The cheaper phenolic switch sections on the PTO can cause alignment issues when cycling.   Relays can be replaced but are also a failure mode.  I started to rewire mine since I picked up a power supply rack but haven’t finished.   

Jack wa7dia has had his on in the past couple of weeks but again ran into an issue he is trying to debug.    Wonder if the vibration in the planes they were used in actually helped to wiggle things back into alignment!!

I don’t know the last time Pete, Wb2jwu fired his up.  He kept it tethered to 3885 to eliminate the switch issue above…

Steve, kd3ht

From: [email protected] [mailto:[email protected]] On Behalf Of Ray Fantini
Sent: Sunday, December 31, 2023 12:40 PM
To: [email protected]
Subject: [Milsurplus] AN/ARC-38 question

when referring to an ARC-38 or an ARC-38A they have two components that make up the frequency selection, the simple part is the Collins PTO assembly that feeds the radio. from what I recall it operates from 2 to 4 MHz and the radio uses doublers beyond that for the other bands. Thats simple being it's a fairly standard PTO, but that's driven by a mechanical assembly that interprets the commands from control head and uses maybe one of the most complex and unexplainable processes for setting the PTO to the correct frequency. It has cryptic numbers on wheels, eccentric cams and planetary gears. I have no idea how it works; I have a running ARC-38 that I use on AM and am happy that the mechanical computer still clicks along but if it failed beyond cleaning and lubricating, I would be screwed. Just looking at the servos of the PA and variable IF you have to think that those engineers back in the middle of the last century must have been giants compared to what we are doing today. I have always regarded my fully working ARC-38 and 180 antenna tuner a marvel of analog technology.

Have to wonder in today's world of doing everything digitally if anyone would still be able to develop such a mechanically complex system ?

Ray F/KA3EKH