[ARC5] Transmitter capacitors

[Roy Morgan]

On Dec 27, 2015, at 7:14 PM, AKLDGUY . <neilb0627 at gmail.com> wrote:

> Each ARC-5 transmitter has two large variable caps...
> On the side of each capacitor is a machined plate
> that is bolted to the studs through which the worm
> drives pass.
> What is the function of the plates?

Neil,

I just had a look at a stripped transmitter chassis here (5.7 - 9 mc) and I see what you are asking about.  Here is my take:

The axial play in the tuning shaft must be kept to a minimum, especially at the oscillator tuning capacitor.  The studs you see need to be very rigidly held in position to accomplish this.  The plate across the top of the two studs adds a lot of rigidity to the whole thing.  Both the variable capacitors have spring loaded anti backlash driven gears of considerable robustness.  The two capacitors appear to be identical, at least in shape.  There has been a report that the oscillator capacitor is made of low-thermal-coefficient metal and so it might actually be different in that respect compared to the plat tuning cap.

It seems to me that the tracking of the oscillator frequency vs. dial calibration is more important than the tracking of the final tank resonant frequency

Note that there is an axial play adjustment at the rear stud of the oscillator capacitor - an adjustable screw locked in place with a nut, and cemented with glyptal.  This is at the rearmost end of the tuning shaft assembly.  The calibration of the dial in frequency depends on there being no play in this mechanism.  I’m not sure if there is any similar mechanical part in the plate tuning capacitor - I don’t see one in my unit.

If you have a URM-25D, see if the frequency shifts if you pull and push on the frequency set knob.  If it does, this is due to the end play adjustment being a bit loose.  The same thing would happen in the command transmitter if there was axial play in the shaft.

I look forward to the ideas from other folks.

Roy

Seasons Greetings to all!

Roy Morgan
k1lky68 at gmail.com
K1LKY Since 1958