(per request)
The only "modifications" needed to a Command transmitter
are simple and 100% restorable.

1. Replace the three capacitors in the bathtub on the back.
Even if you think they're good, they're bad. Trust me on this.
Leakage in this bathtub cap causes big headaches,
and if it isn't leaking now, it will be.
Here's an easy way:
Unsolder the leads of the bathtub cap, leaving the wires intact.
Make sure multiple wires stay connected to each other, just not the cap.
Coat them with insulation. "Liquid Tape" by GC is good and available
at most electronics parts places, or just use tubing.
Leave the wires where they are as their distributed capacitance
is part of the design.
Get some of those little mylar .047s from the parts house.
They're about the size of a pencil eraser. Trace the wires from
the bathtub to where they connect, then solder the cap between
there and the nearest ground.

2. Unsolder one lead from the antenna relay. Roll the spring
contact from the loading coil around so it's always in contact.
Either put some Cramolyn on this contact or polish it as
detailed below.

3. Check the selector relay and make sure that the osc. B+
contact (the "short" one, nearest the coil)
makes before the 1625 cathode contact. Carefully bend
them to make sure. You key the rig with this relay
through it's contact on the back plug-it's called the "selector."
Put a "spike killer" diode across the relay coil winding
(anode to positive voltage lead) to keep the inductive
kick off the filaments and keying leads.

That's it. All the hole drilling and output hacking and
"de-TVIing" of the 50s and 60s was completely unwarranted.

If you want to make it easy to tune and remove
a source of possible "chirp," the next steps are
some work, but well worth the effort-

All the roller coil assembly metal parts (less screws)
are plated in silver. This gets oxidized (that black stuff).
When you're loading the rig, you'll notice "intermittent" power
out as you roll over the gunk. If your loading point is
cruddy with this black stuff, it will heat when you key
and change the loading enough to introduce some chirp
(yes, believe it or not).

Go to Wally World and get some cream silver polish.
Don't waste your money on the "dip" stuff.
Put a towel or other "parts catcher" on your bench.
Completely disassemble the roller coil unit.
Pay close attention so you can get it back together
in the right order.
The leaf-spring contact on the coil axle (ceramic support end)
needs to go back right-side-up, so you might want to mark it
or just notice how it goes back in.
Use a linen or other lint-free cloth to polish the coil,
roller bar, roller, axle leaf spring contact etc.
Use a "Q" tip to polish the inside of the roller wheel as well.
Once all the crud is polished away, rinse off all the polish
with clean water and allow the parts to completely dry-
especially the coil. Don't want water under the turns.

As you reassemble, lubricate the thumb-wheel end and thumb-wheel
axles with petroleum jelly or graphite. Do not lube the
ceramic-to-coil joint, but do lubricate the roller coil axle and
leaf-spring contact with paste Cramolyn or other
good contact lube. Be careful not to overtighten the screws
that are threaded into the ceramic.
Just finger-tight them and lock them in place with some
paint or fingernail polish. Seal the roller-bar screws
with paint as well. Now enjoy easy tuning.

On "chirp-" This is a classic MOPA rig. While you can expect
some small amount of chirp, I've got many transmitters in which
it is difficult to hear any at all. I've never had one that,
after I finished restoring it, had what I call "bad" chirp.
Noticeable chirp in an "ARC-5" transmitter
can usually be traced to a few sources.
Here they are in order of their usual appearance:

1. Poor power supply regulation.
Your supply needs guts enough to maintain 500-600 VDC at 200 mils
and 24 VDC at about 2.5 amps.
(if you don't want to use DC on the fils, put a diode and
filter cap at the relay)

2. Leaky bathtub caps. Reduces osc. drive, messes with
Osc B+, reduces grid bias to the 1625s. See above.

3. Cruddy output tuning network or antenna connections.
Anything that changes the loading- like heating the crud
between the roller and coil-
is going to pull the freq of an MOPA rig.

3. Low under-load emission in the 1626 Osc.
Swap it out with a known-good one.

4. Grid emission in one or both 1625s. Especially
noticeable in one that's been run at 800+ volts.
Swap with known good ones.

I've seen changed resistor values in the osc. stage
and misc. other stuff, but these four are the usual bandits.
While I've heard of the other mica caps going bad, I've
worked on many dozens and never seen it happen. I've
also never seen that custom "button" cap in the MO stage go bad.

I hope this will help out.
Next post for the power supply.

73 DE Dave Stinson AB5S