73,
Mike  KC4TOS